Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Regulation of cerebral and systemic blood flow in humans with high level spinal cord injury: the infleunce… Phillips, Aaron 2013

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
24-ubc_2013_fall_phillips_aaron.pdf [ 3.66MB ]
Metadata
JSON: 24-1.0074203.json
JSON-LD: 24-1.0074203-ld.json
RDF/XML (Pretty): 24-1.0074203-rdf.xml
RDF/JSON: 24-1.0074203-rdf.json
Turtle: 24-1.0074203-turtle.txt
N-Triples: 24-1.0074203-rdf-ntriples.txt
Original Record: 24-1.0074203-source.json
Full Text
24-1.0074203-fulltext.txt
Citation
24-1.0074203.ris

Full Text

     REGULATION OF CEREBRAL AND SYSTEMIC BLOOD FLOW IN HUMANS WITH HIGH LEVEL SPINAL CORD INJURY: THE INFLUENCE OF ALPHA1-AGONIST MIDODRINE HYDROCHLORIDE by Aaron Alexander Phillips B.Sc., The University of Western Ontario, 2007 M.Sc., Brock University, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR of PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Experimental Medicine)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) August 2013 ? Aaron Alexander Phillips, 2013 ii  ABSTRACT RATIONALE: Spinal cord injury (SCI) is a devastating condition, often resulting in severe autonomic dysfunction. Secondary to autonomic dysfunction, many individuals with SCI have low blood pressure, which likely challenges the brain?s capacity to maintain suitable perfusion. Individuals with SCI suffer from a myriad of conditions that may be associated with dysfunctional regulation of cerebral blood flow (CBF), including cognitive dysfunction, orthostatic intolerance and stroke risk. Current knowledge regarding CBF regulation in SCI is limited. AIMS: To comprehensively examine cerebrovascular control in those with SCI, including: 1) the CBF response to cognition, 2) the acute CBF response to tilt, 3) the relationship between baroreflex sensitivity and CBF regulation, 4) the role arterial stiffening plays in baroreflex sensitivity, and 5) to examine the influence of increasing blood pressure through oral administration of 10 mg midodrine (alpha1-agonist). The first investigation found that the CBF response to cognition is absent in those with SCI when compared with able-bodied individuals. When restoring blood pressure to normal levels with midodrine, the CBF response to cognition was improved by 70% which was reflected in a 13% improvement in cognitive function. The second investigation showed that midodrine improved orthostatic tolerance 59% in SCI, and mitigated the reduction of CBF during tilt. The CBF response to tilt was well-maintained in SCI. The third investigation assessed the relationship between common carotid artery stiffness and baroreflex sensitivity in those with SCI. Elevated arterial stiffness (+12%) in those with SCI were negatively related to reduced in baroreflex sensitivity (R2=0.45; p<0.05). Administration of midodrine did not influence baroreflex sensitivity or arterial stiffness. SUMMARY: Studies one and two provide evidence that the CBF response to metabolic stimuli (i.e., cognition) is reduced in SCI, while CBF is effectively maintained in response to alterations in blood pressure. Furthermore, study three shows that reduced baroreflex sensitivity in SCI is closely related to increased arterial stiffness in the SCI population. It is concluded that CBF regulation is altered in those with SCI, and may be related to clinical outcomes such as cognitive decline and orthostatic intolerance.    iii  PREFACE Part I of the literature review (Chapter 2) has been previously published as: Phillips, A.A., Krassioukov, A.V., Ainslie, P.N., Warburton, D.E.R. (2013) Baroreflex function after spinal cord injury. Journal of Neurotrauma. 29,15: 2431-2445. I wrote the manuscript. Krassioukov, Ainslie and Warburton provided editorial feedback on this publication.  Part II of the literature review (Chapter 2) has been accepted for publication as: Phillips, A.A., Ainslie, P.N., Krassioukov, A.V., Warburton, D.E.R. Cerebral blood flow regulation after spinal cord injury. Journal of Neurotrauma. (Acceptance ID NEU-2013-2972). I wrote the manuscript. Krassioukov, Ainslie and Warburton provided editorial feedback on this publication.  The project in Chapter 3 received ethical approval from the UBC Clinical Research Ethics Board (Certificate #H11-02823). Portions of Chapter Two have been previously published as: Phillips, A.A., Krassioukov, A.V., Zheng, M.M.Z., Warburton, D.E.R. Neurovascular coupling of the posterior cerebral artery in spinal cord injury: A pilot study. Brain Sciences. (WM, CD, IR) 3, 2: 790-799. I identified the research question, designed the study, collected the data, analyzed the data, and wrote the manuscript for the publication. Krassioukov, Zheng and Warburton assisted with data analysis and provided editorial feedback on the manuscript.   The project in Chapter 4 received ethical approval from the UBC Clinical Research Ethics Board (Certificate # H11-00048).   The project in Chapter 5 received ethical approval from the UBC Clinical Research Ethics Board (Certificate # H11-00048).         iv  TABLE OF CONTENTS ABSTRACT ................................................................................................................................ ii PREFACE ................................................................................................................................... ii TABLE OF CONTENTS ........................................................................................................... iv LIST OF TABLES ..................................................................................................................... ix LIST OF FIGURES ..................................................................................................................... x LIST OF ABBREVIATIONS ................................................................................................... xii ACKNOWLEDGEMENTS ..................................................................................................... xiv CHAPTER ONE - INTRODUCTION ........................................................................................ 1 STATEMENT OF THE PROBLEM ................................................................................................ 2 OVERVIEW ..................................................................................................................................... 2 CHAPTER TWO - LITERATURE REVIEW ............................................................................ 4 PART I ? CEREBROVASCULAR CONTROL AFTER SCI ................................................ 4 SUMMARY ...................................................................................................................................... 4 INTRODUCTION ............................................................................................................................ 4 REGULATION OF CEREBRAL BLOOD FLOW .......................................................................... 6 Cerebral autoregulation ................................................................................................................. 8 Cerebrovascular reactivity ............................................................................................................ 9 Neurovascular coupling .............................................................................................................. 10 CEREBRAL BLOOD FLOW AFTER SPINAL CORD INJURY ................................................. 10 Cardiovascular consequences of spinal cord injury .................................................................... 11 Static cerebral autoregulation...................................................................................................... 13 v  Dynamic cerebral autoregulation ................................................................................................ 17 Cerebrovascular reactivity .......................................................................................................... 18 Neurovascular coupling .............................................................................................................. 19 FUTURE DIRECTIONS ................................................................................................................ 20 CONCLUSIONS ............................................................................................................................. 22 PART II - BAROREFLEX FUNCTION AFTER SCI .......................................................... 30 SUMMARY .................................................................................................................................... 30 INTRODUCTION .......................................................................................................................... 30 PHYSIOLOGY OF THE BAROREFLEX ..................................................................................... 32 TECHNIQUES TO MEASURE BAROREFLEX SENSITIVITY ................................................. 34 BAROREFLEX FUNCTION AFTER SPINAL CORD INJURY ................................................. 42 High lesion level ......................................................................................................................... 44 Low lesion level .......................................................................................................................... 50 FUTURE DIRECTIONS ................................................................................................................ 52 CONCLUSIONS ............................................................................................................................. 53 PART III ? MIDODRINE HYDROCHLORIDE .................................................................. 62 THESIS OVERVIEW, AIMS AND HYPOTHESES ........................................................... 64 CHAPTER THREE ? REGIONAL NEUROVASCULAR COUPLING AND COGNITIVE PEFORMANCE IN THOSE WITH LOW BLOOD PRESSURE SECONDARY TO HIGH-LEVEL SPINAL CORD INJURY: IMPROVED BY ALPHA-1 AGONIST MIDODRINE HYDROCHLORIDE ................................................................................................................ 66 INTRODUCTION .......................................................................................................................... 66 METHODS ..................................................................................................................................... 68 vi  Protocol ....................................................................................................................................... 68 Data acquisition .......................................................................................................................... 71 Cognitive assessment .................................................................................................................. 71 Data analysis ............................................................................................................................... 71 Statistical analysis ....................................................................................................................... 72 RESULTS ....................................................................................................................................... 72 Cerebral blood flow velocity response ........................................................................................ 73 Able-bodied versus spinal cord injured - verbal fluency task ................................................. 74 Effect of midodrine in SCI .......................................................................................................... 74 Verbal fluency task ................................................................................................................. 74 Visual task ............................................................................................................................... 74 Cognitive Function ...................................................................................................................... 75 DISCUSSION ................................................................................................................................. 85 Clinical implications ................................................................................................................... 87 Limitations .................................................................................................................................. 87 CONCLUSIONS ............................................................................................................................. 88 CHAPTER FOUR ? CEREBRAL BLOOD FLOW CONTROL IN HIGH LEVEL SPINAL CORD INJURY: THE EFFECT OF MIDODRINE HYDROCHLORIDE .............................. 89 INTRODUCTION .......................................................................................................................... 89 METHODS ..................................................................................................................................... 91 Protocol ....................................................................................................................................... 92 Data acquisition .......................................................................................................................... 93 Data analysis ............................................................................................................................... 94 vii  Statistical analysis ....................................................................................................................... 95 RESULTS ....................................................................................................................................... 96 Able-bodied versus spinal cord injured - homeostatic response to progressive tilt ................ 96 Acute haemodynamic responses to orthostatic challenge ....................................................... 96 Transfer function metrics of dynamic cerebral autoregulation ............................................... 96 Spinal cord injured individuals with and without midodrine ...................................................... 97 Homeostatic response to progressive tilt................................................................................. 97 Acute haemodynamic responses to tilt .................................................................................... 97 Transfer function metrics of cerebral pressure-flow relationships .......................................... 97 Orthostatic tolerance ................................................................................................................... 97 Transfer function metrics and blood pressure ............................................................................. 97 Vasomotor control and transfer function metrics........................................................................ 97 DISCUSSION ............................................................................................................................... 104 Able bodied vs. spinal cord injured .......................................................................................... 104 Static cerebral autoregulation in able-bodied and spinal cord injured individuals ............... 104 Dynamic cerebral pressure-flow relationships in able-bodied and spinal cord injured individuals ............................................................................................................................. 104 The effect of midodrine on cerebral blood flow in spinal cord injured individuals .................. 106 The effect of midodrine on static cerebral autoregulation .................................................... 106 Orthostatic tolerance ................................................................................................................. 107 Cerebral pressure-flow relationships and baroreflex function .................................................. 108 Limitations ................................................................................................................................ 109 CONCLUSION ............................................................................................................................. 109 viii  CHAPTER FIVE ? ASSOCIATION BETWEEN INTEGRATED CARDIOVAGAL BAROREFLEX SENSITIVITY AND CAROTID ARTERY MECHANICS IN SPINAL CORD INJURY: EFFECT OF MIDODRINE HYDROCHLORIDE .................................... 111 INTRODUCTION ........................................................................................................................ 111 METHODS ................................................................................................................................... 113 Protocol ..................................................................................................................................... 114 Data acquisition ........................................................................................................................ 115 Data analysis ............................................................................................................................. 116 Statistical analysis ..................................................................................................................... 116 RESULTS ..................................................................................................................................... 117 Relationship between common carotid artery parameters and baroreflex sensitivity ............... 117 Spinal cord injured individuals with and without midodrine .................................................... 117 DISCUSSION ............................................................................................................................... 120 Relationship between common carotid artery parameters and baroreflex sensitivity ............... 120 Spinal cord injured with and without midodrine....................................................................... 121 Limitations ................................................................................................................................ 122 CONCLUSIONS ........................................................................................................................... 122 CHAPTER SIX ? GENERAL SUMMARY AND CONCLUSIONS .................................... 123 INTEGRATION AND INTERPRETATION OF MAJOR FINDINGS ............................. 123 FUTURE STUDIES ............................................................................................................ 125 REFERENCES ........................................................................................................................ 128  ix  LIST OF TABLES Table 1. Summary of studies highlighting cerebral blood flow parameters in those with spinal cord injury. .................................................................................................................................... 23 Table 2. Summary of studies examining cardiovagal baroreflex function in humans with spinal cord injury. .................................................................................................................................... 55 Table 3. Summary of studies potentially highlighting sympathetic baroreflex function in humans with spinal cord injury. ................................................................................................................. 59 Table 4. Individual demographic information for spinal cord injured participants. ..................... 70 Table 5. Selected cardiovascular variables for spinal cord injured and able-bodied participants. 70 Table 6. Seated haemodynamic variables for spinal cord injured individuals with and without midodrine and able-bodied participants. ....................................................................................... 73 Table 7. Selected correlations (r) between haemodynamic parameters and VF scores. ............... 76 Table 8. Transfer function metrics of mean arterial pressure and cerebral blood flow velocity in the supine and upright position. .................................................................................................. 102 Table 9. Selected cardiovascular variables for SCI and AB participants. .................................. 115 Table 10. Individual demographic information for SCI individuals........................................... 115 Table 11. Homeostatic haemodynamic variables for spinal cord injury before and after midodrine and able-bodied participants. ...................................................................................................... 118 Table 12. Relationships between cardiovagal baroreflex sensitivity and common carotid artery stiffness in able-bodied and spinal cord injured individuals in the upright position. ................. 119     x  LIST OF FIGURES Figure 1. Illustration of primary pathways relevant to human cerebral blood flow control. .......... 7 Figure 2. Illustration of theoretical mechanisms of cerebral blood flow control that would be impacted by high level spinal cord injury (SCI). .......................................................................... 13 Figure 3. Illustration of theoretical mechanisms of cerebral blood flow control that would be impacted by high level spinal cord injury (SCI). .......................................................................... 15 Figure 4. An example of data required for cross-spectral analysis. .............................................. 35 Figure 5. An example of data required for sequence analysis. ..................................................... 36 Figure 6. An example of data required for the modified Oxford technique. ................................ 39 Figure 7. An example of the neck-cuff set-up. ............................................................................. 40 Figure 8. An example of the data generated from a Valsalva maneuver. ..................................... 41 Figure 9. An example of data generated from repeated squat-stand maneuver. ........................... 42 Figure 10. Illustration of where baroreflex dysfunction may occur after spinal cord injury. ....... 44 Figure 11. Summary of evidence examining baroreflex function in those with high level spinal cord injury. .................................................................................................................................... 46 Figure 12. Summary of evidence examining baroreflex function in those with low level spinal cord injury. .................................................................................................................................... 52 Figure 13. Cerebrovascular response to visual-spatial task. ......................................................... 78 Figure 14. Cerebrovascular response to verbal fluency task. ....................................................... 80 Figure 15. The relationship between changes systemic and cerebral parameters and markers and changes in verbal fluency (VF) scores in those with SCI. ............................................................ 82 Figure 16. The relationship between middle cerebral artery pulsatility ratio (MCA-PR) and verbal fluency scores..................................................................................................................... 84 xi  Figure 17. Illustration of study procedures. A) Order of testing; B) Tilt-testing protocol. .......... 93 Figure 18. Steady-state haemodynamic responses to tilt. ............................................................. 99 Figure 19. Time-domain dynamic responses to tilt. ................................................................... 101 Figure 20. Effect of midodrine on orthostatic tolerance. ............................................................ 103    xii  LIST OF ABBREVIATIONS AB Able-bodied AP Arterial pressure ACE Angiotensin converting enzyme au Arbitrary units BRS Baroreflex sensitivity CA Cerebral autoregulation BP Blood pressure CPP Central perfusion pressure CVP Central venous pressure CBF Cerebral blood flow cm Centimetre(s) CO2 Carbon dioxide CSP Carotid sinus pressure dCA Dynamic cerebral autoregulation ES Effect size Gup Baroreflex gain to increasing blood pressure Gdown Baroreflex gain to decreasing blood pressure HFRRI high frequency RRI power HLSCI High-level spinal cord injury HR Heart rate HUT Head-up tilt Hz Hertz ICP Intracranial pressure kg Kilogram(s) LBNP Lower body negative pressure LFRRI low frequency RRI power LLSCI Low level spinal cord injury MAP Mean arterial pressure MCA Middle cerebral artery  MCAv Middle cerebral artery blood flow velocity xiii  mmHg Millimetres of mercury ms Milliseconds MSNA Muscle sympathetic nervous system activity N/A Not applicable PaCo2 Carbon dioxide tension PCA Posterior cerebral artery PETCO2 End-tidal partial pressure of carbon dioxide PI Pulsatility index PR Pulsatility ratio s Second(s) SBPLF low frequency power of systolic blood pressure sCA Static cerebral autoregulation SCI Spinal cord injury SNA Sympathetic nervous system activity SNP Sympathetic preganglionic neurons SV Stroke volume TPR Total peripheral resistance TVC Total vascular conductance Xe Xenon             xiv  ACKNOWLEDGEMENTS First, I would like to sincerely thank all the individuals who participated in our research trial.   I would also like to recognize with much appreciation my entire thesis committee for their general support and attention. Dr. Darren Warburton placed a great deal of trust and belief in my abilities from the day I began studying in his laboratory. I would like to thank him for providing me with extensive and truly remarkable academic and professional opportunities throughout my studies. Dr. Philip Ainslie?s academic guidance and enthusiasm gave me the foundation and focus to reach for the high-hanging fruit. I would like to thank him for his unwavering support and empathy. Dr. Andrei Krassioukov?s clinical research expertise as well as excitement for this project gave me a great deal of pride in my studies and motivated me to persevere. I would like to thank him for his patience, knowledge and time invested in these projects. I would like to also recognize with appreciation Dr. Shannon Bredin for financial support throughout my studies.   Several laboratory mates also assisted with the progression of these studies and served as extremely supportive peers. In particular I would like to thank Dr. Anita Cote, Dr. Christopher West, Annie Zheng, Taylor Drury, Heather Foulds, Danielle Beaudoin, Dr. Jamie Burr, and Alis Bonsignore for their collaboration and friendship.   Ren?e, thank you for the serenity and understanding you have provided me through this somewhat chaotic chapter of my life. Words cannot express how important this has been.   A special gratitude is reserved for my immediate family (Scott, Christine, Sarah and Elizabeth). Without your support I would not have achieved what I have to date. Every step was met with equal doses of excitement and encouragement ? instilling me with pride and energy to do my best work at each successive milestone.    With the end of this part of my life, I look forward to the future, and feel a sense of pride at what has been accomplished.  ?That it will never come again is what makes life so sweet? ? Emily Dickinson 1  CHAPTER ONE - INTRODUCTION Spinal cord injury (SCI) is a devastating chronic condition, often resulting in severe dysfunction of autonomic cardiovascular control (Krassioukov 2009). Presently, cardiovascular disease is the number one killer of people with SCI (Garshick et al., 2005; Myers et al., 2007). Secondary to autonomic dysfunction, many individuals with SCI have low blood pressure, which likely challenges the brains capacity to effectively maintain cerebral blood flow (CBF) (Claydon et al., 2006). Those with SCI suffer from  increased risk of stroke (Wu et al., 2012), reduced cognitive function (Wecht et al., 2012), and orthostatic intolerance (Gonzalez et al., 1991); all of which are likely related to impaired CBF regulation.   Neurovascular coupling is (NVC) described as the hyperemic response to increased metabolic demand in the brain, and represents a important marker of cerebrovascular control (Willie et al., 2011b). Furthermore, NVC is theorized to mediate the relationship between CBF dysfunction and cognitive decline (Novak and Hajjar 2010). To date, it is unknown if the CBF response to increased cerebral metabolic demand is altered in the SCI population.   Cerebral autoregulation describes the brains ability to maintain perfusion in response to alterations in blood pressure. Static cerebral autoregulation (sCA; i.e., the ability to maintain CBF in response to steady-state changes in blood pressure) has been shown to be well maintained in SCI, although only global markers of CBF have been examined (i.e., xenon clearance rate, middle cerebral artery blood flow velocity; (Nanda et al., 1974; Nanda et al., 1976; Handrakis et al., 2009)). Examining arteries that are part of the vertebrobasilar system (i.e., arterial system responsible for perfusion of the medulla oblongata, which contains associated autonomic control centres and discrete regions responsible for consciousness (Shin et al., 1999)) might be more relevant to orthostatic tolerance as opposed to more global CBF measures. In addition to only examining relatively global changes in CBF, there is limited evidence on dynamic cerebral autoregulation (dCA; i.e., the ability to maintain CBF in response to changes in blood pressure occurring over less than five seconds) in SCI, and these are limited to transfer function metrics that are currently under scrutiny as to whether they truly measure dCA (Tzeng et al., 2012).   Baroreflex function is inversely related to cerebral autoregulation in the able-bodied population, suggesting a redundant system designed to maintain CBF (Tzeng et al., 2010). As recently reviewed, cardiovagal baroreflex sensitivity (BRS; i.e., the capability of the vagal 2  component of the autonomic nervous system to detect and respond to changes in blood pressure), is reduced in those with SCI (Phillips et al., 2012c). Cardiovagal BRS may be reduced due to a mechanical component (i.e., increased stiffness of the aorta and or/carotid arteries containing stretch-receptors), or a downstream neural mechanism. It is known that central arterial stiffness is increased in SCI (Phillips et al., 2012b); however, the role arterial stiffness plays in reduced BRS in this population is uncertain. It is also unknown if the previous reported reductions in BRS in SCI (Phillips et al., 2012c) are related to compensatory alterations in cerebral autoregulation.        Clearly, the current understanding of CBF regulation in SCI is limited. In addition to the limited knowledge on NVC and cerebral autoregulation in SCI, it is also unclear if dysfunction in CBF control is due simply to low blood pressure (i.e., perfusion pressure) or cerebral regulatory dysfunction (i.e., control arising from blood vessels of the brain). Midodrine hydrochloride, an alpha1-aderenoreceptor agonist is indicated for use treating orthostatic hypotension, and leads to increased blood pressure. Midodrine, increases blood pressure in SCI, but does not cross the blood-brain barrier and has little ? receptor affinity (McTavish and Goa 1989). As such, midodrine-induced increases in blood pressure allow insight into whether cerebrovascular regulatory parameters are improved by simply increasing blood pressure, or if dysfunction is innate to the cerebrovascular vessels. STATEMENT OF THE PROBLEM  There is a lack of evidence regarding CBF regulation in those with SCI. Specifically, evaluations of regional CBF (i.e., incorporation of CBF evaluations both large-scale and those local to the vertebrobasilar system) and the relationship between CBF and cognition are completely lacking. In addition, the relationship between blood pressure and CBF evaluated using perturbations (i.e., head-up tilt as opposed to spontaneous oscillations) in SCI has not been evaluated. Finally, there is no information on the link between BRS and CBF regulation, in SCI, and whether increased arterial stiffness in this population disrupts the fundamental inverse relationship.  OVERVIEW  Chapter two includes a review of the literature pertaining to CBF control and BRS in those with SCI. The first investigation is presented in Chapter three where regional NVC is examined in those with SCI as compared to AB. The mitigating influence of restoration of blood pressure with midodrine in SCI is also explored. Chapter four examines regional static and dynamic cerebral autoregulation in those with SCI. This chapter also explored the relationship between 3  cerebral autoregulation and orthostatic tolerance in SCI. Finally, this chapter describes the relationship between BRS and cerebral autoregulation in SCI. Chapter five builds upon findings from Chapter four, by investigating the relationship between carotid artery stiffness and BRS in SCI, and explores the role increased stiffness plays on the relationship between CBF control and baroreflex function. The impact midodrine exerts on BRS and arterial stiffness in also quantified. Chapter six integrates the findings of these investigations, discusses their relevance and impact, and finally provides recommendations for further study.                     4  CHAPTER TWO - LITERATURE REVIEW PART I ? CEREBROVASCULAR CONTROL AFTER SCI SUMMARY Significant cardiovascular and autonomic dysfunction occurs following a spinal cord injury (SCI). Two major conditions arising from autonomic dysfunction are orthostatic hypotension and autonomic dysreflexia (i.e., severe acute hypertension). Effective regulation of cerebral blood flow (CBF) is essential to offset these drastic changes in cerebral perfusion pressure. In the context of orthostatic hypotension and autonomic dysreflexia, the purpose of this review is to critically examine the mechanisms underlying effective CBF following an SCI and propose future avenues for research. Although only 16 studies have examined CBF control in those with high level SCI (above the 6th thoracic spinal segment), it appears that CBF regulation is markedly altered in this population. Cerebrovascular function is comprised of three major mechanisms: 1) cerebral autoregulation, (i.e., ?CBF/? blood pressure); 2) cerebrovascular reactivity to changes in PaCO2 (i.e. ?CBF/arterial gas concentration); and 3) neurovascular coupling (i.e., ?CBF/? metabolic demand). While static cerebral autoregulation appears to be well maintained in high level SCI, dynamic cerebral autoregulation, cerebrovascular reactivity, and neurovascular coupling seem to be markedly altered. Several adverse complications after high level SCI may mediate the changes in CBF regulation including: systemic endothelial dysfunction, sleep-apnea, dyslipidemia, decentralization of sympathetic control, and dominant parasympathetic activity. Future studies are needed to describe whether altered CBF responses after SCI aid or impede orthostatic tolerance. Further, simultaneous evaluation of extra- and intra cranial CBF, combined with modern structural and functional imaging, would allow for a more comprehensive evaluation of CBF regulatory processes. We are only beginning to understand the functional effect of dysfunctional CBF regulation on brain function on individuals with SCI, which are likely to include increased risk of transient ischemic attacks, stroke and cognitive dysfunction.  INTRODUCTION The incidence of spinal cord injury (SCI) ranges from 14-58 per million in nations where data are available, tending to be highest in regions with more elderly individuals, and greater population access to motor vehicles (Chiu et al., 2010). As such, developing nations are expected 5  to have marked increases in SCI prevalence in the near future (Liu et al., 2011b). Although SCI is widely considered a condition primarily associated with a loss of motor ability, SCI also results in other important health concerns such as severe cardiovascular dysfunction (Scott et al., 2011). After SCI, supraspinal regulation of autonomic function is disrupted (Krassioukov 2009). Owing to the dissociation between autonomic function and supraspinal control, many of those living with SCI have low resting blood pressure (BP), hypotensive bouts during an orthostatic challenge (such as changing postures quickly), and suffer uncontrolled bouts of hypertension, a condition known as autonomic dysreflexia (Krassioukov 2009). In addition, dysfunctional BP control is associated with impaired cognitive function, (Davidoff et al., 1985; Jegede et al., 2010; Wecht et al., 2012). Together, these effects can severely impact quality of life, and can be a major limitation in effective physical therapy and rehabilitation from injury (Illman et al., 2000). It is suspected that all of the above issues could at least be partially mediated by impaired cerebral perfusion secondary to ineffective BP control (Phillips et al., 2012c).  It is now clear that cardiovascular diseases are among the most common causes of death in those with SCI (DeVivo et al., 1999). Recently, epidemiological evidence shows stroke risk is two to three times greater in those with SCI, even after controlling for a number of cardiovascular risk factors such as hypertension, diabetes, arrhythmia, and coronary artery disease (Wu et al., 2012). Brief disruptions of cerebral blood flow (CBF) caused by impaired vascular control may cause irreversible neuronal cell death (Endres et al., 2003; Aries et al., 2010). Conversely, inadequate counter-regulation against excessive cerebral perfusion can cause intracranial hypertension and predispose to hemorrhagic stroke (Compton et al., 1987; Sloan et al., 1989). As recently reviewed baroreflex control after SCI and highlighted that the function of this feedback system is significantly impaired in those with SCI (Phillips et al., 2012c). Consequently, poor BP control in those with SCI makes appropriate regulation of CBF crucial for preventing stroke. As a negative relationship exists between baroreflex function and CBF regulation, a review of the current literature examining CBF control in those with SCI may shed light on the overall trends and underlying pathophysiology in this complex and diverse condition (Tzeng et al., 2010). There is evidence that CBF dysfunctions contribute to the variety of clinical conditions that not only cloud present clinical challenges for management of individuals with SCI, but could even be life threatening in nature. For example, poor control over CBF during orthostatic hypotension can result in transient ischemic attack or even stroke (Eigenbrodt et al., 6  2000).  Furthermore, CBF regulatory dysfunction could also result in cognitive deficits in these individuals and negatively affect quality of life (Pulmuter et al. 2012). Here, the current understanding of CBF regulation in humans is reviewed, and the related alterations following SCI.  REGULATION OF CEREBRAL BLOOD FLOW It is often highlighted that the brain receives a disproportionate amount of blood relative to its mass. Specifically, the brain receives approximately 20% of cardiac output although it only makes up around 2% of body mass (Siesjo 1978; Edvinsson and Krause 2002). This relationship highlights the extremely elevated metabolic demands occurring in cerebral tissue (Edvinsson and Krause 2002). The brain also utilizes over 20% of resting oxygen and glucose (Siesjo 1978). Sufficient and appropriate matching of brain blood flow to metabolic demand is essential as cerebral ischemia and syncope can result after only three seconds and neuronal death can occur after approximately five minutes of disrupted CBF (Edvinsson and Krause 2002).    Regulation of CBF is achieved through several factors including metabolic, myogenic, neurogenic, and systemic control (Paulson et al., 1990) (Figure 1). All these factors interact to regulate CBF through the adjustment of cerebrovascular resistance (Aaslid et al., 1989). Together, cerebral autoregulation, cerebrovascular reactivity, and neurovascular coupling are commonly used parameters to reflect the complex interplay of regulatory factors.   7   Figure 1. Illustration of primary pathways relevant to human cerebral blood flow control. Cardiac output (Qc) and total peripheral resistance (TPR) together generate mean arterial pressure. Cerebral perfusion pressure (1) is the difference between mean arterial pressure and intracranial pressure (2; ICP) when central venous pressure (3; CVP) is lower than ICP.  Neurogenic control over cerebral  vascular tone (4) is widely disputed, however there is some limited evidence suggesting an autonomic influence on dynamic cerebral autoregulation (Dawson 2000, Purkayastha et al., 2012, Ogoh 2008), as well as potentially cerebrovascular reactivity (Jordan et al. 2000).  There is also controversial evidence that Qc alters cerebral blood flow independently of CPP (5).  When CVP and/or ICP are elevated, venous outflow from the brain is likely also partially ?regulated? by a Starling resistor, due to the relatively static dimensions and enclosed nature of the cranium (6).  Cerebral blood flow is altered primarily in response to changes in pH (7; also metabolism and other factors), employing both endothelial-dependent, and endothelial-independent pathways (8) in order to provide redundant regulation of cerebral [H+] in order to maintain optimal enzymatic pH within the brain. Endothelial function appears to partially mediate cerebrovascular reactivity to hypercapnia (Thompson, Peebles), possibly hypocapnia (Geary 1998, Zhang 2001), as well as neurovascular coupling (Piknova et al. 2011), but not cerebral autoregulation (Tzeng 2011, Rosengarten 2007).   8  Cerebral autoregulation Cerebral autoregulation, which describes the ability of the brain to maintain CBF under a variety of perfusion pressures, can be divided into static and dynamic components. Static CA (sCA) is used to describe CBF during steady-state conditions (Tiecks et al., 1995) and is traditionally considered to be held relatively constant over a wide range of perfusion pressures (i.e., mean arterial pressures (MAP) from 60 to 150 mmHg) (Lassen 1959; Larsen et al., 1994). Lucas et al. (2010) however, recently reported a relatively ?pressure passive? CBF response to both increased and decreased BP induced pharmacologically (Lucas et al., 2010). The assumption that diameter remained constant during the infusion of vasoactive substances was made based on the results of Giller et al. (1993) who measured the outer diameter during craniotomy and found changes in diameter of 4% (Giller et al., 1993). However, volumetric blood flow through the internal carotid artery during steady state phenylephrine infusions showed increases in middle cerebral artery blood velocity, but apparently no measurable change in internal carotid artery flow (Ogoh et al., 2011), raising the possibility that elevations in flow velocity may reflect increase in arterial vascular tone, potentially of the insonated vessel itself. Such changes would invalidate the use of transcranial Doppler as an accurate surrogate of CBF. Discrepancies between these studies have not been resolved. Nevertheless, cerebral autoregulation has been shown to be reduced in acute ischemic stroke (Eames et al., 2002), chronic traumatic brain injury (Bailey et al., 2013), carotid artery stenosis (White and Markus 1997), and autonomic failure (Blaber et al., 1997).  Dynamic CA (dCA) on the other hand, describes the ability of the cerebrovascular system to oppose short term changes in perfusion pressure over a time period of less than five seconds (Tiecks et al., 1995; Zhang et al., 1998). Both sCA and dCA are thought to be regulated by a combination of myogenic, metabolic and neurogenic control mechanisms (for review see: (Paulson et al., 1990)); however, sCA and dCA and may rely on relatively different influences from the various regulatory factors (Dawson et al., 2000). For example, it has been suggested that neural control over cerebral vascular tissue more heavily influences dCA than sCA (Dawson et al., 2000), while endothelial function has been shown to not relate to dCA (Zhang et al., 2004). Myogenic control, on the other hand, is thought to play a role primarily in passive, compliance-based dCA (i.e., transfer function analysis phase) and less of a role in active, resistance-based dCA (i.e., gain and coherence) (Tzeng et al., 2011; Tzeng and MacRae 2013). Further, although 9  on a continuum, dysfunction in dCA is thought to occur prior to sCA in some pathological scenarios (Tiecks et al., 1995). Very recently, Pukayastha et al. showed, using prazosin (alpha-1  antagonist), that sympathetic control is important to dCA (namely LF gain) at rest and during exercise (Purkayastha et al., 2012). Ogoh et al. (2008) also showed that the MCAv responsiveness to thigh cuff deflation was reduced after generalized alpha-adrenergic blockade (Ogoh et al., 2008). Furthermore, evidence showing increased transfer function analysis gain after cholinergic blockade through glycopyrrolate in the same frequencies as sympathetic control (i.e., > 0.05 Hz) suggests that both the parasympathetic and sympathetic system are active in dCA that may predominantly oppose rising and falling perfusion pressures, respectively (Hamner et al., 2012). However, it should be noted that recent evidence in both humans (Aaslid et al., 2007; Schmidt et al., 2009; Tzeng et al., 2010) and animals (Cassaglia et al., 2008b) support the idea of hysteresis in CA. In other words, the brain can more effectively buffer hypertension as compared to hypotension. If this is the case for SCI remains unknown, but if comparable to healthy humans would provide a means to defend against autonomic dysreflexia.  Cerebrovascular reactivity The index of cerebrovascular reactivity (i.e., relative change in cerebral blood flow in response to altered blood gas concentration) allows for quantification of the sensitivity of the intracranial vessels to a given stimulus. Most commonly, stimuli include either pharmacological or ventilatory driven alterations of arterial blood gas content (i.e., oxygen or carbon dioxide). A reduced CBF response to hypo- and hypercapnia has been shown to be related to obstructive and central sleep apnea (Reichmuth et al., 2009; Burgess et al., 2010), hypertension (Serrador et al., 2005), carotid artery stenosis (Widder et al., 1994), congestive heart failure (Xie et al., 2005), and have independent predictive value for future ischemic stroke (Silvestrini et al., 2000; Markus and Cullinane 2001; Vernieri et al., 2001).  The overarching control of CBF is via PaCO2. Specifically, CBF adjusts roughly 3.8%?mmHg-1 change in PaCO2 partial pressure, with a slightly more robust response to hypercapnia in comparison to hypocapnia (Reivich 1964; Harper and Glass 1965; Grubb Jr et al., 1974; Kemna et al., 2001; Ainslie and Duffin 2009). Briefly, CBF is thought to preferentially alter CO2 and [H+] at the level of the brain stem in order to maintain pH within a narrow range (Ainslie and Duffin 2009). Maintenance of cerebral pH helps stabilize respiratory and chemoreceptor centres, which are also sensitive to [H+]. The mechanisms through which this is 10  achieved are only partially understood but thought to be the result of activated potassium channels in smooth muscle as well as rapid adjustment of several key vasoactive substances such as nitric oxide and prostaglandin (Ainslie and Duffin 2009).  Neurovascular coupling The close temporal and regional linkage between neural activity and CBF response is termed neurovascular coupling. Cerebral blood flow responses in the MCA, posterior cerebral artery and anterior cerebral artery to visual, verbal and cognitive tasks have been widely measured (Aaslid 1987; Schuepbach et al., 2002; Rosengarten et al., 2007a; Schuepbach et al., 2007; Willie et al., 2011c). Prior work shows altered neurovascular coupling in those with hypotension (Duschek and Schandry 2004), Type I diabetes (Rosengarten et al., 2002), and Alzheimer?s disease (Rosengarten et al., 2007b); however, no study has directly related the neurovascular coupling response to cognitive function. The metabolic portion of CBF regulation is related directly to increased oxygen availability and reduced carbon dioxide during periods of increased CBF (Filosa 2010). Similarly to working muscle, a complex interplay occurs whereas nitric oxide and other endothelial metabolites work to match blood flow provision to metabolic demand (Zonta et al., 2003; Payne 2006). This functional hyperemia employs intimate interactions between cerebral vascular cells such as endothelium and smooth muscle, with neurons and glia (i.e., astrocytes, microglia, oligodendrocytes) to match neural activation with CBF (Iadecola 2004).  CEREBRAL BLOOD FLOW AFTER SPINAL CORD INJURY A total of 16 studies have examined CBF control in those with SCI (Eidelman et al., 1972; Nanda et al., 1974; Nanda et al., 1976; Yamamoto et al., 1980; Gonzalez et al., 1991; Houtman et al., 2000; Houtman et al., 2001; Catz et al., 2006; Catz et al., 2007a; Catz et al., 2007b; Handrakis et al., 2009; Wilson et al., 2010; Sahota et al., 2012; Wecht et al., 2012; Phillips et al., 2013b). In the current section, the literature examining these CBF regulatory indicators will be highlighted and the potential underlying mechanisms will be discussed (Table 1). Specifically, it will be discussed how CA, cerebrovascular reactivity and neurovascular coupling are influenced by HLSCI. The vast majority of these studies have examined CBF in those with HLSCI, with only one including a lower level SCI group (i.e., <T6) (Sahota et al., 2012). As such, specific focus will be on studies examining those with HLSCI. Also, one of the studies will not be discussed as the researchers employed a liquid-meal ingestion to alter BP, which did not alter BP 11  in their HLSCI group, precluding the evaluation of a CBF response (Catz et al., 2006; Previnaire and Soler 2010). Several reports support the idea that those with HLSCI complain of orthostatic intolerance in the early stages after injury; however, these complaints reduce in frequency as duration of injury occurs, even with persistent and severe orthostatic hypotension (Guttmann 1953; Guttmann et al., 1963; Johnson and Spalding 1974; Nanda et al., 1974). It has been speculated that CBF regulatory process may adapt to overcome profound orthostatic hypotension in chronic HLSCI (Houtman et al., 2001). Considering the neurogenic, vascular, metabolic, and respiratory changes known to occur after SCI, it is plausible that CA is altered in this population. Cardiovascular consequences of spinal cord injury The autonomic nervous system is comprised of both sympathetic and parasympathetic divisions. Parasympathetic nervous outflow occurs from cranial nerves III, VII, IX, and X superiorly and S2-4 inferiorly. Only cranial nerves IX and X participate in cardiovascular control, and as they exit above the spinal cord level, any parasympathetic influence on cerebrovascular following SCI tone would be intact (Krassioukov and Weaver 1996). On the other hand, sympathetic nervous outflow occurs from the sympathetic preganglionic neurons (SPNs) localised within T1-L2 spinal segments of the spinal cord. These neurons synapse into the sympathetic paravertebral ganglia (ganglionic neurons in the sympathetic chain) (Krassioukov and Claydon 2006), and finally postganglionic fibers innervate target organs in the periphery. After SCI, tonic supraspinal control to the spinal SPNs is lost and sympathetic outflow from the spinal cord below the level of injury is severely disrupted (Krassioukov 2009). Drastic changes in systemic cardiovascular regulation occur in those with lesion level above the 6th thoracic spinal cord segment. Lesions above T6 are associated with a loss of supraspinal control over the heart and splanchnic blood vessels (Teasell et al., 2000); both of which are required for effective long- and short-term BP regulation (Claydon et al., 2006). Further to this, sympathetic control of the cerebral blood vessels is transmitted through the superior cervical ganglion, which originates from spinal nerves T1-T4 (1970). As such, any SCI occurring above T4 would be expected to result in partial or complete (>T1) abolishment of sympathetic cerebrovascular control.  Also highly relevant to cerebrovascular regulation, endothelial function, estimated by flow mediated dilatation, has been shown to be systemically reduced after SCI (Stoner et al., 2006), in some cases in as little as six-weeks post-injury (Thijssen et al., 2006). Nitric oxide is the primary substance causing vasodilatation in response to sheer-stress (Joannides et al., 1995), 12  and leads to smooth muscle relaxation and increased blood flow (Maiorana et al., 2003). The production of nitric oxide relevant to vascular control occurs in endothelial cells (Knowles and Moncada 1994) and mediates the relationship between cerebral vascular tone and the majority of signaling messengers (i.e. acetylcholine, ADP, ATP, bradykinin, endothelin, sodium fluoride, oxytocin, vasopressin etc.) (Feletou et al., 2011). Reduced nitric oxide however does not appear to influence cerebral autoregulation but is associated with reduced cerebrovascular reactivity (Thompson et al., 1996; Geary et al., 1998; Zhang et al., 2001; Peebles et al., 2008). Following this, it could be suspected that systemic endothelial dysfunction influences the regulation of CBF (Figure 2).        13   Figure 2. Illustration of theoretical mechanisms of cerebral blood flow control that would be impacted by high level spinal cord injury (SCI).  Briefly, decreased central sympathetic control of systemic blood vessels leads to greatly reduced cerebral perfusion pressure. Exacerbating the hypotension, those with SCI above T5 also have impaired central sympathetic control over the heart. Systemic endothelial (Endo) dysfunction may also lead to decreased CO2 reactivity, neurovascular coupling, and potentially cerebral autoregulation. As mentioned, in Figure 1, direct neurogenic control of cerebral vascular tone is controversial, but cerebral vessels are innervated by sympathetic fibres originating from the T1-4 level (superior cervical ganglia). As such SCI above T5 would result in impaired or absent central sympathetic control of cerebrovasculature. ? denotes where cerebrovascular dysfunction may occur after spinal cord injury (1-endothelial dysfunction, 2- altered direct neural cerebrovascular influence 3- sympathetic neural control over total peripheral resistance (TPR), 4- sympathetic neural control over cardiac output (Qc)). *Note that when a complete injury occurs, sympathetic vasomotor control is disrupted below the lesion level.  Static cerebral autoregulation Early studies from Nanda et al. examined CBF using inhaled 133Xenon clearance rate during supine-to-seated, and supine-to-raised leg/LBNP maneuver reported that CBF was well maintained during an average 18 mmHg drop in MAP (Nanda et al., 1974; Nanda et al., 14  1976)(Figure 3). Handrakis and colleagues provided more insight by measuring MCAv after ACE-inhibitor infusion combined with 45 degrees tilt (Handrakis et al., 2009); results revealed that MCAv decreased to a similar rate in chronic HLSCI as compared to AB although the MAP drop in HLSCI was markedly higher (p = 0.02). More recent work has shown that sCA (albeit using the relationship between MAP and MCA(diastolic)) was similar between HLSCI and AB in response to head-up tilt, although the HLSCI group reported greater decreases in end-tidal CO2 (Sahota et al., 2012). These authors showed an interesting correlation between supine plasma noradrenaline concentrations (but not low frequency oscillations in systolic BP) and sCA, suggesting a relationship between sCA and autonomic completeness (Sahota et al., 2012). Using a similar 133Xenon technique in primarily acute SCI patients (i.e., 92% <12 months since injury), Yamamoto et al. showed that the sCA  ( = ?CBF/?MAP) response to 30 degree tilt was impaired in acute HLSCI (i.e., greater changes in CBF for a given change in MAP); however those who had injuries occurring within six months of examination had significantly impaired sCA as compared to those closer to the chronic phase (Yamamoto et al., 1980). As CBF is the final common pathway before the development of syncope, it is not surprising that reduced static CBF control is a major factor related to the increased risk of orthostatic intolerance after SCI. In support of this, Gonzalez and colleagues showed that orthostatically intolerant acute HLSCI had greater decreases in MCAv during steady-state 80 degrees tilt when compared to a tolerant chronic HLSCI group (Gonzalez et al., 1991).  15   Figure 3. Illustration of theoretical mechanisms of cerebral blood flow control that would be impacted by high level spinal cord injury (SCI). 1- Nanda et al., 1974; 2 ? Nanda et al., 1976; 3 ? Handrakis et al., 2009; 4- Houtman et al., 2001; 5-Sahota et al., 2012; 6-Yamamoto et al., 1980; 7-Wilson et al., 2010.   A landmark study by Houtman and colleagues examined steady-state CBF velocity response to 5-minute long bouts of progressive LBNP in a group of HLSCI (Houtman et al., 2001). This study showed remarkably poor MAP control in HLSCI and large reductions in MCAv, but similar changes in cerebrovascular resistance (i.e., ?MAP/?CBF) as compared to AB indicating similar sCA. The large reductions in CBF velocity resulted in a trend for lower cerebral oxygenation in HLSCI (Houtman et al., 2001). The finding that CBF is reduced more than perfusion pressure during LBNP has long been shown in AB, and although compelling evidence now exists to the contrary, was originally thought to result from systemic sympathetic activation of both muscular and cerebral vasculature during central hypovolemia (Aaslid et al., 2007; Cassaglia et al., 2009; Schmidt et al., 2009). The majority (i.e., 8/10) of the HLSCI group 16  in Houtman?s later work however had a decentralized superior cervical ganglion (as the result of complete injury above T1) and likely would not be capable of sympathetically mediated increases in cerebral vascular resistance (Houtman et al., 2001). As both HLSCI and AB individuals reported similar changes in end-tidal CO2 (i.e., a surrogate marker of PaCO2), Houtman?s findings support the contention that cerebrovascular changes during orthostatic challenge are primarily related to changes in PaCO2 (Novak et al., 1998; Blaber et al., 2001; Howden et al., 2004).  Finally, two interesting studies examined the CBF velocity response to increasing BP in those with HLSCI following two minutes of cold water immersion of the hand  (Catz et al., 2007b) or foot (Catz et al., 2007a). Although not originally interpreted in this manner, it is likely that both these studies induced autonomic dysreflexia in HLSCI (Groothuis and Hopman 2008). As such, it is expected that in their HLSCI group (C4-C7), unopposed sympathetic outflow would have occurred not only in the systemic vascular bed, but also the cerebrovascular tissue (Krassioukov and Weaver 1996). Both of these studies reported similar average cerebrovascular resistance during autonomic dysreflexia in HLSCI when compared to AB. Furthermore, these studies also included a group of participants with injuries ranging T4-T6, which would be expected to have preserved central sympathetic control over the cerebrovasculature (1970), and showed that when the nociceptive stimulus is applied below the injury level (i.e., the foot), increases in cerebrovascular resistance do not occur during autonomic dysreflexia (Catz et al., 2007a); supporting the idea that sympathetic cerebrovascular control helps to mitigate flow increases during transient periods of increased perfusion pressure (Cassaglia et al., 2008; Schmidt et al., 2009).  Together, the available literature suggests that sCA is well maintained in those with chronic HLSCI, but may be impaired in the acute phase of injury. Although the early studies showed that absolute CBF response is similar in HLSCI as compared to AB, more recent work shows that dysfunctional BP control leads to larger fluctuations in CBF, but similar changes in resistance (Houtman et al., 2001). In one study, a small sub-set of three HLSCI suggests a strong systemic influence of sCA, as abdominal binders and compression stockings improved this metric (Yamamoto et al., 1980). It is likely that cerebrovascular regulatory factors other than sympathetic control allow for preserved sCA in response to decreasing BP in HLSCI with sympathetic decentralization and endothelial dysfunction (i.e., the myogenic influence). In 17  response to autonomic dysreflexia, those with injuries above the superior cervical ganglion are able to increase cerebrovascular resistance to match BP increases, while those with lower level lesions (with preserved central sympathetic control over cerebrovasculature) do not; likely due to baroreflex mediated sympathetic withdrawal above the lesion level (Teasell et al., 2000).   Dynamic cerebral autoregulation Only three studies have examined dCA in those with HLSCI, all of which employed spontaneous transfer function analysis between MAP and MCAv and one with an additional analysis of MAP and PCAv (Wilson et al., 2010; Sahota et al., 2012; Phillips et al., 2013b) (Figure 3). Recently, Wilson et al. showed that supine coherence in the VLF range was reduced in their group of six individuals with HLSCI, while gain and phase were similar (Wilson et al., 2010). Later it was shown that upright MAP-MCAv coherence was reduced in the LF range (i.e., 0.07-0.2 Hz) (Phillips et al., 2013b). Similarly, reduced PCAv coherence in the LF and VLF range was reported (Phillips et al., 2013b). Sahota et al. showed that supine  LF gain was increased and upright LF phase was increased, both indicative of reduced sCA (Sahota et al., 2012). The latter study evaluated only the LF transfer function analysis for MAP-MCAv when coherence was >0.5 (Sahota et al., 2012). No existing studies examine dCA using a model of perturbed BP/CBF SCI, in order to improve the reliability of TFA (Katsogridakis et al., 2012).  Considering the combined findings of these studies, spontaneous dCA is altered in those with HLSCI, and is possibly related to both sympathetic decentralization of the cerebrovasculature and increased parasympathetic tone after HLSCI (Claydon and Krassioukov 2008). A fundamental inverse relationship has recently been shown between both spontaneous and non-spontaneous dCA and cardiac BRS in healthy AB (Tzeng et al., 2010). According to this relationship, and the recent review highlighting that cardiac BRS is markedly reduced in HLSCI, it could be expected that coherence and MAP-MCAv gain would be reduced while phase would be increased (Phillips et al., 2012c). It is possible that cardiovascular and autonomic pathology after HLSCI leads to a loss of this fundamental compensatory relationship (Tzeng et al., 2010). However, more work should be completed using larger sample sizes, incorporating VLF ranges as well as driving BP changes using either oscillatory LBNP or tilt in order to increase confidence in transfer function analysis estimates (Claassen et al., 2009).   18  Cerebrovascular reactivity  Early work using 133Xenon as well as more current work using transcranial Doppler shows that the CBF response to hypercapnia is preserved in those with HLSCI (Eidelman et al., 1972; Yamamoto et al., 1980; Wilson et al., 2010). On the other hand, the response of CBF to hypocapnia does not appear as consistent. For example, Eidlelman reported that the CBF decrease in response to hypocapnia was abolished after HLSCI. In contrast, other reports showed that the hypocapnic response of CBF is preserved (Nanda et al., 1974; Nanda et al., 1976; Wilson et al., 2010). The preliminary study in this group by Eidelman et al. (Eidelman et al., 1972) however may be confounded as four of the nine HLSCI participants to have BP recorded during hyperventilation showed BP changes meeting the criteria for autonomic dysreflexia (Krassioukov and Claydon 2006); changes that would induce systemic, and more controversially, cerebral vasoconstriction (Teasell et al., 2000). The vasoconstriction induced by autonomic dysreflexia would thereby augment hypocapnia induced vasoconstriction (Peebles et al., 2012).  Differences between Eidelman and Nanda in terms of hypocapnic CBF response in HLSCI are difficult to explain but may be due to different techniques for measuring CBF (i.e., modified vs. non-modified 133Xenon inhalation) (Eidelman et al., 1972; Nanda et al., 1974; Nanda et al., 1976).   Although hampered by a low sample size, Wilson et al. (2010) reported differences in subtle parameters of CBF between HLSCI and AB during both hypo- and hypercapnia. Pulsatile flow (pulsatility index) of MCAv decreased significantly more in response to hypercapnia in the HLSCI group, while the cerebrovascular resistance response to hypocapnia was reduced (Wilson et al., 2010). These values suggest nuanced cerebrovascular changes occur after HLSCI, which influence the CBF response to PaCO2. In support of the idea that the hypocapnic CBF response is blunted in HLSCI, Sahota et al. also showed that end-tidal CO2 decreased significantly more in HLSCI as compared to AB during upright tilt, but diastolic CBF velocity changed similarly (Sahota et al., 2012). Although end-tidal CO2 decreases may have been secondary to greater increases in dead space and/or decreases in cardiac output) during head up tilt in SCI compared to AB (Anthonisen and Milic-Emili 1966; Gisolf et al., 2004), this differential response between HLSCI and AB suggests a reduced CBF sensitivity to hypocapnia.   Pulsatile flow patterns represent an area of increasing interest within the field of CBF regulation, which has been suggested as a primary mechanism through which the brain maintains 19  perfusion during low mean perfusion pressure, such as severe hemorrhage, and orthostatic hypotention; the latter of which is widespread in HLSCI (Levine et al., 1994; Teasell et al., 2000; Rickards et al., 2007; Rickards et al., 2011). Hypercapnia preferentially alters the small distal resistance vessels, which regulates pulsatile flow (Wei et al., 1980) and the adaptation of this vascular bed in HLSCI, as shown by  Wilson et al., may represent an adaptation to chronically altered haemodynamic and/or respiratory patterns in those with HLSCI as a means to maintain cerebral perfusion in the face of reduced perfusion pressure (Wilson et al., 2010). Reduced cerebrovascular reactivity, especially to hypocapnia, has been previously related in patients and healthy individuals to sleep-apnea (Xie et al., 2005; Ainslie et al., 2007). Sleep-apnea has been reported to occur in up to 40% of those with HLSCI, and cerebrovascular reactivity may be an important mediating factor (McEvoy et al., 1995).  Clearly, further examination of this relationship is warranted, especially considering the increased risk of stroke in the HLSCI population and the independent predictive value of cerebrovascular reactivity for stroke (Thompson et al., 1996).   Neurovascular coupling  Neurovascular coupling describes the hyperemic response to cognition. Only two studies have examined neurovascular coupling in those with HLSCI.  Wecht and colleagues showed that mean MCAv and cerebrovascular resistance did not change when completing a Stroop test in either AB or HLSCI (Wecht et al., 2012). This technique is quite different from the established neurovascular coupling procedures, as Wecht and colleagues reported mean values from three repeated 45s long Stroop tests, and it is not clear if an eyes-closed resting period occurred, or if the peak response (as opposed to the average) was different between groups. In many other studies, albeit using more established protocols, MCA, anterior cerebral artery, and posterior cerebral artery CBF velocity, has been shown to increase in healthy controls during cognitive tasks and visual motor stimuli while cerebrovascular resistance decreases (Schuepbach et al., 2002; Moody et al., 2005; Willie et al., 2011c). Regardless, these authors reported a trend suggestive of a differing response in controls compared to combined HLSCI and LLSCI (p = 0.08), and a significant positive relationship between changes in cerebrovascular resistance during cognition and Stroop test performance in the AB group (indicating a relationship between neurovascular coupling and cognitive performance); this relationship did not occur in the HLSCI group (Wecht et al., 2012). Recently, it has been revealed that a change in posterior cerebral 20  artery CBF velocity during visual cortex stimulation is essentially absent in those with HLSCI (Phillips et al., 2013c). It has also been shown that the hyperemic response to cognition improves after administration of an alpha-1 agonist (midodrine hydrochloride; does not cross the blood brain barrier), albeit still reduced as compared to matched controls (See Chapter 3).Together, the scant available literature suggests that the neurovascular coupling response is heavily dependent on the underlying BP and systemic vascular tone, with HLSCI and decentralized systemic sympathetic vascular control resulting in an inability to match CBF to increases in neuronal activity. This preliminary data seems to link the plethora of evidence showing cognitive function is declined in those with hypotension with and without SCI (Duschek et al., 2003; Duschek and Schandry 2004; Duschek et al., 2005; Novak and Hajjar 2010; Rose et al., 2010). Other than hypotension, it is possible that nitric oxide availability (Piknova et al., 2011), glucose intolerance (Myers et al., 2007; Gandhi et al., 2010) , and dyslipidemia (Vichiansiri et al., 2012) also mediate the relationship between altered neurovascular coupling and HLSCI. Interestingly, cessation of statin therapy, which acutely increases stroke risk (Blanco et al., 2007), rapidly reduced neurovascular coupling through what is thought to be endothelium dysfunction (Rosengarten et al., 2007a).  FUTURE DIRECTIONS Many interesting potential directions of future research exist in order to better evaluate CBF regulation in HLSCI who are at increased risk of stroke, suffer from orthostatic hypoperfusion, and experience autonomic dysreflexia. Most apparent to us, true orthostatic tolerance in those with HLSCI has yet to be examined, although it has been suggested that this population has remarkable orthostatic tolerance given extremely reduced perfusion pressure (Houtman et al., 2001). The use of LBNP to presyncope in those with HLSCI would evaluate this contention, and concurrent measurement of PaCO2, BP and CBF would shed light on potential underlying mechanisms.    Recently, the beat-by-beat measurement of diameter and blood flow velocity in the internal carotid and vertebral arteries has emerged as a viable technique to measure CBF regulation. These extra-cranial assessments of CBF are free of the assumption that the three main cerebral arteries do not change diameter under most conditions (Serrador et al., 2000). Furthermore, as extra-cranial arteries provide CBF regulatory influence (Faraci and Heistad 1990; Willie et al., 2012), this technique, in addition of transcranial Doppler, allows for a more 21  global and comprehensive assessment of CBF control. Also, magnetic resonance imaging (e.g., pulsed arterial spin labelling, blood-oxygen-level-dependent contrast) allows for more localized assessments of specific regions should also be used to evaluate CBF control in those with HLSCI (D'Esposito et al., 2003; Hendrikse et al., 2004). The use of LBNP (or tilt), applied at specific frequencies where dCA is thought to be active, should also be examined in those with HLSCI; as this would increase the confidence in the transfer function analysis metrics. It has recently been demonstrated that various measures of dCA are often poorly, or even contradictorily related, thus it is paramount that a variety of dCA measures are used before concluding on whether or not dCA is impaired, improved or unchanged after HLSCI (Tzeng et al., 2012).  The evaluation of cerebrovascular reactivity presents an especially clinically relevant area of future research in the HLSCI population, considering the independent stroke risk value of this metric and the elevated stroke risk in the HLSCI population (Silvestrini et al., 2000; Wu et al., 2012). It should be investigated whether altered cerebrovascular reactivity is secondary to, or implicated in, the increased prevalence of sleep-apnea in those with HLSCI or endothelial dysfunction. Evaluating cerebrovascular reactivity and endothelial function in HLSCI before and after continuous positive airway pressure therapy and/or statin administration may shed light on this relationship (Diomedi et al., 1998). From a neurovascular coupling perspective, very little is currently known. Future directions should include using established neurovascular coupling procedures while evaluating the MCA, and posterior cerebral artery during a variety of different stimuli. Clearly relating cognitive performance and neurovascular coupling parameters is also required, not only in HLSCI but the AB population as well.   HLSCI offers a relevant model to evaluate the influence of sympathetic control on CBF control. For example, evaluating the influence of systemic sympathetic control and oscillatory CBF patterns on CBF regulation is possible using a model including only those with injuries above T6. On the other hand, specifically examining those with injuries at T3-4 allow for a model with theoretically preserved central sympathetic control of cerebral but not systemic blood vessels (Pick 1970).    Finally, the result of long term CBF regulatory dysfunction in SCI needs to be evaluated. The accumulated cerebrovascular trauma induced by the large swings in perfusion pressure may predispose to transient ischemic attacks, leading to cognitive deficits, depressive symptoms and increased risk for stroke. 22    CONCLUSIONS The available literature indicates that CBF control is altered in those with HLSCI (Figure 3; Table 1), but may improve with recovery from injury. Specifically, neurogenic and vascular changes occurring after HLSCI are most consistently implicated for CBF regulatory dysfunction through both direct and mechanistic evidence. Although sCA appears to be preserved, spontaneous metrics of dCA are different in those with HLSCI as compared to AB. Further to this, preliminary information suggests that the CBF response to changes in PaCO2 is also altered; however, this needs to be confirmed with a larger sample size and confirmed autonomic completeness of injury. Finally, although very limited evidence exists, significant reductions in the neurovascular coupling are apparent in those with HLSCI. Some potential causes of CBF regulatory change in HLSCI include sympathetic decentralization, vascular dysfunction as well as sleep-related breathing problems.          23  Table 1. Summary of studies highlighting cerebral blood flow parameters in those with spinal cord injury. Details Techniques Resting CBF CA CBFRx NVC Notes Eidelman et al. 1972 n = 12 tetraplegic (C5-7)  n = 5 paraplegic   (T2-4) United Kingdom 133Xe inhalation during hypercapnia (increased inspired CO2), and hypocapnia (hyperventilation), and bladder stimulation (n = 3) (induced autonomic dysreflexia).   N/A N/A ? CO2up ? CO2down  N/A Compared individuals with SCI above C7 to those below T2 (i.e., no able-bodied control). During hyperventilation 4/12 tetraplegics developed autonomic dysreflexia according to reported blood pressures. Tetraplegic and paraplegic groups were comprised of acute and chronic SCI participants. Nanda et al. 1974 n = 7 HLSCI (C4-T3)  n = 7 IOH   n = 10 AB United Kingdom Modified 133Xe inhalation during sitting-up (n = 4; C4-7), and hypocapnia (n = 7). N/A ? sCA ? N/A sCA was compared between IOH and HLSCI only. CBFRx was compared to AB only. All participants were in the chronic phase of SCI. Nanda et al. 1976 n = 7 HLSCI (C4-T3)  n = 13 AB United Kingdom Modified 133Xe inhalation during sitting-up/ lower body negative pressure (n = 7), and hypocapnia (n = 7). N/A ? sCA ? N/A The study above (Nanda et al., 1974) may have used the same participants. All participants were in the chronic phase of SCI. 24  Details Techniques Resting CBF CA CBFRx NVC Notes Yamamoto et al. 1980 n = 13 tetraplegic n = 4 paraplegic  (T3-12) n = 21 control USA Modified 133Xe inhalation during hypercapnia (increased inspired CO2; n = 11 tetraplegics, n = 22 paraplegics) and 30? upright-tilt (n = 10 tetraplegics, n = 4 paraplegics). ? ? sCA ? N/A HLSCI group was comprised of acute and chronic SCI participants. Self reported trend for CBFRx to be reduced in tetraplegics (no P-value reported). Paraplegics had similar resting CBF and sCA as compared to AB. Paraplegics were all in the acute phase of SCI while tetraplegics were acute and chronic combined.  Gonzalez et al. 1991 n = 10 tetraplegic (C5-7)OH+  n = 10 tetraplegic (C5-5)OH- Transcranial Doppler of the MCA during progressive tilt (0?, 30?,60?,80?) . N/A N/A N/A N/A Completeness of injury did not relate to the development of OH. The tetraplegic OH- group had larger decreases in CBF velocity at 80? tilt. All participants were in the acute phase of SCI. Houtman et al. 2000  n = 11 HLSCI (C4-T4) n = 10 AB The Netherlands Cerebral oxygenated, deoxygenated and total hemoglobin concentration by near infrared spectroscopy supine and after 70? head-up tilt      Cerebral oxygenation hemoglobin concentration decreased significantly in HLSCI in response to head-up tilt but not AB. All participants were in the chronic phase of SCI. 25  Details Techniques Resting CBF CA CBFRx NVC Notes Houtman  et al. 2001  n = 8 HLSCI (>T4) n = 8 AB The Netherlands Transcranial Doppler of the MCA as well as cerebral oxygenated, deoxygenated and total hemoglobin concentration during progressive LBNP. N/A N/A N/A N/A Only six HLSCI and AB had the MCA successfully isonated. CBF velocity decreased more in HLSCI during LBNP as compared to AB, but cerebrovascular resistance was similar between groups.  Oxygenation index tended to decrease more in HLSCI (p = 0.08). All participants were in the chronic phase of SCI. Catz et al. 2007  n = 11 tetraplegic (C4-C7) n = 10 paraplegic (T4-T6) n = 13 AB Israel Transcranial Doppler of the MCA before, during, and after cold water immersion of the foot.  ? N/A N/A N/A This procedure likely induced autonomic dysreflexia in the tetraplegic group as well as many of the paraplegics. Cerebrovascular resistance increased in tetraplegics and AB during foot immersion but not in paraplegics. Tetraplegic and paraplegic groups were comprised of acute and chronic SCI participants. 26  Details Techniques Resting CBF CA CBFRx NVC Notes Catz et al. 2008  n = 11 tetraplegic (C4-C7) n = 10 paraplegic (T4-T6) n = 13 AB Israel Transcranial Doppler of the MCA before, during, and after cold water immersion of the hand. ? N/A N/A N/A This procedure likely induced autonomic dysreflexia in the tetraplegic group. Cerebrovascular resistance increased in all three groups during hand immersion. Tetraplegic and paraplegic groups were comprised of acute and chronic SCI participants. Catz et al. 2008  n = 11 tetraplegic (C4-C7) n = 10 paraplegic (T4-T6) n = 13 AB Israel Transcranial Doppler of the MCA before, during, and after a standardized meal.  ? N/A N/A N/A Meal led to increased blood pressure in AB, decreased in paraplegics and oscillating BP tetraplegics. Cerebrovascular resistance tended to increase in response to the meal in tetraplegics and AB but not paraplegics (p = 0.08). Group differences were not detected. Tetraplegic and paraplegic groups were comprised of acute and chronic SCI participants. 27  Details Techniques Resting CBF CA CBFRx NVC Notes Handrakis et al. 2009  n = 7 HLSCI (C5-T1) n = 7 AB USA Transcranial Doppler of the MCA during combined 45? tilt and ACE inhibitor (1.25 mg enalaprilat). ? ?sCA N/A N/A All participants were in the chronic phase of SCI. Wilson et al. 2010 n = 6 tetraplegic (C5-C7) n = 14 AB New Zealand/Canada Spontaneous transfer function analysis of blood pressure and MCAv (transcranial Doppler) in the supine position. Transcranial Doppler of MCAv during hypercapnia (increased inspired CO2), and hypocapnia (hyperventilation).   ? ?dCA ?* ? Theoretically, decreased coherence in VLF range indicates an improvement of dCA, however this relationship is still being investigated. Trend for baseline MCA velocity to be reduced in tetra (p = 0.09). Baseline MCA velocity pulsatility index was 27% higher in tetraplegics. *During hypocapnia, pulsatility index was increased 78% in tetraplegics (p = 0.02). Cerebrovascular reactivity to combined hyper- and hypocapnia was reduced 35% in tetraplegics (p = 0.04). All participants were in the chronic phase of SCI. 28  Details Techniques Resting CBF CA CBFRx NVC Notes Wecht et al. 2012 n = 7 tetraplegic (C4-C8) n = 6 paraplegic (T2-T10) n = 16 AB USA    Transcranial Doppler of the MCA during Stroop cognitive test.  ? N/A N/A ?* *Trend for the AB CBFv response to be different (i.e., increase vs. decrease) from combined tetraplegic and paraplegic groups during cognitive task. Larger decreases in cerebrovascular resistance during cognition were related to higher Stroop colour scores in AB but not either SCI group.  Sahota  et al. 2012 n = 19 HLSCI n = 7 LLSCI n = 16 Canada Spontaneous transfer function analysis of blood pressure and MCAv (transcranial Doppler) in supine and upright position.  ? ?dCA ? ? The decrease in EtCO2 in response to tilt was greater in HLSCI compared to AB. Sample sizes were reduced to 4-5 HLSCI participants for transfer function analysis. Decreased dCA is based on an increase in LF gain (supine) and a decrease in LF phase (upright) in HLSCI. Autonomic completeness related to sCA (noradrenaline) and dCA (low frequency systolic blood pressure). All participants were in the chronic phase of SCI. 29  Details Techniques Resting CBF CA CBFRx NVC Notes Phillips et al. 2013 n = 8 HLSCI n = 8 AB Canada Spontaneous transfer function analysis of blood pressure and MCAv (transcranial Doppler) as well as PCAv in supine and upright position. ? ?MCA dCA ?PCA dCA ? ? Coherence was reduced for both MCAv and PCAv in SCI. BP-MCAv phase was reduced in HLSCI, however BP-MCAv gain was similar. BP-PCAv phase and gain were similar between groups.  Phillips et al. Under Revision n = 7 HLSCI (C4-T1) n = 7 AB Canada Transcranial Doppler of the posterior cerebral artery during visual spatial cerebral activation (reading). ? N/A N/A ? All participants were in the acute phase of SCI. HLSCI was comprised of both acute and chronic participants.  ?, significant increase compared to controls, ;?, significant decrease compared to controls; ?, no difference compared to controls; SCI, spinal cord injury; HLSCI, high level SCI ( ?T6); LLSCI, low-level SCI (<T6); AB, able-bodied, CBF; cerebral blood flow; sCA, static cerebral autoregulation; dCA, dynamic cerebral autoregulation CBFRx; cerebrovascular reactivity; NVC, neurovascular coupling; MCA, middle cerebral artery; LBNP; lower body negative pressure; IOH, idiopathic orthostatic hypotension; EtCO2, end-tidal carbon dioxide.   30  PART II - BAROREFLEX FUNCTION AFTER SCI SUMMARY Significant cardiovascular and autonomic dysfunction occurs after SCI. It is now recognized that cardiovascular disease is a leading cause of morbidity and mortality in SCI. Patients with SCI may also suffer severe orthostatic hypotension and autonomic dysreflexia. Baroreflex sensitivity (i.e., the capability of the autonomic nervous system to detect and respond efficaciously to acute changes in blood pressure) has been recognized as having predictive value for cardiovascular events as well as playing a role in effective short-term regulation of blood pressure. The purpose of this paper is to review the mechanisms underlying effective baroreflex function, describe the techniques available to measure baroreflex function, and summarize the literature examining baroreflex function after SCI. Finally, the potential mechanisms responsible for baroreflex dysfunction after SCI will be discussed and future avenues for research proposed. Briefly, although cardiovagal baroreflex function is reduced markedly in those with high level lesions (above the 6th thoracic level) the reduction appears partially mitigated in those with low lesion levels. Although no studies have examined the sympathetic arm of the baroreflex in those with SCI, despite this being arguably more important to blood pressure regulation than the cardio-vagal baroreflex, nine articles have examined sympathetic responses to orthostatic challenges; these findings are reviewed. Future studies are needed to describe whether dysfunctional baroreflex sensitivity after SCI is due to arterial stiffening or a neural component. Further, measurement of forearm vascular conductance and/or muscle sympathetic nerve activity is required to directly evaluate of the sensitivity of the sympathetic arm of the baroreflex in those with SCI.   INTRODUCTION The prevalence of SCI ranges from 14-58 per million in nations where data are available, tending to be highest in regions with more elderly individuals and greater access to motor vehicles (Chiu et al., 2010). As such, developing nations are expected to have marked increases in SCI prevalence in the near future (Liu et al., 2011b). Although SCI is widely considered a condition primarily associated with a loss of motor ability, SCI also results in other important limitations such as severe cardiovascular dysfunction (Scott et al., 2011). After SCI, supraspinal regulation of autonomic function is disrupted. Owing to the dissociation between autonomic function and 31  supraspinal control, many of those living with SCI have low resting blood pressure, hypotensive bouts during an orthostatic challenge (such as changing postures quickly), and suffer uncontrolled bouts of hypertension, a condition known as autonomic dysreflexia (Krassioukov 2009). Related to both of these conditions, cardiovascular diseases are some of the most common causes of death in those with SCI (DeVivo et al., 1999; Eigenbrodt et al., 2000; Wu et al., 2012).  The autonomic nervous system is comprised of both sympathetic and parasympathetic divisions. Parasympathetic nervous outflow occurs from cranial nerves III, VII, IX, and X superiorly and S2-4 inferiorly. As the cranial nerves do not transmit through the spinal cord, vagal (cranial nerve X) control of heart rate is preserved after SCI (Krassioukov 2009). Sympathetic nervous outflow occurs from the T1-L2 level into the sympathetic paravertebral ganglia (sympathetic chain) (Krassioukov 2006). After SCI, sympathetic control in regions below the lesion level are severely disrupted (Krassioukov 2009). Drastic changes in cardiovascular regulation occur in those with lesion levels above the 6th thoracic spinal nerve. Lesions above T6 are associated with a loss of supraspinal control over the heart and splanchnic blood vessels (Teasell et al., 2000); both of which are required for effective long- and short-term blood pressure regulation (Claydon et al., 2006).   The baroreflex is a complex and multi-factorial negative feedback system that is integrated in concert with respiration, and circulating blood gases.  Arterial stretch-receptors provide surrogate information on current blood pressure to the nucleus tractus solitarius, which then influences efferent autonomic nerve traffic (Krassioukov and Weaver 1996). This system employs both the sympathetic and parasympathetic autonomic divisions to regulate blood pressure within a narrow range over a wide variety of environmental conditions and body positions (Raven 2008). The importance of the baroreflex system to cardiovascular regulation has been supported by studies showing that when sino-aortic autonomic control is interrupted, there is an increase in blood pressure variability, as well as frequent bouts of orthostatic hypotension (Timmers et al., 2003). Cerebral oxygen delivery, which is the final common pathway leading to syncope, relies on effective blood pressure maintenance through the baroreflex (as well as cerebral autoregulation) in order to maintain sufficient cerebral blood flow (Ogoh et al., 2008; Tzeng et al., 2010). Another case for the necessity of a functional baroreflex system can also be made through evidence from evolutionary biology showing that the system 32  (although in a more rudimentary form) is even present among the simplest vertebrates, and with a similar complexity to humans in reptiles (Karemaker and Wesseling 2008).   The purpose of this review is to summarize and evaluate the current literature examining baroreflex function in those with SCI. Further, SCI-related changes in baroreflex function in relation to lesion level will be evaluated, with specific emphasis on those with lesions above and below the 6th thoracic spinal segment. In conducting this review, it was hypothesised that both cardiovagal vasomotor baroreflex function would be reduced in SCI, albeit to a greater extent in those with high level injury. PHYSIOLOGY OF THE BAROREFLEX It is pertinent to first understand the normal functioning of the baroreflex in order to appreciate alterations occurring after SCI. Currently, it is well established that the baroreflex is comprised of two interdependent systems (Taylor et al., 1995; Fu et al., 2009) that work in concert as one reflex system to provide short-term regulation of blood pressure. The first, a low pressure system, is made up of cardiopulmonary stretch receptors located in the heart and lungs, which augments sympathetic nervous system activity in response to reductions in central venous pressure and volume. The second, a high pressure baroreflex system, consists of stretch-receptors located in the tunica adventitia of the aortic arch and carotid bulbs (Fadel et al., 2003). These spray-like nerve endings generate a more rapid rate of depolarization, and hence increase the frequency of action potentials in afferent nerves during periods of increased wall distension (Stanfield and Germann 2008). The signal is transmitted from the carotid bulb via the glossopharangeal nerve (cranial nerve IX) and the aortic arch via the vagal nerve (cranial nerve X) to the nucleus of the solitary tract in the medulla oblongata (Krassioukov and Weaver 1996). This transmission, which provides surrogate information on systemic blood pressure, is integrated with other afferent information in order to modulate efferent nervous activity transmitted through the vagal nerve and sympathetic chain, to target organs, with the aim of rapidly maintaining blood pressure at a set-point (Stanfield and Germann 2008). Specifically, increases in blood pressure lead to increased vagal tone, and sympathetic inhibition, which consequently results in decreased vascular tone, venous return, cardiac contractility and heart rate (while the reverse actions occur in response to reductions in blood pressure) (Pang 2001).  Baroreflex sensitivity (BRS), describes the rate and magnitude by which the system outputs (e.g., heart rate, vasomotor tone) respond to changes in system input (blood pressure, stretch-33  receptor loading/unloading) (Willie et al., 2011a). Hence, a more sensitive baroreflex system will have more rapid and greater responses to the same change in blood pressure, and more effectively maintain blood pressure within the desired range.    In addition to the integration of the low and high pressure baroreflex systems, it is important to understand that the baroreflex is integrated with chemo and pulmonary afferent information in order to produce a suitable efferent response. As such, the baroreflex is influenced by both respiration and arterial blood gases. For example, inspiration decreases the cardiovagal baroreflex response whereas the opposite is true for expiration (Eckberg and Orshan 1977). In fact the magnitude of baroreflex resetting (adjusting blood pressure around a new set-point) is related to inspiratory time, suggesting a fundamental ?gating? relationship between baroreflex adjustment and respiration (Steinback et al., 2009). Furthermore, hypoxic peripheral chemoreflex activation leads to a resetting of both the sympathetic and parasympathetic baroreflex to a higher blood pressure but may or may not affect the sensitivity of the baroreflex per se (Simmons et al., 2007; Steinback et al., 2009). These brief examples highlight that chemoreceptor, baroreceptor and pulmonary afferent information is tightly regulated leading to a complex interdependent relationship where baroreceptor (and pulmonary) afferents modulate the chemoreflex influence on autonomic function (Van De Borne et al., 2000).   The recent explosion of studies examining baroreflex function is in large part due to clinical interest stemming from evidence showing that cardiovagal BRS holds prognostic value for cardiovascular events in a number of clinical populations (La Rovere et al., 2011; Yufu et al., 2011). Furthermore, longterm disruption of the cardiovagal baroreflex function has led to increases in blood pressure variability with little or no influence on mean arterial blood pressure (Ogoh et al., 2006). Resting measures of cardiovagal BRS are also relatively simple to carry out, requiring concurrent recordings of beat-to-beat blood pressure and R-R intervals. As such, the literature is disproportionally represented by studies examining the cardiovagal branch of the baroreflex. However, there is limited value of the cardiovagal branch of the baroreflex in preventing syncope or orthostatic hypotension. Although those individuals with complete autonomic failure show severe and marked reductions in blood pressure during orthostatic challenges, blood pressure responses to orthostatic challenge are similar, if not identical, before and after complete vagal efferent blockade (Low 2003; Ogoh et al., 2006). Considering Poiseuille?s law, blood pressure is affected to the fourth power by arterial diameter and only 34  linearly by increases in flow (heart rate derived changes in cardiac output). As such, it is not surprising that the vasomotor branch of the baroreflex is much more important than the vagal branch for maintenance of mean arterial blood pressure.  TECHNIQUES TO MEASURE BAROREFLEX SENSITIVITY Although it is likely impossible to study so called "low pressure" or "high pressure" systems in isolation (Taylor et al., 1995; Fu et al., 2009), several techniques are available to measure baroreflex function; some with considerable limitations for use in SCI. Spontaneous techniques from 3-5 min duration recordings of resting values tend to be the most common due to inexpensive software and their non-invasive nature. Two options exist for evaluating spontaneously occurring baroreflex function. First, cross-spectral analysis of an input and output (for example systolic blood pressure and R-R interval for cardiovagal BRS) can be evaluated in the low frequency range (0.04-0.15 Hz), with the gain and phase used to determine the sensitivity and timing of the relationship between the measures (DeBoer et al., 1987) (Figure 4). The second, commonly referred to as the sequence technique, involves calculating linear regressions between spontaneously occurring changes in input and the resulting output (Fritsch et al., 1986) (Figure 5).  With no external perturbation of blood pressure, however, it is debated whether resting (i.e., closed-loop) measures of arterial blood pressure are accurately detecting baroreflex function, or are the result of oscillatory influences from other factors independently influencing blood pressure and heart rate (Diaz and Taylor 2006).  35   Figure 4. An example of data required for cross-spectral analysis. A minimum of five minute long recordings of resting (A) R-R interval and (B) blood pressure or muscle sympathetic nervous system activity are evaluated using cross spectral analysis. Cross spectral power in the low frequency range (i.e., around 0.1 Hz) where coherence is greater than 0.5 provides an estimate of closed-loop baroreflex sensitivity. Spectral power of R-R interval (C), spectral power of systolic blood pressure (D)   36   Figure 5. An example of data required for sequence analysis. From continuous recordings of (A) R-R interval and (B) systolic blood pressure, linear regressions are generated between the two measures, with each solid line representing a sequence relating changes in R-R interval and systolic blood pressure over a number of cardiac cycles. The dotted line (C) represents the average regression and provides an estimate of closed-loop baroreflex sensitivity. Typically, in order to have a sequence included in the average regression, R-R interval variation should greater than 5 ms and changes in blood pressure should be greater than 0.5 mm Hg over a duration of four heart beats.   Convincing evidence against spontaneous transfer analysis comes from a recent paper by Kamiya et.al (Kamiya et al., 2011). These authors used an animal model to characterize and compare the open-loop and closed-loop baroreflex. To simulate the open-loop characteristics carotid sinus pressure (CSP) was perturbed using random white noise thereby isolating the carotid pressure from the arterial pressure. To simulate the closed-loop characteristics the CSP was matched to the aortic pressure (AP). In both situations they simultaneously measured renal sympathetic nerve activity (SNA) and AP. This enabled them to independently examine the neural (CSP to SNA) and peripheral (CSP to AP) arcs of the baroreflex, which reflect the 37  feedback and feed-forward components of the baroreflex respectively, as well as the total arc (neural and peripheral). In brief their results indicated that the spontaneous baroreflex as assessed under closed-loop conditions predicted the peripheral but not the neural arc, whereas the open-loop condition could predict both. Given that the baroreflex is a feedback mechanism they subsequently argued that the spontaneous baroreflex bore no resemblance to baroreflex function. Moreover the lack of reliability of the spontaneous baroreflex, led them to advocate the use of open-loop methods for assessing baroreflex function. In this context the open-loop methods, which involve large and dynamic perturbations of blood pressure sufficient to overcome internal noise and engage the baroreflex are deemed a more reliable method of assessing baroreflex function. To overcome this limitation, other more invasive techniques also exist. The modified Oxford technique, for example, likely the most accurate method, involves bolus injections of phenylephrine and sodium nitroprusside to increase and then decrease blood pressure over a two minute duration trial (Hunt et al., 2001) (Figure 6). Alternatively, manipulation of the carotid baroreceptors using positive and negative pressures using a modified neck collar is a non-pharmacological approach (Ernsting and Parry 1957; Fadel et al., 2003). Both these techniques involve potential risk for the participant, including carotid plaque rupture/embolization, and transient hypo- and hypertension. The neck cuff-technique also does not allow for measurement of the aortic baroreceptors (Fadel et al., 2003) (Figure 7), although differentiation between the carotid and aortic stretch receptor afferent signal cannot be made with the Oxford technique either. Another technique to evaluate baroreflex function in an open-loop model involves using cross spectral analysis during relatively large oscillatory blood pressure changes at (either repeated squat stands or repeated short duration bouts of LBNP) 0.05 and 0.1 Hz  (Figure 8) (Zhang et al., 2009a). These large oscillatory perturbations of blood pressure provide similar or greater coherence between blood pressure and heart rate but appear to show different information as compared to spontaneous indicators. Specifically, reduced gain (i.e., baroreflex sensitivity) during the squat-stand maneuver suggests feed forward mechanisms may play a role in spontaneously derived baroreflex gain. Also, enhanced gain in the 0.05 Hz frequency of perturbation  as compared to the faster 0.1 Hz provides evidence that the cardiovagal baroreflex may be more active in mitigating blood pressure changes at specific frequencies (Zhang et al., 2009a). Collectively, the lack of reliability of the spontaneous baroreflex (Laude et al., 2004; Tzeng et al., 2009) supports the use of open-loop methods for assessing baroreflex function. In 38  this context the open-loop methods, which involve large and dynamic perturbations of blood pressure sufficient to overcome internal noise and engage the baroreflex are deemed a more reliable method of assessing baroreflex function. A final way in which baroreflex sensitivity can be evaluated involves using the Valsalva maneuver. Briefly, this technique involves evaluating the blood pressure and heart rate response during phase II and IV of the maneuver (Korner et al., 1976; Vogel et al., 2005). Most commonly the Valsalva estimates of BRS involve calculating the slope between the blood pressure and heart rate changes during phase IV (Eckberg and Sleight 1992), however blood pressure responses during phase II and IV can help indicate vasoconstrictor effectiveness (Korner et al., 1976; Vogel et al., 2005) (Figure 9). Changes in both the sympathetic and parasympathetic wings of the autonomic nervous system can be measured using any of these techniques with simultaneous recordings of vascular resistance or muscle sympathetic nervous system activity (MSNA) with blood pressure and heart rate (Parati et al., 2000). These techniques can evaluate the baroreflex mediated output response to either or both increasing (Gup) and decreasing (Gdown) blood pressure.    39   Figure 6. An example of data required for the modified Oxford technique. Arterial pressure (A), systolic blood pressure (B) and R-R interval (C) responses after bolus injections of sodium nitroprusside (NP) and phenylephrine (PE).The relationship between increasing or decreasing blood pressure and changes in R-R interval or muscle sympathetic nervous system activity provide an estimate of open-loop baroreflex sensitivity. Note the reduced R-R interval response to changes in systolic blood pressure shown in (D) as compared to (E).   40   Figure 7. An example of the neck-cuff set-up. Positive (A) or negative (B) pressure serve to compress or distend carotid stretch receptors which provide surrogate afferent information on short term blood pressure changes. The relationship between carotid distending pressure and either R-R interval, vascular resistance or muscle sympathetic nervous activity changes provide information on carotid baroreceptor sensitivity.  The addition of vascular resistance or recordings of muscle sympathetic nervous activity allows the calculation of the vascular arm of the baroreflex as well as the cardiac arm (via the changes in R-R interval).  41   Figure 8. An example of the data generated from a Valsalva maneuver. The quantitative Valsalva maneuver is performed by blowing with an open glottis into a mouthpiece connected to the mercury column of a sphygmomanometer with an air leak (grey shaded portion). A 40-50 mmHg pressure is maintained for 15 seconds (A). Blood pressure recovery in phase II and cardiopressor response in phase IV are indices of vasoconstrictor and contractile integrity. Baroreceptor mediated tachycardia in phase II and bradycardia in phase IV determines if cardiovagal reflexes are intact. Cardiovagal baroreflex sensitivity is generally reported as the ratio of R-R interval changes divided by systolic blood pressure changes over phase II and transitioning from phase III to IV.  42   Figure 9. An example of data generated from repeated squat-stand maneuver. Changes in blood pressure, heart rate, and end-tidal CO2 in a young subject under resting conditions (A), and during active squat-stand maneuvers at 0.1 (B) and 0.05 (C) Hz. Note the large and coherent oscillations in blood pressure and heart rate during these maneuvers relative to resting conditions.  BAROREFLEX FUNCTION AFTER SPINAL CORD INJURY After SCI, a significant disruption of the autonomic nervous system occurs. Similarly to motor deficits after SCI, the level of spinal lesion greatly influences the amount of cardiovascular regulation possible after injury . Although vagal influence on chronotropy is maintained after SCI (Mathias and Bannister 2002), several reports  have shown that the sensitivity of the cardiovagal baroreflex is impacted (Table 2). As cardiovagal BRS has shown predictive value for future cardiovascular events in able-bodied individuals, these findings support epidemiological studies indicating that cardiovascular risk is increased in those with SCI (La Rovere et al., 1998; Ormezzano et al., 2008; Yufu et al., 2011). As mentioned previously, the cardiovagal baroreflex 43  is unlikely to influence the overall absolute mean arterial blood pressure response to orthostatic challenge. However, evidence indicates that in able-bodied individuals, the vagal influence on heart rate (and therefore cardiac output) plays an important role in the short-term regulation of blood pressure (100% in the first 2-3 s after stimulus) but only a minor role after that (23%) (Ogoh et al., 2003). Further, the cardiovagal baroreflex is intimately related to the brain?s ability to maintain effective perfusion (cerebral autoregulation) (Ogoh et al., 2010; Tzeng et al., 2010). As such,  it is reasonable  to suggest that abnormal cardiovagal baroreflex function after SCI is associated with the reduced orthostatic tolerance found after injury (Ogoh et al., 2006).   The loss of the sympathetic branch of the baroreflex is far more detrimental to blood pressure regulation after SCI, as the descending sympathetic pathway becomes disrupted and results in significant reductions in the control of vasomotor tone below the lesion level (Figure 10) (Furlan et al., 2003; Krassioukov and Claydon 2006). Those with lesion levels above the 6th thoracic spinal level have more severe autonomic dysfunction as compared to those with lesions below this level (Mathias and Bannister 2002). This is due partially to decreased sympathetic activity within the sympathetic post-ganglionic fibres, which innervate the splanchnic vascular bed, originating from below the 6th thoracic spinal segment. and therefore not having supra-spinal descending communication (Mathias and Bannister 2002). Additionally, autonomic dysfunction (e.g., autonomic dysreflexia) results from a lack of descending supra-spinal inhibition during periods of noxious or non-noxious stimuli reaching the spinal cord below the level of injury such as catheterization, bladder distension, bowel evacuation, and even a tight shoelace  (Teasell et al., 2000; Mathias and Bannister 2002; Krassioukov and Claydon 2006). As with motor impairment, there is a high level of variability in the level of autonomic dysfunction, even for individuals with the same lesion level  which is probably due to variability in the number of preserved descending autonomic pathways synapsing on sympathetic pre-ganglionic neurons below the level of injury (Teasell et al., 2000; Mathias and Bannister 2002; Krassioukov 2009).  44   Figure 10. Illustration of where baroreflex dysfunction may occur after spinal cord injury. Briefly neural signals from brain (central command), as well as afferent input from the aortic and carotid stretch-receptors, central and peripheral chemoreceptors, and low pressure pulmonary centres converge in the cardiovascular control centre. The complex interplay of these factors regulates blood pressure set-point and influence the rate and amplitude of autonomic response to acute blood pressure changes (baroreflex sensitivity). This is achieved through rapid alterations of HR (heart rate), SV (stroke volume), and most importantly TVC (total vascular conductance) by adjusting autonomic outflow to cardiac and vascular tissue.  ? denotes where baroreflex dysfunction may occur after spinal cord injury (1- arterial stiffening, 2- integration in the nucleus tractus solitarius, 3-sympathetic descending pathway disruption, 4-sino atrial node transmission). *Note that when complete a complete injury occurs, sympathetic vasomotor control is disrupted below the lesion level.  High lesion level The literature search revealed 12 studies that have examined vagally-mediated baroreflex function in those with high-level SCI (Table 2). Five of these 12 articles reported a decrease in cardiovagal BRS; one showed an increase; and the remaining seven demonstrated no difference between individuals with SCI and able-bodied controls.  As SCI is a low prevalence condition, 45  typically studies of reduced statistical power are published. The average Cohen?s d value for effect size (i.e., the difference between the two means divided by the pooled standard deviation) is 0.29 ? 0.7 in studies investigating cardiovagal baroreflex function in humans with high level SCI. These values show that the SCI and able-bodied groups differed on average by approximately 0.3 ? 0.7 standard deviations. Considering that a Cohen?s d value of 0.29 is considered a small effect and the variability between articles was more than twice the mean, these values do not compellingly illustrate that BRS is reduced in those with a  high lesion. This is not entirely surprising considering not only the small sample sizes, but also the extreme heterogeneity of SCI (Scott et al., 2011). Interestingly, the two studies using the neck cuff technique reported very high Cohen?s d values at 1.8 and 0.99, suggesting greater sensitivity when using this technique (Convertino et al., 1991; Koh et al., 1994).   If indeed cardiovagal BRS is reduced in high-level SCI, some consideration of this condition is deserved. Cardiovagal BRS is influenced by any of the following mechanisms: a) arterial stiffening reducing the input from the stretch receptors, b) a reduction in the signal transmission and integration in the nucleus tractus solitarius, or c) changes in the efferent signal transmission at the SA node (Eckberg and Sleight 1992) (Figure 11). As neither the vagal or glossopharyngeal nerves are damaged during high level SCI, it is speculated that reductions in cardiovagal BRS are associated primarily with arterial stiffening in this population. Increased arterial stiffening leads to decreased activation of arterial stretch receptors for a given change in intra-arterial pressure; therefore, directly reducing the sensitivity of the system (Monahan et al., 2001; Mathias and Bannister 2002; Mattace-Raso et al., 2007). Arterial stiffening is enhanced by physical inactivity, which is highly prevalent in SCI (Myers et al., 2007). These pathways are well known and have been reviewed recently (Seals et al., 2009). However, studies have reported that arterial stiffness is increased in those with SCI, even when matched for physical activity patterns (Wong et al., 2007; Miyatani et al., 2009; Phillips et al., 2012b). Mechanisms responsible for increased stiffness specific to SCI population include: reduced shear stress leading to reduced arterial calibre and increased wall thickness; endothelial cell glucose insensitivity; and sympathetic dysfunction (Fronek et al., 1978) (Alan et al., 2010; Rowley et al., 2012). Further evidence of an arterial-mediated alteration in BRS exists in a study showing that upon movement from supine to 45 degrees of head up tilt, high lesion level SCI have greater 46  reductions in carotid arterial diameter and blood flow velocity as compared to both low lesion level SCI and AB controls (Wecht et al., 2004).    Figure 11. Summary of evidence examining baroreflex function in those with high level spinal cord injury. ? indicates where baroreflex function in likely to occur in those with high level spinal cord injury.   Alternatively, the neural component may also be influenced after SCI. Those with high level SCI have chronically elevated renin concentrations and suggested that this may impact sino-atrial vagal sensitivity (Mathias et al., 1980; Wecht et al., 2006). Furthermore, autonomic regulatory centres in the brain require sufficient perfusion in order to maintain effective blood pressure regulation (Lanfranchi and Somers 2002). Also, considering that individuals with chronic hypotension also have reduced cerebral perfusion it is plausible that the neural BRS pathway is also disrupted in those with high level SCI (Duschek and Schandry 2004). Indeed 47  marked inability to maintain cerebral blood flow during orthostatic challenge has been shown in high level SCI (Houtman et al., 2001; Bluvshtein et al., 2011; Sahota et al., 2012).      The one study to show a paradoxically significant increase in cardiovagal BRS used a very similar technique (power spectral transfer function analysis) and had a relatively large sample size (n =14, T3 and above) (Munakata et al., 2001). The authors speculated that cardiovagal sympathetic afferent activity may be reduced during head-up tilt in high level injury group due to reductions in venous return and cardiac dimensions, thus decreasing sympathetic afferent and the subsequent efferent activity. However, it is well established that resting SNS activity in a complete lesion above T6 is severely disrupted and that outside of autonomic dysreflexia, sympathetic tone is essentially null (Stjernberg et al., 1986). As such, it is very unlikely that the above postulation explains this paradoxical finding as sympathetic outflow to the heart typically occurs at the T4-5 level and all participants had complete injury levels at T3 or above (Munakata et al., 2001). Alternatively, these contrary spontaneous cardiovagal BRS results may be due to the very uncommon technique of examining BRS gain from a broad frequency range including both the low and mid-frequency ranges (i.e., 0.02-0.4). Also, Munakata reported that 5/14 high lesion level individuals had BRS gain generated in the high frequency range, due to low coherence over the low and mid-range frequencies; the prior of which has been shown to have increased coherence due to respiration (Tzeng et al., 2009). On the other hand, research has illustrated that when chronic vagal tone is increased in canines, cardiovagal BRS is increased (Zhang et al., 2009b). In addition, it has been shown that increases in supine cardiac vagal tone in those with high level SCI (Claydon and Krassioukov 2008). These studies provide an explanation for the number of articles that reported preserved cardiovagal BRS in those with SCI, if it is assumed that arterial stiffness was increased.    The T6 level has been shown to be an important lesion level due to the loss of descending supraspinal sympathetic signals in the crucial splanchnic region. Accordingly, SCI above the 6th thoracic vertebrae leads to immense disturbances in autonomic cardiovascular regulation. After high level SCI, sympathetic outflow is differentially influenced depending on both the lesion level and the completeness of injury. It is important to note that sympathetic vasomotor innervation persists after high level SCI. A combination of poor supraspinal sympathetic regulation, as well as hypersensitivity to alpha-1, alpha-2, agonists and angiotensin II after high level SCI leads to inappropriate adrenergic responses (Krum et al., 1992; Mathias and 48  Bannister 2002). Only eight articles have examined the sympathetic baroreflex response in those with high level SCI, and none have used established blood pressure perturbing techniques known to provide the most accurate and reliable results (Table 3) (Diaz and Taylor 2006). Sympathetic dysfunction below the lesion level has made measuring the sensitivity of the sympathetic baroreflex difficult. Several studies, using indirect estimates derived from frequency analysis of heart rate and systolic blood pressure, suggested a reduced sympathetic response to an orthostatic challenge in those with high level SCI. These measures cannot be taken to directly relate to sympathetic activity, and the use of frequency analysis derived measures to quantify sympathetic activation is contentious. While originally reported to be significantly related using group data during lower body negative pressure (Pagani et al., 1997), Ryan et al. has shown recently that on an individual basis low frequency power of systolic blood pressure (SBPLF) does not correlate with recordings of MSNA (Ryan et al., 2011). Similarly, the low frequency component of the ratio of low frequency RRI power to high frequency RRI power (LFRRI/HFRRI) has been shown to be related to MSNA in two early articles, although both have limitations in that either 1) the statistical analysis may not be suitable (Pagani et al., 1997), or 2) only 40% of participants reported a significant relationship (Saul et al., 1990). Also in opposition of using LFRRI/HFRRI, Cooke et al. has shown in able-bodied individuals that the changes in LFRRI/HFRRI that occur during sympathetic activation are more due to decreases in the HF denominator (thought to arise primarily from respiratory mediated increases in vagal efferent activity) and not the LF numerator (Cooke et al., 2008). Taking into consideration the controversy surrounding these measures, studies examining spectral derived indicators of sympathetic tone in those with high level SCI still deserve discussion and indeed SBPLF, and LFRRI/HFRRI are reduced in those with high level SCI during orthostatic challenges (Table 3). As the amplitude of vagal withdrawal is usually the same (Wecht et al., 2006) or increased (Claydon and Krassioukov 2008) in this population, reduced LFRRI/HFRRI may indicate an attenuation of the sympathetic cardiac response during an orthostatic challenge. These studies, which have employed indirect indicators of sympathetic activity, are supported more directly by work showing that the norepinephrine response to a nitroprusside bolus (rapid decrease in blood pressure) was severely reduced in high level lesion SCI. Together, there is compelling evidence for a dysfunctional sympathetic response to blood pressure changes and orthostatic challenges in this population (Koh et al., 1994). This evidence should be interpreted with caution however, as these studies do not 49  necessarily show a mitigated baroreflex-mediated sympathetic response (which would occur within 5 seconds after orthostatic challenge), as much as they highlight the functional inadequacy of the sympathetic autonomic branch during haemodynamic challenges in those with high level SCI.    Due to the loss of sympathetic vasomotor tone below the lesion level and the susceptibility to orthostatic hypotension, those with high level SCI rely disproportionately on the renin-angiotensin-aldosterone system to regulate blood pressure (Popa et al., 2010). As such, larger increases in renin have been found during orthostatic challenge in tetraplegics (Wecht et al., 2005). The increase in renin-angiotensin dependency, however, appears to be more related to blood pressure responses due to poor vasomotor response, and less to baroreflex function specifically. Improving blood pressure response through nitro-L-arginine methyl ester administration led to mitigated increases in aldosterone (trend for smaller increases in renin) during orthostatic challenge in those with high lesion level SCI (Wecht et al., 2009).   It should be noted that one study did not find a reduced response of SBPLF during orthostatic challenge in high-level lesions, a finding potentially explained by a weaker orthostatic stimulus than that used in other studies (Bluvshtein et al., 2011). Collectively, the overall findings in this field of study can be explained through work by Stjernberg et al. showing that resting MSNA in the peroneal nerve is extremely low and almost non-existent in those with SCI (Stjernberg et al., 1986). This highlights that the ability to produce effective vasomotor tone is lost below the level of SCI. Severely reduced vasomotor control is also displayed in work by Houtman et al. that demonstrated an essentially passive blood pressure and cerebral blood flow response to lower body negative pressure in those with high SCI level, while in able-bodied individuals with intact vasomotor control, mean arterial pressure is well maintained (Houtman et al., 2001). In this study, it was illustrated that blood pressure and cerebral blood flow decreased in a remarkably linear magnitude in relation to the suction applied to the legs (Houtman et al., 2001). The most interesting aspect of this article is the finding that cerebral oxygenation was markedly decreased in the high lesion group as compared to able-bodied individuals; however, both groups reported similar prevalence of syncope, suggesting that downstream mechanisms related to the hypoxic threshold for syncope may have adapted. Other work has highlighted this possibility, showing that subtle markers of cerebral haemodynamic adaptation (dynamic cerebral autoregulation as well as cerebral blood flow pulsatility index) occurs in those with high level 50  SCI (Wilson et al., 2010). This association provides support of the fundamental inverse relationship between BRS and cerebral autoregulation (Tzeng et al., 2010).   It is unfortunate that so few studies have investigated the sympathetic branch of the baroreflex system in those with high-level lesions, as this would provide insight into the functioning of the baroreflex branch most important to the widespread orthostatic intolerance present within this population. More studies on this topic may shed light on why blood pressure is so drastically reduced after orthostatic challenge and why alpha-1 agonist administration not only improves resting blood pressure, but also mitigates the blood pressure reduction during orthostatic challenge (Wecht et al., 2011).   Low lesion level Marked variability exists within seven articles that have examined cardiovagal baroreflex function in low level SCI (Table 2). For example, three studies showed a reduction in cardiovagal BRS. Of these, two employed the Valsalva technique to measure BRS (Grimm et al., 1998; Houtman et al., 1999), while the remaining article used a spontaneous indicator of BRS(Grimm et al., 1998). As such, 80% of the studies using spontaneous indicators of BRS failed to show a significant reduction in those with low lesion level SCI. The average Cohen?s d value for effect size is 0.55 ? 1.11 in studies investigating spontaneous baroreflex function in humans with low level SCI. Taking into account the variability in the available evidence, it is still not clear if cardiovagal BRS is reduced in those with low level SCI. Again, the two studies employing phase IV Valsalva for BRS evaluation reported high Cohen?s d values of 2.3 and 1.0 (Grimm et al., 1998; Houtman et al., 1999).   Similar to high lesions, those with low level SCI tend to be more physically inactive in comparison to able-bodied individuals (Myers et al., 2007). Furthermore, lower resting supine vagal tone has been shown in those with low SCI (Claydon and Krassioukov 2008). Therefore, following the same principles outlined for the high level lesion SCI group (i.e., physical activity and resting vagal tone), there is a strong rationale for finding reduced sensitivity of the baroreflex (Wecht et al., 2004). From a haemodynamic perspective, cerebral perfusion is likely maintained in low level SCI, and cardiovagal tone is preserved (Houtman et al., 2001; Bluvshtein et al., 2011). Following this, the neural pathway of the baroreflex is likely not disrupted (Claydon and Krassioukov 2008).  Both studies using phase IV Valsalva derived-values reported reduced BRS in low lesion SCI. It is difficult to synthesize these findings with the overall trends found using 51  spontaneous BRS measures. Published work (Zollei et al., 2003) comparing the Valsalva technique to the modified Oxford method have shown the two measures to be correlated, but not to provide similar results. When compared to spontaneous BRS techniques, the values from the Valsalva technique were not associated with those reported using the spectral method, and related significantly to only Gdown of the sequence method (Zollei et al., 2003). It has been suggested that BRS markers derived from the Valsalva maneuver provide different information regarding the baroreflex compared to spontaneous derived markers. Spontaneous BRS is thought to be representative of tonic cardiovagal activity whereas BRS measures derived from perturbed blood pressure (Valsalva, modified Oxford) are associated with the phasic relationship between vagal tone and blood pressure changes (Parlow et al., 1995). Therefore, it may be the case that phasic BRS is influenced after SCI whereas tonic BRS is not. The clinical value of phasic versus tonic BRS has yet to be determined.   Several articles have attempted to measure the sympathetic branch of the baroreflex system in those with low level SCI. These articles have shown that the orthostatic response in SBPLF and LFRRI/HFRRI was also reduced in those with low level SCI (Table 3). Those with low-level SCI also do not have tonic vasomotor outflow below the lesion level and therefore would have reduced amplitude of SBP oscillations. Indeed, this group is expected to have more systemic vasomotor tone as compared to those with higher lesion levels as they have less vascular tissue under disrupted sympathetic control (Stjernberg et al., 1986). Bluvshtein et al. reported that LFRRI/HFRRI was similar between low lesion level SCI and able-bodied controls, however the sample size was quite small and the study may have been under-powered  (Bluvshtein et al., 2011). These findings highlight that the cardiac and vasomotor response to orthostatic challenge is disrupted, even after low level SCI. Interpretation of these studies should be made with caution as neither of the markers of SNS activity truly represents BRS, as they are simply measures of resting autonomic tone in different orthostatic positions.  In general, baroreflex dysfunction in those with low level lesions is not as severe as in those with high level lesions (Figure 12). This is thought to be due to the aforementioned mitigating factors, such as greater physical activity levels, preservation of some sympathetic vasomotor tone, and maintenance of cerebral perfusion.   52   Figure 12. Summary of evidence examining baroreflex function in those with low level spinal cord injury. ? indicates where baroreflex function in likely to occur in those with high level spinal cord injury.  FUTURE DIRECTIONS In order to focus the clinical and research efforts to improve BRS, the issue of whether dysfunctional BRS after SCI is due primarily to increases in arterial stiffening or to a more downstream neural component needs to be elucidated. The resolution of this issue would also aid in the development of new technologies, and the advancement of techniques currently undergoing testing such as a transcutaneous bionic baroreflex system (Yoshida et al., 2008). It should be noted that a number of articles are insufficiently powered to assess baroreflex function in SCI. This issue should be more widely acknowledged and discussed in papers where sample size is an issue.   The physical activity levels of the low level SCI groups were not mentioned in most research studies, and only one article attempted to control for physical activity levels (Legramante et al., 2001). There is often much more access to, and participation from, SCI 53  participants who are highly active, while it is exceedingly difficult to recruit SCI subjects with a more physically inactive lifestyle. Physically active individuals with SCI tend to have reduced arterial stiffness and would also be expected to have increased exposure to gravitational challenge as compared to their inactive counterparts (resulting from generally fewer orthostatic challenges in daily living) (Phillips et al., 2012b). As both of these factors are known to be related to reduced BRS, this selection bias may have led to more similar cardiovagal BRS (Scott et al., 2011). Further to this, spontaneous techniques for evaluating BRS, especially those derived through the spectral method are confounded by the feed forward relationship between increasing heart rate and blood pressure (Diaz and Taylor 2006). As such, spontaneous BRS measures, particularly for the purpose of stratifying for cardiovascular risk, should be used with caution, while non-spontaneous measures of BRS need to be more widely employed (Diaz and Taylor 2006).      As those with chronic hypotension and SCI are thought to have decrements in cognitive function associated with reduced brain blood perfusion, the relationship between baroreflex function, cerebral haemodynamic regulation and cognitive function in SCI also needs to be examined. Baroreflex sensitivity directly influences cerebral perfusion, and appropriate treatment of BRS after SCI may mitigate the reductions in cognitive function (Wecht et al., 2012).  In both high and low level SCI, direct measurement of the sympathetic branch of the baroreflex is warranted in order to evaluate the role reduced sensitivity plays in orthostatic hypotension. Although peroneal nerve MSNA is not viable, brachial nerve MNSA or brachial artery vascular resistance are potential sites to measure vasomotor responses to blood pressure and/or arterial diameter changes.  CONCLUSIONS The available literature indicates that BRS is disrupted in those with both low and high level SCI. Baroreflex function is more consistently disrupted in those with high level lesions, due to severe dysfunction of the sympathetic as well as parasympathetic branches. The overall findings are hampered by small sample sizes and several spontaneous and ramped BRS techniques which detect different components of the baroreflex. Baroreflex sensitivity in those with low level SCI is less clear. Arterial stiffening has been shown to be increased in both high and low level SCI although it is interesting that BRS is not consistently reduced in these populations. It is plausible that the increased vagal tone, which has been shown to improve cardiovagal BRS (Zhang et al., 54  2009b), may ameliorate the decline resulting from increased arterial stiffening. Focus should be directed to examining the sensitivity of the sympathetic branch of the baroreflex, due to the lack of information currently available.   55  Table 2. Summary of studies examining cardiovagal baroreflex function in humans with spinal cord injury. Details  Variable High Lesion d ES Low Lesion D ES Notes Convertino et al. 1991  n = 16  USA Supine carotid-vagal (neck-cuff) ? 1.8 0.67     Krum et al. 1992  n = 8 Australia G-up of modified Oxford technique. ?*      *Trend of reduced BRS  p = 0.18 Koh et al. 1994  n = 8  USA Supine carotid-vagal (neck-cuff) ? 0.99 0.44     Vagal response to modified Oxford technique (n = 3). ?      Grimm et al. 1998 n  = 12 USA Seated heart rate response to phase IV Valsalva    ? 2.32 0.76  Spontaneous cardio-vagal    ? 3.63 0.88 Houtman et al. 1999  n = 11 high lesion n = 10 paraplegics  (T4-5 included) Netherlands Seated heart rate response to phase IV Valsalva ?* 1.12 0.49 ? 1.03 0.46 *Trend for high lesion group to be reduced compared to AB. High lesion greater spontaneous BRS than paraplegics Supine spontaneous cardio-vagal ? 0.6 0.29 ? 0.79 0.37 56  Details  Variable High Lesion d ES Low Lesion D ES Notes Iellamo et al. 2001 n = 9 Italy Supine spontaneous cardio-vagal ? 0.05 0.03     Seated spontaneous cardio-vagal ? 0.29 0.14    Munakata et al. 2001      n = 14 high lesion n = 12 low lesion         (T4 included) Japan Supine spontaneous cardio-vagal ? 0.12 0.01 ? 0.11 0.06 High level group included injuries from C3-T3, while low injury level was T4-L1. Spontaneous cardio-vagal during 60? HUT ? -0.78 -0.36 ? -0.19 -0.1 Legramante et al. 2001  n = 8 high lesion  n = 7 low lesion Italy Supine spontaneous cardio-vagal ? 0.18 0.09 ? 0.24? 0.12? Similar reduction in BRS after HUT in all groups. During HUT, low lesion SCI had reduced ratio of baroreflex to non-baroreflex sequences; high lesion group is further reduced. High lesion group have reduced non-baroreflex sequences when supine, but increased during HUT as compared to low lesion group and AB. Spontaneous cardio-vagal during 70? HUT ? 0.31 0.16 ? 0.14? 0.07? 57  Details  Variable High Lesion d ES Low Lesion D ES Notes Gao et al. 2002  n = 9 Australia Semi-recumbent cardio-vagal ? -0.09 -0.05     Aslan et al. 2007  n = 5 high lesion n = 5 low lesion USA Supine spontaneous cardio-vagal ? -0.08 -0.04 ? 1.0 0.45 All acute patients. Low lesion SCI had reduced BRS in supine recovery from orthostatic challenge. High lesion group had similar cardio-vagal response to AB during progressive tilt. Castiglioni et al. 2007  n = 33 (T5-L4) Italy Supine spontaneous cardio-vagal    ? 0.21? 0.10? Baroreceptor effectiveness index was reduced in SCI. Heart rate response to blood pressure (phase) was delayed in SCI  Claydon and Krassioukov 2008  n = 9 high lesion level n = 8 paraplegic (T2-T11) Canada   Supine spontaneous cardio-vagal ? -0.47 -0.23 ? 0.21 0.10  Seated spontaneous cardio-vagal ? 0.46 0.23 ? 0.13 0.07 58    ?; significant increase compared to able-bodied controls, ?; significant decrease compared to able-bodied controls, ? no difference when compared to able-bodied controls, SCI; spinal cord injury, BRS; baroreflex sensativity, SCI; spinal cord injury, HUT; head -up tilt. AB; able-bodied, ES; effect size, d; Cohen?s d value. High lesion refers to those with injuries at or above the 6th thoracic vertebrae while low lesion level denotes those with injures below the 6th thoracic level unless otherwise stated, ? denotes that effects size values were calculated from graphical estimates and not numerical data.  59  Table 3. Summary of studies potentially highlighting sympathetic baroreflex function in humans with spinal cord injury. Details  Variable High Lesion Low Lesion Notes Guzetti et al. 1994  n = 6-8 Italy RRI LF/ RRI HF  response to passive tilt of 40? to 80? (n=8) ?   SBPLF response to passive tilt of 40? to 80? (n=6) ?  Koh et al. 1994  n = 8  USA Norepinephrine response to modified Oxford technique (n = 3) ?   Houtman et al. 1999  n = 9 tetraplegic  n = 9 paraplegic Netherlands SBPLF response to HUT ? ?  Wecht et al. 2003  n = 19 New York City SBPLF response to progressive HUT  ?  RRI LF/ RRI HF  response to progressive HUT  ? 60  Details  Variable High Lesion Low Lesion Notes Wecht et al. 2006  n = 7 high lesion n = 7 low lesion USA RRI LF/ RRI HF after 45? HUT ? ?  Aslan et al. 2007  n = 5 high lesion n = 5 low lesion USA SBPLF response to progressive HUT ? ? SBPLF decreased in high lesion group during HUT, was similar in low lesion group and was increased in AB. Claydon and Krassioukov 2008  n = 9 high lesion level n = 8 paraplegic (T2-T11) Canada  SBPLF  during sit up test ?? ? RRI LF/ RRI HF increased AB, but not high or low level lesion groups in response to sit-up test. SBPLF during sit up test was lowest in high level, and intermediate in low level as compared to controls.  RRI LF/ RRI HF during  sit up test ? ? Handrakis et al.  2009  n = 9 USA SBPLF response  after combined ACE inhibitor and 45? HUT  ?  Had reduced SBPLF after tilt while able-bodied individuals had increased values. 61  Details  Variable High Lesion Low Lesion Notes Bluvshtein et al. 2011  n = 11 high lesion n = 10 low lesion Israel RRI LF/ RRI HF  response to 35? HUT ? ? RRI LF/ RRI HF increased in both groups similarly to non-injured controls. Trend for a decrease in RRILF in high lesion only. ?; significant increase compared to able-bodied controls, ?; significant decrease compared to able-bodied controls, ? no difference when compared to able-bodied controls, SCI; spinal cord injury, RRI LF/ RRI HF; ration of LF power of RRI to HF power of RRI, HUT; head -up tilt.  SBPLF; spectral power of blood pressure in the LF region, AB; able-bodied. High lesion refers to those with injuries at or above the 6th thoracic vertebrae while low lesion level denotes those with injures below the 6th thoracic level unless otherwise stated. 62  PART III ? MIDODRINE HYDROCHLORIDE Midodrine hydrochloride is a sympathomimetic selective alpha-1 adrenergic receptor agonist, and is a pharmaceutical therapy indicated for treatment of orthostatic hypotension (Pittner et al., 1976). Midodrine acts on arterioles and veins to increase total peripheral resistance and reduce venous capacitance, thereby increasing venous return and increasing cardiac output (Thulesius et al., 1979; Zachariah et al., 1986; McTavish and Goa 1989). Midodrine itself is not active, however after enzymatic hydrolysis desglymidodrine is formed which directly acts on alpha-1  adrenoreceptors (Pittner et al., 1976). Desglymidodrine does not cross the blood-brain barrier and exerts minimal cardiac effect (Pittner et al., 1976; Schatz 1984). Also, desglymidodrine has a bioavailability of 93% (Grobecker et al., 1987). Desglymidodrine is structurally very similar to norephinephrine (McTavish and Goa 1989). Peak plasma concentration of desglymidodrine is approximately 60-90 minutes after oral administration (Pittner et al., 1976). The half life of desglymidodrine is three hours in those with normal renal function (Pittner et al., 1976), which is longer than other similar drugs (Marini et al., 1984; Fouad-Tarazi et al., 1995). The primary side effect of midodrine is supine hypertension (Grubb et al., 1999). Other side-effects of midodrine are generally less severe than those reported by similar drugs but can include confusion, anxiety, piloerection reactions (i.e., goose-bumps, tingling, itching of the scalp) (McTavish and Goa 1989; Robertson and Davis 1995). Hesitancy and retention can occur as well after midodrine administration due to constriction of urethral sphincter (Jankovic et al., 1993). Clearly, other medications that impact alpha receptors should be avoided as they can possibly have additive or antagonizing actions and can dangerously magnify or reduce the effect of midodrine.   Midodrine is the only pharmaceutical therapy approved for the treatment of dizziness during standing, and has been shown to decrease the occurrence of syncopal episodes in AB individuals with vasovagal syncope in a number of uncontrolled and two randomized trials (Grubb et al., 1999; Mitro et al., 1999; Perez-Lugones et al., 2001; Samniah et al., 2001). Very recently however in a randomized cross-over trial, midodrine failed to show reduced syncopal episodes in a group of AB individuals who did not respond to non-pharmaceutical therapy (Romme et al., 2011). Although the quality of evidence is considered low (Logan and Witham 2012), an upcoming multicenter, international, randomized, placebo-controlled study of midodrine in the prevention of vasovagal syncope is underway and should provide a high level analysis of the usefulness of midodrine (Raj et al., 2012). In SCI, the effect of midodrine on 63  orthostatic hypotension and symptoms of presyncope have been studied by five uncontrolled comparative (i.e., drug vs. no drug) studies, generally supporting the use of midodrine for the reduction of orthostatic hypotension, but not necessarily orthostatic tolerance or cerebral hypoperfusion (Senard et al., 1991; Barber et al., 2000; Mukand et al., 2001; Wecht et al., 2010; Wecht et al., 2011). In fact, the two studies designed to examine cerebrovascular responses to tilt after midodrine in SCI did not have AB controls, measured self reported symptoms of presyncope as opposed to orthostatic tolerance, focused on only sCA, and did not examine regulation of arteries of the vertebrobasilar system (Wecht et al., 2010; Wecht et al., 2011); arteries that may be more related to orthostatic tolerance as they perfuse the medulla oblongata which contains associated autonomic control centers as well as discrete regions responsible for consciousness (Shin et al., 1999).                       64  THESIS OVERVIEW, AIMS AND HYPOTHESES This literature review has described, compared and discussed the existing scientific literature examining baroreflex and cerebral blood flow control in those with SCI. Following this review, it is clear that the current knowledge is lacking in several key areas. Specifically, no data exists on the metabolic component of CBF control, while very little is understood about regional CBF autoregulation. There is also no information regarding why baroreflex function is reduced in SCI (i.e., mechanical or neural components), and whether it is fundamentally related to CBF control in this population (as it is in AB). The central aim of this thesis is to evaluate CBF regulation in SCI at rest and during orthostasis, before and after increasing blood pressure via midodrine hydrochloride. Within the literature review, three main research foci that will serve as the outline of this thesis were highlighted: 1. The regional CBF response to metabolic demand in those with SCI before and after increasing blood pressure.  2. Spontaneous and non-spontaneous metrics of regional cerebral autoregulation in those with SCI during orthostatic challenge: the effect of in increasing blood pressure 3. The neural and mechanical components of the cardiovagal baroreflex during orthostatic challenge in those with SCI, and the effect of increasing blood pressure  With these primary research foci, this thesis is divided into three study chapters each with a specific hypothesis:  Chapter 3 (Study 1): Regional neurovascular coupling and cognitive performance in those with low blood pressure secondary to high-level spinal injury: improved by alpha-1 agonist midodrine hydrochloride. Hypothesis 1. Those with SCI will have impaired neurovascular coupling and cognitive function as compared to able-bodied individuals (AB).  Hypothesis 2. Increasing blood pressure through the administration of midodrine will improve neurovascular coupling and cognitive function in SCI.   Chapter 4 (Study 3): Cerebral blood flow control in high level spinal cord injury: the effect of midodrine hydrochloride. 65  Hypothesis 1. Static cerebral autoregulation would be preserved SCI while dynamic cerebral pressure-flow relationships would be altered compared to AB. Hypothesis 2. Increasing blood pressure via midodrine in SCI would not influence static CA, but would partially normalize dynamic cerebral pressure-flow metrics to values reported in AB. Hypothesis 3. Orthostatic tolerance would be improved in SCI after midodrine. Hypothesis 4. Dynamic cerebral pressure-flow relationships would be uncoupled from baroreflex function in SCI.  Chapter 5 (Study 4):  Association between integrated cardiovagal baroreflex sensitivity and carotid artery mechanics in spinal cord injury: effect of midodrine hydrochloride. Hypothesis 1. Arterial stiffness will be increased in those with SCI and that increased arterial stiffness will be related to reduced cardiovagal baroreflex sensitivity. Hypothesis 2. Midodrine will not influence arterial stiffness or cardiovagal baroreflex sensitivity in those with SCI.               66  CHAPTER THREE ? REGIONAL NEUROVASCULAR COUPLING AND COGNITIVE PEFORMANCE IN THOSE WITH LOW BLOOD PRESSURE SECONDARY TO HIGH-LEVEL SPINAL CORD INJURY: IMPROVED BY ALPHA-1 AGONIST MIDODRINE HYDROCHLORIDE INTRODUCTION Spinal cord injury (SCI) is a devastating clinical condition often resulting in disturbed motor sensory, and autonomic function below the level of injury. As a result of disrupted descending autonomic spinal pathways after SCI, sympathetic vasomotor tone is impaired, often resulting in arterial hypotension (Teasell et al., 2000). Those with injury at or above T6 spinal segment, due to a myriad of potential mechanisms (including decentralization of sympathetically mediated vasomotor tone in the splanchnic region and legs) have more profound autonomic disturbances including resting hypotension and orthostatic hypotension as compared to those  with lower level SCI (Krassioukov and Claydon 2006).   An abundance of evidence, from both animal and human studies, demonstrated that there is an association between low blood pressure and a variety of cognitive impairments (Elias et al., 2010; Hartman et al., 2012). The high prevalence of cognitive dysfunction (i.e., between 10 and 60%) in SCI (Davidoff et al., 1992; Dowler et al., 1995; Dowler et al., 1997) is likely to be at least partially due to widespread hypotension (Noreau et al., 2000; Cariga et al., 2002; Claydon et al., 2006; Myers et al., 2007). This notion is further highlighted by a significant positive relationship between systolic blood pressure (SBP) and cognitive function in the SCI population (Jegede et al., 2010).  It has been speculated that cognitive dysfunction in chronic hypotension is due to reduced blood flow perfusion in the cerebrovasculature (Perlmuter et al., 2012). Resting cerebral blood flow (CBF), however, has been shown to be similar at rest in SCI as compared to AB (Nanda et al., 1974; Nanda et al., 1976; Handrakis et al., 2009). Although resting CBF may be preserved after SCI, cerebrovascular reserve (i.e., the ability of the cerebrovascular system to respond to acute increases in metabolic demand, mechanical or neural stimuli) may be affected, resulting in an insufficient CBF response (Novak and Hajjar 2010). Supporting this contention, elegant work by Harper and Glass (1965) showed that progressive hypotension abolishes the CBF capacity to dilate or constrict (Harper and Glass 1965).   67   Indeed, in individuals with chronic hypotension it has been reported that there is a blunted increase (i.e., peak response 40% lower) in middle cerebral artery (MCA) mean velocity during a preparatory phase before cognitive testing (i.e., five seconds prior to reaction time) (Duschek and Schandry 2004). Interestingly, it was demonstrated in the normotensive controls that  increases in MCA flow velocity during the preparatory phase were related to reaction time (Duschek and Schandry 2004). This protocol for evaluating the MCA flow velocity response to cognition has never been reported, and the ?cognitive task? was that of 20 individual simple-reaction time tests, each divided by 50 seconds; likely resulting in limited cerebral activation as compared to more complex processes (Schubert and Szameitat 2003), such as those found in everyday living.  Neurovascular coupling (NVC; i.e., the CBF response to cognition) is a well-established protocol examining the metabolic component of cerebrovascular reserve, and is considered one of three fundamental CBF regulatory measures (Willie et al., 2011b). Impaired NVC, which is associated with stroke and hypertension, disrupts both the delivery of substrates to activated cerebral tissue and the removal of harmful by-products, and is likely to play a integral role in cognitive dysfunction (Girouard and Iadecola 2006). As cerebral perfusion pressure is a critical component of CBF control, it is possible that NVC mediates the relationship between reduced blood pressure and cognitive function in SCI (Harper and Glass 1965). Wecht et al. partially examined this relationship by showing the MCA response to cognition (i.e., Stroop test) was not different between AB and SCI (Wecht et al., 2012). Although the internal carotid/MCA provides >75% of total CBF (Lindegaard et al., 1987), arteries responsible for perfusing the brainstem (i.e., vertebral/posterior cerebral arteries (PCA)) have been shown to be more responsive to various cognitive stimuli (Azevedo et al., 2011; Willie et al., 2011c; Sato et al., 2012; Willie et al., 2012). Considering the different responsiveness of the posterior cerebral vasculature, a global evaluation of NVC incorporating an assessment of both the MCA and PCA is likely to increase sensitivity for detecting changes in cerebrovascular reserve.    There is a lack of studies examining NVC in either those with low blood pressure or SCI. To further shed light on the mechanism of cognitive dysfunction in low blood pressure secondary to SCI, NVC was examined with and without the administration of midodrine hydrochloride, an alpha-1-agonist, to increase blood pressure to AB level in the SCI group (Low et al., 1997). The primary hypothesis was that those with SCI will have an impaired NVC response and cognitive 68  function as compared to AB. Second, it was hypothesized that increasing blood pressure through the administration of midodrine will improve NVC and cognitive function in SCI.  METHODS Ten individuals (7 male, 3 female) with motor complete SCI participated in this study (C4-T5, AIS A and B; Table 4). Eight participants were <1 year post-injury while 2 were >1 year post-injury (Table 4). The control group was composed of 10 age- and sex-matched individuals (AB). All testing took place at GF Strong Rehabilitation Centre, in Vancouver, Canada. Participants with SCI were approached by a research coordinator after being notified by an attending physician that the patient met the inclusion criteria. Control participants were recruited with posters placed around the University of British Columbia campus. Participant characteristics are presented in Table 5.    All participants were instructed to abstain from exercise and alcohol for 24 h before testing. No caffeine was permitted the day of testing. Additionally, participants were requested to abstain from all other medications on the day of testing, and to only have a small meal approximately one hour before testing. All foods ingested were monitored. The majority of participants had a single juice box or water, while two had a small yogurt. Those who were smokers or had a history of CVD were excluded from participation. All participants provided written informed consent in accord with the Clinical Research Ethics Board at the University of British Columbia, who approved this study.  Testing took place over two days, each separated by at least 48 hours, taking place between 10 am-12 pm. The two testing days were identical except for the administration of 10 mg midodrine on one of the days. In order to remove the learning effect, the order of days (i.e., whether baseline or midodrine trial went first) was randomized.  Protocol  A visual task was employed to activate the occipital lobe (while measuring PCAv), and a verbal fluency task to preferentially activate the left cerebral cortex (while measuring MCAv). The order of which was randomized. Occipital stimulation was achieved by reading from a magazine (Azevedo et al., 2007). Three minutes of baseline data, as well as a mock practice were recorded to ensure haemodynamic homeostasis. Following this, 10 cycles occurred, each consisting of 30 s eyes-closed followed by 30 s reading. An auditory stimulus provided notification of ?reading? 69  and ?eyes-closed? periods. The velocity response was averaged for all 10 trials for each participant.   The verbal fluency task was adapted from a previously described methodology (Moody et al., 2005). This test had the combined benefit of having been shown to preferentially activate the left MCA, and be a valid and ubiquitous marker of cognition (Tombaugh et al., 1999; Moody et al., 2005). A PowerPoint (Microsoft, Redmond, WA) slide show was used consisting of a series of 10 slides presenting a single letter for 30 seconds followed by the words ?Eyes Closed? for 30 seconds. The ten letters were M, H, O, I, W, B, T, F, A, S. The letters were separated into two stages (i.e., self-reported word totals versus verified word totals) separated by two minutes, the order of which was randomized.  The self?reported word total stage involved 7 cycles, where the participant recorded the number of words they generated on their right hand (those with limited hand motion were instructed to imagine counting on right hand) in response to letters M-T. Immediately after the eyes-closed period began, the participant was instructed to report how many words they had generated.  The verified total words section involved letters F, A, S, and consisted of the participants stating each word orally while a tester recorded each word. The two-stage approach was completed for two reasons: 1) to ensure accurate verbal fluency scores and 2) ensure accurate PETCO2 during cognition. The haemodynamic response to the 7 cycles of the M-T stage were averaged and used for analysis.  On the day of assessment using midodrine, a 10 mg oral dose was administered. The set-up for post-midodrine testing was initiated precisely 75 minutes after midodrine administration in order to test at the time of peak effect, which based on pharmacokinetics is approximately one hour after oral intake (Wright et al., 1998). A 10 mg dose was chosen as this has been shown to lead to the greatest improvements in orthostatic hypotension and symptoms of orthostatic intolerance, with no more side effects than a 5 mg dose (Wright et al., 1998). Each participant was tested in the seated position after 15 minutes of quiet rest.        70  Table 4. Individual demographic information for spinal cord injured participants. Participant No. (SCI) SCI Level DOI (weeks) AIS Grade Age (yr) Stature (cm) Mass (kg) Education Sex 1 T5 11 A 27 158.0 58.0 5 F 2 T5 324 A 43 165.5 66.0 4 F 3 C4 6.5 A 47 175.5 79.0 2 M 4 C5 144 A 36 180.5 70.0 5 M 5 C5 7 A 17 175.0 54.0 0 M 6 C5 7 B 42 175.0 71.0 0 M 7 C5 5 A 19 189.0 70.5 1 M 8 C5 10 B 28 178.0 94.0 0 M 9 C6 8 A 22 162.0 45.5 1 F 10 C7 11 A 26 177.0 74.0 0 M DOI, duration of injury; AIS grade, American Spinal Injury Association Impairment Scale, Education = years of post secondary.  Table 5. Selected cardiovascular variables for spinal cord injured and able-bodied participants. Variable  AB (n = 10) SCI (n = 10) P Value Age (years) Mass (kg) BMI (kg?m-2) Education (years post high-school)  Sex (# female) TBI (#) 31 ? 11 71 ? 15 24.5 ? 3.5 1.8 ? 2.1 3 0 29 ? 10 68 ? 14 22.6 ? 3.7 2.3 ? 1.6 3 1 0.44 0.57 0.12 0.38 N/A N/A AB, able-bodied controls; SCI, high level spinal cord injury; BMI, body mass index, TBI; traumatic brain injury; N/A, not applicable.      71  Data acquisition For each participant, brachial blood pressure was measured (BpTRU-BPM-100, Coquitlam; VSM Medical, Vancouver, BC, Canada) on the right brachial artery (before and after both the baseline and post-drug procedure). Measurement of blood pressure occurred three times, with the latter two being averaged. The following were sampled at 1000 Hz using an analog-to-digital converter (Powerlab/16SP ML 795; ADInstruments, CO Springs, CO) interfaced with data acquisition software on a laptop computer (LabChart 7 ADInstruments): beat-by-beat blood pressure via finger photoplethysmography (Finometer PRO, Finapres Medicine Systems, Amsterdam, Netherlands), electrocardiogram (ML 132; ADInstruments), end-tidal partial pressure of carbon dioxide (PETCO2; CO2 Analyzer Gold Edition-17515, Ventura, CA), velocity in the left middle cerebral artery (MCAv) or right posterior cerebral artery (PCAv; Doppler-Box, Compumedics DWL, Singen, Germany). These arteries were insonated using a 2Mhz probe mounted on the temporal bone and a fitted head-strap. As described in depth elsewhere (Willie et al., 2011c), the P1 segment of the PCA was insonated at depths between 60-70 mm, while the MCA was insonated from 45-55 mm. Arteries were confirmed using ipsilateral common carotid artery compression, ensuring an increase in PCA velocity and decrease in MCA velocity.   Cognitive assessment To evaluate cognitive assessment from verbal fluency outcome the following metrics were calculated: 1) average number of words recorded for letter F,A,S (VFAVE); 2) total number of words generated to letters F, A, and S (VFFAS) and 3) average number of self-reported words (VFSELF). Data analysis All signals were visually-inspected for artifacts or noise and corrected by linear interpolation. The CBFv signals were filtered by a low-pass filter with a cut-off frequency of 10 Hz (LabChart 7). All haemodynamic variables were sampled at a (heart) beat-by-beat basis (as detected by the ECG) while PETCO2 was sampled on a breath-by-breath basis (as detected by the peak of the first derivative of the PETCO2 waveform). All signals were then transferred to Excel (Microsoft) where custom designed cubic spline interpolation allowed for re-sampling at 5 Hz. Systolic and diastolic blood pressure (DBP), were then recorded as well as peak MCAv/PCAv and minimum MCAv/PCAv. From this mean arterial pressure as (2*DBP+SBP)/3 and mean CBF (CBFvmean) 72  as (2*CBFv minimum+CBFv maximum)/3 were calculated. This also allowed for the calculation of cerebrovascular conductance (CVC; CBFvmean/MAP). Cerebral pulsatility index was also calculated according to the measure of Gosling (Gosling and King 1974), and normalized cerebral pulsatility index for systemic pulsatility index to generate pulsatility ratio (Levine et al., 1994). The latter  is more directly associated with local cerebrovascular responses (Xu et al., 2011). All the above metrics were calculated five times per second, and all averaged trials (i.e., 10 trials for the visual task, and seven for the verbal task) for each participant.  In addition to looking at absolute changes during the activation period, 15-25 s of the eyes-closed period was used as baseline values to calculate percent changes. Average and peak values from the 30 second activation period are reported. The activation period was binned into 6 - 5 s long averages for analysis (Figures 13 and 14).    Statistical analysis Following confirmation of normal distribution, midodrine-free and midodrine values were compared using paired-sample t-tests while midodrine free and midodrine data were compared to the control group using independent-sample t-tests. Bivariate correlations were also performed. A P-value less than 0.05 was considered significant. Data are reported as mean ? standard deviation. Previous data indicated a required sample size of 8-10 per group when comparing neurovascular coupling responses between groups (Willie et al., 2011). To date, no study has examined the reliability of TCD-based NVC.  RESULTS Resting haemodynamic variables are presented in Table 6. Briefly, MAP, SBP and DBP increased with midodrine administration while HR was reduced. Midodrine increased blood pressure up to values found in AB controls (Table 6). Both MCA and PCA mean flow velocity were similar between AB and SCI at baseline       73   Table 6. Seated haemodynamic variables for spinal cord injured individuals with and without midodrine and able-bodied participants. Variable AB (n = 10) SCI (n = 10) SCImido (n = 10) SBP (mmHg) DBP (mmHg) MAP (mmHg) HR (beats?min-1) MCAvmean (cm?s-1) PCAvmean (cm?s-1) MCAcvc (cm?s-1?mmHg-1) PCAcvc (cm?s-1?mmHg-1) MCA-PR (au) PCA-PR (au) PETCO2 (mmHg) LFSBP (mmHg2) supine LFSBP (mmHg2) upright 119 ? 15 77 ? 11 92 ? 14 71 ? 12 61 ? 13 34 ? 13 0.68 ? 0.19 0.39 ? 0.15 1.62 ? 0.13 1.69 ? 0.61 35 ? 3.6 12.6 ? 12.3 24.3 ? 18.8 97 ? 11* 57 ? 10* 70 ? 10* 77 ? 20 58 ? 13 38 ? 8 0.85 ? 0.23 0.56 ? 0.13* 1.72 ? 0.41 1.45 ? 0.69 33 ? 3 1.8 ? 2.2* 2.7 ? 2 .4* 117 ? 15# 70 ? 10# 85 ? 10# 65 ? 12# 66 ? 10 36 ? 13 0.78 ? 0.15 0.43 ? 0.15# 1.82 ? 0.44 1.77 ? 0.52 34 ? 3 3.8 ? 4.6* 1.8 ? 2.3*? AB, able-bodied controls; SCI, high level spinal cord injury without midodrine; SCImido, high level spinal cord injury with midodrine; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial blood pressure; HR, heart rate; MCAvmean, middle cerebral artery mean blood flow velocity; PCAvmean, posterior cerebral artery mean blood flow velocity; MCAcvc, middle cerebral artery cerebral vascular conductance; PCAcvc, posterior cerebral artery cerebral vascular conductance; PR, pulsatility ratio; PETCO2, partial pressure of end-tidal oxygen; LFSBP; low frequency power of systolic blood pressure. *different from AB (p < 0.05); # different from SCI (p < 0.05), ?different from SCImido (p = 0.06).   Cerebral blood flow velocity response  Able-bodied versus spinal cord injured - visual task In AB both MAP and PETCO2 were unchanged during the visual task (Figure 13). In contrast, in AB, PCA mean flow velocity and PCAcvc were elevated (all p < 0.01). In addition, PCA-pulsatility ratio decreased (1.70 ? 0.66 vs. 1.58 ? 0.56 au; p = 0.006) during the visual task.  74   In those with SCI without midodrine, there were no changes in MAP or PETCO2 during the visual task. Also, although PCA mean flow velocity did not increase (38.2 ? 8.7 vs. 39 ? 8.0 cm?s-1; p = 0.51) in the SCI, PCA-pulsatility ratio was reduced (1.43 ? 0.66 vs. 1.35 ? 0.63, p = 0.003 au; Figure 13). No other calculated parameter changed during the visual task in SCI without midodrine.    As compared to AB, those with SCI reported similar average changes in MAP and PETCO2, but reduced PCA mean flow velocity and PCAcvc response (p = 0.009, p = 0.012) Peak changes in PCA mean flow velocity (p = 0.018) and cvc (p = 0.03) were also mitigated in SCI as compared to AB (Figure 13). Able-bodied versus spinal cord injured - verbal fluency task Able-bodied individuals had an increase in MCA mean flow velocity in response to cognition (60.5 ? 12 vs. 63.4 ? 12.7; p = 0.013), but no change in MAP or MCAcvc. Similar to SCI, PETCO2 and MCA-pulsatility ratio were reduced during the activation (34.5 ? 2.7 vs. 33.6 ? 3.4 mmHg; p = 0.016; 1.61 ? 0.41 vs. 1.53 ? 0.17 au; p = 0.04). In SCI without midodrine , MCA mean flow velocity increased over the VF task (58.2 ? 12.9 vs. 60 ? 12.7 cm?s-1; p = 0.004) in SCI. Average PETCO2 also decreased during cognition (33.3 ? 3.5 vs. 32.5 ? 3.7 mmHg; p = 0.016). No change in MAP, or MCAcvc occurred in response to cognition (Figure 14), although MCA- pulsatility ratio decreased (1.73 ? 0.43 vs. 1.67 ? 0.42 au; p = 0.003). No difference between SCI and AB occurred during the reading task (Figure 14). Effect of midodrine in SCI Verbal fluency task There was no change in the NVC response to verbal fluency in SCI with midodrine, although the peak change in MCA- pulsatility ratio tended to occur later with midodrine (9.5 ? 9.3 vs. 16.8 ? 9.9 s; p = 0.06). Visual task With midodrine in SCI, PCA mean flow velocity and PCAcvc increased during the visual task (36 ? 13 vs. 39 ? 15 cm?s-1; p = 0.001; 0.44 ? 0.16 vs. 0.48 ? 0.18 cm?s-1?mmHg-1; p = 0.001). PCA- pulsatility ratio was also reduced in SCI with midodrine during the visual task (1.83 ? 0.60 vs. 1.78 ? 0.59 au; p = 0.036; Figure 13). Also, PETCO2 decreased during the visual task (34.5 ? 2.7 vs. 33.8 ? 3.1 au; p = 0.032).  75   Changes in PCA mean flow velocity over the activation period were 70% greater with midodrine administration in SCI as compared to without midodrine (p = 0.03), while the PCA- pulsatility ratio response trended to be reduced (p = 0.088). The peak change in PCAv also trended to be greater with midodrine (p = 0.066). Changes in PETCO2 and MAP in response to visual activity were not different with midodrine as compared to SCI.  PCA mean flow velocity average response over activation period was lower with midodrine as compared to AB in response to visual activation (p = 0.06), and from 10-20 s of the activation period specifically (Figure 13). Cognitive Function Midodrine administration resulted in improved VFAVE (8.4 ? 3.6 vs. 9.5 ? 2.9 words; p = 0.04), and VFFAS (25.4 ? 11 vs. 28.6 ? 8.8 words; p = 0.045) but not VFSELF (p = 0.465) in the SCI group. Without midodrine, SCI had lower VFAVE (8.4 ? 3.6 vs. 11.3 ? 2.1 words; p = 0.023), and VFSELF (10.0 ? 3.8 vs. 13.0 ? 3.2 words; p = 0.038) but not VFFAS (p = 0.08) as compared to AB. All verbal fluency scores were highly correlated (VFFAS vs. VFAVE, r=0.94; VFAVE vs. VFSELF, r=0.88; VFFAS vs. VFSELF, r=0.81; all p < 0.001).  Those with the greatest increases in resting blood pressure, as a result of midodrine, reported the largest improvements in cognitive function (Figure 15). Similarly, those who had the largest reduction in resting MCA conductance reported the largest improvements in cognitive function (Figure 16). MCAv pulsatility ratio was consistently negatively associated with increased VFFAS (Figure 16, Table 7).             76  Table 7. Selected correlations (r) between haemodynamic parameters and VF scores. Variable (n = 30) VFFAS r VFAVE r Ave %?MCAvmean Peak %?MCAvmean Ave %?MAP 0.20 0.13 -0.06 0.18 0.09 -0.03 0-5 s %?MCA-PR 5-10 s %?MCA-PR 10-15 s %?MCA-PR 15-20 s %?MCA-PR 20-25 s %?MCA-PR 25-30 s %?MCA-PR -0.14 -0.30 -0.31 -0.37* -0.38* -0.43* -0.19 -0.31 -0.38* -0.45* -0.43* -0.47* MCAvmean; middle cerebral artery mean blood flow velocity; MAP, mean arterial pressure; MCAv, middle cerebral artery blood flow velocity; PR, pulsatility ratio. *P ? 0.05.  77      78  Figure 13. Cerebrovascular response to visual-spatial task.   Mean arterial pressure (MAP), posterior cerebral artery (PCA) mean flow velocity (vmean), and cerebrovascular conductance (cvc), and partial pressure end-tidal carbon dioxide (PETCO2) response during verbal-fluency testing. Thin black line represents spinal cord injury before midodrine (SCI), thick black line represents spinal cord injury after midodrine (SCImido), grey line represents able-bodied individuals. The black filled rectangle represents the 30 s activation period, whereas the 6 smaller empty rectangles represent 5 s bins from the activation period. MAP and PETCO2 did not change significantly in response to the visual task. PCAvmean and PCAcvc did not significantly increase in SCI. PCAvmean and PCAcvc increased significantly during activation in SCI after midodrine, however this response was still reduced as compared to the significant response in AB.          79      80  Figure 14. Cerebrovascular response to verbal fluency task.   Mean arterial pressure (MAP), middle cerebral artery (MCA) mean flow velocity (vmean), and cerebrovascular conductance (cvc),  and partial pressure end-tidal carbon dioxide (PETCO2) response during verbal-fluency testing. Thin black line represents spinal cord injury before midodrine (SCI), thick black line represents spinal cord injury after midodrine (SCImido), grey line represents able-bodied individuals. The black filled rectangle represents the 30 s activation period, whereas the 6 smaller empty rectangles represent 5 s bins from the activation period. Although MCAv, MCAcvc and PETCO2 significantly changes during the activation period, there were no significant differences between AB, SCI, or SCImido with respect to their responses.   81      82  Figure 15. The relationship between changes systemic and cerebral parameters and markers and changes in verbal fluency (VF) scores in those with SCI.  Individuals with larger increases in mean arterial pressure (MAP) also had larger increases in VF. Also, those with greater reductions in resting conductance in the middle cerebral artery (MCAcvc) had larger increases in VF.              83      84  Figure 16. The relationship between middle cerebral artery pulsatility ratio (MCA-PR) and verbal fluency scores. Small diamonds represent spinal cord injury before midodrine (SCI), large black diamonds represent spinal cord injury after midodrine (SCImido), grey diamonds represents able-bodied individuals. Notice that larger and later decreases in pulsatilty ratio are associated with increased cognitive function. 85  DISCUSSION  This study examined the NVC response in the middle and posterior cerebral arteries in SCI with and without midodrine, as well as in matched AB control group. The primary findings are: 1) NVC of the PCA is severely impaired in SCI as compared to AB, but is improved by increasing blood pressure up to AB levels; 2) MCA NVC is similar in SCI to AB; 3) One aspect of cognition, verbal fluency, improves after increasing blood pressure in SCI; 4) Larger decreases in resistance during NVC is associated with improved verbal fluency.   This is the first study to examine CBFv of the PCA in those with SCI. In a seated position, although PCAv was maintained, PCAcvc was elevated. An increase in CVC indicates increased conductance at the pial level, in order to maintain CBF in the posterior region (Ursino et al., 1998). Reductions in blood flow in the posterior region may cause an interruption of the blood supply to the medulla oblongata, which contains autonomic control centres, and discrete regions responsible for consciousness (Shin et al., 1999). An increase in PCAcvc to preferentially maintain perfusion of the cardiovascular and respiratory centres has been highlighted in response to head-up tilt in AB (Tatu et al., 1996; Sato et al., 2012), and an exaggerated adaptation of this response may explain the widely reported ?remarkable tolerance to orthostatic hypotension? in the SCI population (Mathias and Bannister 2002).   With regard to the PCA NVC response, the blunted effect in SCI is likely secondary to reduced cerebral perfusion pressure. In support of this contention, increasing blood pressure to AB levels resulted in a normalization of PCAcvc and improvement in the PCA NVC response (Figure 13). The NVC response may have a minimum threshold for cerebral perfusion pressure, below which NVC does not occur. A similar minimum perfusion threshold relationship was demonstrated in an early animal study showing the hyperemic response to increased PaCO2 is reduced and eventually absent as blood pressure decreases (Harper and Glass 1965). More recently in humans, attenuated cerebrovascular reactivity to hypercapnia during pharmacologically-induced hypotension has been shown in two studies using ganglionic blockade (Przybylowski et al., 2003) and sympathetic blockade (Ainslie et al., 2012). This study extends these findings, showing that the metabolic response (i.e., NVC) is also largely dependent on the extent of perfusion pressure.   In this study, increasing blood pressure in SCI only partially improved the NVC-PCA response however, suggesting other CBF regulatory mechanisms may be dysfunctional in SCI. 86  Attenuated NVC, even after midodrine, could be due to a variety of factors outside of low blood pressure, including reduced nitric oxide availability (Piknova et al., 2011), glucose intolerance (Myers et al., 2007; Gandhi et al., 2010) , and dyslipidemia (Vichiansiri et al., 2012).   This study clearly showed that NVC of the MCA is preserved to AB levels in those with SCI. These findings appear to be in opposition to that reported by Jegede et al. (2010), however differences may be explained by a combination of a) a dissimilar cognitive task (i.e., verbal fluency vs. Stroop test), b) a more stringent procedure (i.e., seven vs. three cycles averaged), or c) shorter activation period averaged (30 vs. 45 seconds) (Jegede et al., 2010). It is unclear from the Jegede et al. (2010) study if an eyes-closed rest period preceded cerebral activation, or if verbal responses to cognition influenced PETCO2, as the latter was not measured. The current data show that the MCAv response to verbal fluency appears to be unaffected by varying blood pressure in SCI. This study reported similar resting MCA mean flow velocity, suggesting preserved static cerebral autoregulation in SCI (a premise that has been widely supported) (Nanda et al., 1974; Nanda et al., 1976; Handrakis et al., 2009; Phillips et al., 2013a). The present study also showed similar MCAcvc in SCI, suggesting that resting cerebrovascular tone was maintained. Preservation of resting cerebrovascular tone may permit a suitable CBF response to metabolic stimulation (Novak and Hajjar 2010). As NVC of the MCA was well maintained at baseline, increasing perfusion pressure through midodrine exerted little effect. When Duschek and colleagues treated 25 chronic hypotensives (90% female) with an approximately 63% greater dose of midodrine (i.e., 0.4mg?kg-1) than the present study, they reported increased seated MCAv, and a greater MCAv hyperemic response during preparation to cued-reaction time. These authors however, did not compare to a normotensive control group (Duschek et al., 2007). Following this, it is unclear if seated MCAv or MCA NVC is lower in normotensive individuals and improved in chronic hypotensives with midodrine, or if the exaggerated effect on MCAv is due to the high dosage employed. Such higher doses resulted in hypertension for 1-3 hours and related adverse reactions (e.g., persistent supine hypetension, as well as marked paresthesiae and pruritus) (Wright et al., 1998). The current study provides evidence that resting MCAv and NVC of the MCA is similar in those with SCI as compared to AB, and is not altered by increasing blood pressure.    The hypotensive SCI group in the present study reported significantly reduced cognitive function at baseline, which is consistent with the literature (Davidoff et al., 1992; Dowler et al., 87  1997; Duschek et al., 2007). This study also found improved cognition after increasing blood pressure with 10 mg midodrine. Further, the present data shows that hypotensive individuals with SCI who have the largest improvements in blood pressure also have the largest improvements in cognition (Figure 15). Several cross sectional studies have reported reduced cognitive function in hypotensive AB individuals (Duschek and Schandry 2007), and one recent study showed that hypotensive SCI had reduced memory and a trend for slowed attention and processing speed as compared to normotensive SCI (Jegede et al., 2010).  This study is the first prospective work to show that acute increases in blood pressure can improve aspects of cognitive function in those with SCI; thereby establishing a direct relationship similar to that shown in AB (Duschek et al., 2007).   This study reported a significant relationship between change in VF scores with both average change in MCA-pulsatility ratio and time of peak change in MCA-pulsatility ratio, as well as from 10-30 seconds of cerebral activation. This relationship suggests that those who reduce pulsatility ratio the most, especially in the latter part of the activation period generate more words to the verbal fluency test (Woodcock et al., 1972). Although the cerebrovascular resistance results do not match identically, the present study is in general agreement with that of Wecht et al. (2012) who showed that decreases in MCA resistance were associated with increased cognitive function in AB (i.e., Stroop test) (Wecht et al., 2012). Together, these studies indicate that altered cerebrovascular resistance is a link between CBF and cognitive function.  Clinical implications  The finding that NVC is reduced in those with SCI links cognitive decline to reduced cerebrovascular reserve. Improving blood pressure and cerebrovascular reserve through midodrine administration served to improve orthostatic hypotension, enhance NVC, and consequently improve cognitive function. This study provides direct clinical evidence that 10 mg midodrine administration is an efficacious pharmaceutical therapy for the cerebrovascular dysfunction and reduced cognitive function after SCI.   Limitations As is the case when using transcranial Doppler for the assessment of cerebral blood flow, an assumption is made that arterial diameter is maintained constant in order to accurately reflex changes in flow. Indeed, two studies have shown that administration of  similar alpha-1 agonists 88  have not led to reductions in intracranial vessel diameter (Greenfield Jr and Tindall 1968; Johnston et al., 1994) , allowing the present study to fairly assume maintained MCA and PCA diameter with midodrine. With respect to vessels downstream to the MCA and PCA, the blood-brain barrier usually prevents intravascular catecholamines from binding to adrenergic receptors located in cerebral arterioles (MacKenzie et al., 1976).   This was not a blinded, placebo-controlled trial. Although subjects were randomized with respect to the order they had NVC evaluated (in order to mitigate any learning effect of VF tasks), participants were aware of the medications being provided. However, the participants were blinded to the purpose of the study.   It should be noted that both chronic (i.e., injury duration >1 year) participants from the current study have been previously examined for a prior investigation. Both these individuals were highly physically-active as compared to their chronic SCI counterparts (Phillips et al., 2012b). This suggests that the chronic participants in the present study were likely homogenous regarding physical activity levels with the acute participants, which may mitigate the physical activity-mediated cardiovascular decline that occurs over the first two-to-three years after SCI (Bauman and Spungen 2000; Krassioukov and Claydon 2006).  CONCLUSIONS This study shows that NVC, specifically that of the PCA, is impaired in those with SCI but can be partially normalized following blood pressure corrections with 10 mg of midodrine administration. This dose also resulted in an improvement in cognitive function, which was directly related to improvements in blood pressure. Finally, a relationship between cerebrovascular resistance and cognition has been shown that has not been previously reported. This study highlights the inverted-U shape describing the relationship between blood pressure and cerebral function; where in addition to hypertension, low blood pressure leads to cerebrovascular and cognitive decline.  Future studies should examine other CBF regulatory metrics in low blood pressure SCI before and after acutely increasing blood pressure in order to elucidate whether dysfunction is due to reduced perfusion pressure or cerebrovascular dysfunction. Furthermore, the chronic effects of low blood pressure on cerebral function and sophisticated measures of cognition in those with SCI need to be explored. Greater clinical focus on low blood pressure managements needs to occur in the SCI population.    89  CHAPTER FOUR ? CEREBRAL BLOOD FLOW CONTROL IN HIGH LEVEL SPINAL CORD INJURY: THE EFFECT OF MIDODRINE HYDROCHLORIDE INTRODUCTION Spinal cord injury (SCI) is a devastating chronic condition that results in not only motor and sensory deficits but also autonomic dysfunction (Krassioukov and Claydon 2006). As a consequence of decentralization of sympathetic fibres, those with SCI suffer from low resting blood pressure and episodes of severe hypotension when moving upright (orthostatic hypotension) (Claydon et al., 2006). Orthostatic hypotension has been shown to be an important independent risk factor for the development of stroke in able-bodied (AB) individuals (Eigenbrodt et al., 2000), which is 2-3 times more likely in those with SCI (Wu et al., 2012).   A fundamental property of cerebral vessels is their capacity to locally regulate cerebral blood flow (CBF). Effective autoregulation of CBF requires a integrated response from myogenic, neurogenic, metabolic, and systemic factors (Willie et al., 2011b). Brief disruptions of CBF caused by impaired blood pressure control during orthostatic hypotension may cause irreversible neuronal cell death (Endres et al., 2003; Aries et al., 2010). Consequently, poor blood pressure control in those with SCI makes appropriate regulation of CBF crucial for preventing stroke.  A number of studies have examined static cerebral autoregulation (CA; i.e., the ability to maintain CBF during a range of steady-state [0 Hz] blood pressures) in those with SCI and there is consistent evidence that static CA is preserved in this population (Phillips et al., 2013a). In contrast, the dynamic relationship between blood pressure and CBF has only been examined in two studies (Wilson et al., 2010; Sahota et al., 2012). Both static and dynamic CA are thought to be regulated by a combination of myogenic, metabolic and neurogenic control mechanisms (for review see: (Paulson et al., 1990)); however, static and dynamic CA may rely on relatively different influences from the various regulatory factors (Dawson et al., 2000). For example, it has been suggested that neural control over cerebral vascular tissue more heavily influences dynamic as opposed to static CA (Dawson et al., 2000), while endothelial dependence has been shown to not relate to dynamic CA (Zhang et al., 2004). The two studies to report on dynamic CA in SCI only reported spontaneous analyses of cerebral pressure-flow relationships, which may not reflect how the cerebrovasculature responds to rapid perturbations in blood pressure 90  (Panerai 2008; Wilson et al., 2010; Sahota et al., 2012). In addition, the relationship between CBF and blood pressure in a haemodynamically stable, closed-loop situation, is unlikely to simulate the drastic reductions in cerebral perfusion pressure that occur during orthostatic challenges (Tzeng et al., 2012). In both these studies (Wilson et al., 2010; Sahota et al., 2012), only metrics from the middle cerebral artery (MCA) were reported as opposed to arteries of the vertebrobasilar system; arteries that may be more related to orthostatic tolerance as it perfuses the medulla oblongata which contains associated autonomic control centers as well as discrete regions responsible for consciousness (Shin et al., 1999). Furthermore, several studies have recently shown the internal carotid/middle cerebral arterial region is differentially sensitive to orthostatic challenges compared to the vertebral/posterior cerebral artery (PCA) region (Azevedo et al., 2011; Willie et al., 2011c; Sato et al., 2012; Willie et al., 2012). Our understanding of CBF regulation in those with SCI would be enhanced with 1) a regional evaluation of cerebral pressure-flow relationships using both the MCA and PCA; 2) perturbing blood pressure using orthostasis to generate a relevant dynamic stimulus; and 3) an examination of some potential mechanisms that may regulate CBF.   Midodrine, an alpha-1 adrenoreceptor agonist, is a first line of defense to improve orthostatic tolerance in those with acute SCI (Krassioukov et al., 2009). A 10 mg dose of midodrine has been shown to mitigate orthostatic hypotension in SCI, however the influence on CBF is unclear (Wecht et al., 2010; Wecht et al., 2011). One study showed no change in supine CBF velocity of the MCA (MCAv) after midodrine in SCI; however,  reductions in MCAv during an orthostatic challenge were attenuated (Wecht et al., 2010). In a follow-up study from this research group, MCAv was increased at baseline, and was not changed during orthostatic challenge (Wecht et al., 2011). Both of these studies, however, examined the relationship between MCAv in a homeostatic/steady-state situation (i.e., 0 Hz), and these findings are likely to be clarified and enhanced by an evaluation of the dynamic relationship between blood pressure and CBF both at rest and during orthostatic stress.   This study had four main objectives. The first objective was to compare static and dynamic regional CBF responses to orthostasis in SCI and AB. The second objective was to evaluate how midodrine influences the static and dynamic regional CBF responses to orthostasis in SCI. The third objective was to evaluate orthostatic tolerance in SCI before and after midodrine. The final objective was to evaluate the relationship between dynamic systemic blood 91  pressure regulation and the cerebral pressure-flow relationship in SCI.  It was hypothesized that 1) static and dynamic CA in SCI would be preserved in both the MCA and PCA, 2) midodrine would not influence static CA, but would partially normalize dynamic cerebral pressure-flow metrics to values reported in AB, 3) orthostatic tolerance would be improved after midodrine, and 4) dynamic cerebral pressure-flow relationships would be uncoupled from baroreflex function in SCI.  METHODS Ten individuals with SCI participated in this study (C4-T5), AIS A and B; Table 4). Eight participants were <1 year post injury while 2 were >1 year post injury (Table 4). All participants were referred for autonomic testing due to clinical observations. The control group was composed of 10 age- and sex-matched individuals (AB). All testing took place at GF Strong Rehabilitation Centre, Vancouver, Canada. Participants with SCI were approached by a research coordinator after being notified from an attending physician that the patient met study inclusion criteria. Control participants were recruited from posters placed around the University of British Columbia campus. Participant characteristics are presented in Tables 4 and 5.    All participants were instructed to abstain from exercise and alcohol for 24 h before testing. No caffeine was permitted the day of testing. Additionally, participants abstained from all other medications on the day of testing, and to only have a small meal (e.g., a small yogurt) approximately one hour before testing. Those who were smokers or had any history of cardiovascular disease were excluded from participation. All participants provided written informed consent in accord with the Clinical Research Ethics Board at the University of British Columbia, who approved this study.  Participants were tested over two days. Each testing day was separated by at least 48 hours and took place between 10 am-12 pm. The testing days were identical except for the administration of 10 mg midodrine on one of the days. The order of days (i.e., whether baseline or midodrine trial went first) was randomized. On the midodrine day, a 10 mg oral dose was administered. Midodrine is converted to the pharmacologically-active metabolite desglymidodrine, which has a half-life of approximately three hours (Grobecker and Kees 1993). The set-up for post-midodrine testing was initiated precisely 45 minutes after midodrine administration (Figure 17A) in order to conduct physiological assessments at the time of peak response, which is approximately one hour after ingestion (Wright et al., 1998). A 10 mg dose 92  was chosen as this has been shown to elicit the greatest improvements in orthostatic hypotension and symptoms of orthostatic intolerance, with no additional side-effects than a 5 mg dose (Wright et al., 1998). Participants rested quietly in the supine position for 15 minutes prior to initiation of testing.  Protocol Participants were transferred to the tilt table and rested supine for 15 min, while baseline haemodynamic data were recorded (Figure 17b). Participants were progressively tilted from supine to 30?, 45?, and 60?. Five minutes of continuous data were collected at each tilt level and participants were instructed to keep eyes open throughout the test. Transition between tilt levels was achieved in less than 5 seconds. Also, participants were asked about the presence of presyncopal symptoms (i.e., dizziness, light-headedness, nausea) at the beginning, middle and end of each stage. Participants rated their symptoms between 1-10 (with 5 being slightly dizzy and 10 being about to lose consciousness or vomit) and were also instructed to notify the testing team if at any point if their rank became greater than 7/10. The stage and time (s) at which each participant withdrew or was withdrawn from tilt were recorded. Orthostatic tolerance index (OTi) was calculated by the formula [OTi] = [Final tilt degree (?)] x [time (s) the last stage was tolerated]. 93  Tilt Test45 min15 min10 mg midodrine30 minTilt TestBreak/Snack30 minFACULTY OF MEDICINE5 min5 min5 minSupine30?45?60?AB Figure 17. Illustration of study procedures. A) Order of testing; B) Tilt-testing protocol.  Data acquisition For each participant, brachial blood pressure was measured (BpTRU-BPM-100, Coquitlam; VSM Medical, Vancouver, BC, Canada) on the right arm at least two times at each stage of tilt. Beat-by-beat blood pressure via finger photoplethysmography (Finometer PRO, Finapres Medicine Systems, Amsterdam, Netherlands), heart rate (electrocardiogram ML 132; ADInstruments), end-tidal carbon dioxide partial pressure (PETCO2; CO2 Analyzer Gold Edition-17515, Ventura, CA), left MCAv and right PCA blood flow velocity (PCAv; Doppler-Box, Compumedics DWL, Singen, Germany) was measured. Using two - 2 MHz probes mounted bilaterally on the temporal bones using a fitted head-strap, the P1 segment of the PCA was insonated at depths between 60-70 mm, while the MCA was insonated at 45-55 mm. Arteries were confirmed using ipsilateral common carotid artery compression, ensuring an increase in PCA velocity and decrease in MCA velocity. All data were collected at 1000 Hz using an analog-to-digital converter (PowerLab/16SP ML 795; ADInstruments, CO Springs, CO) 94  interfaced with data acquisition software (LabChart 7 ADInstruments) on a laptop computer. Finger photoplethysmograph signal was corrected to the brachial level. Data analysis For evaluation of time-domain dynamic pressure-flow relationships during a clinically-relevant orthostatic stimulus acute changes in blood pressure and CBFv for 15 s prior and 60 s after tilt were evaluated. The blood pressure, MCAv, and PCAv responses occurring after each tilt stage were averaged for each participant. Not every tilt resulted in large decreases in blood pressure in all participants. As such, in order to be included in the average response, the tilt had to result in a decrease of MAP >15 mmHg over the first 30 seconds. All signals were visually inspected for artifacts or noise and corrected by linear interpolation. The CBFv signals were filtered by a low-pass filter with a cut-off frequency of 10 Hz (LabChart 7). All haemodynamic variables were sampled at a heart beat-by-beat basis (as detected by the ECG) while PETCO2 was sampled on a breath-by-breath basis (as detected by the peak of the first derivative of the PETCO2 waveform). All signals were then transferred to Excel (Microsoft, Redmond, WA) where custom designed cubic spline interpolation allowed for re-sampling at 5 Hz. After re-sampling, a mean response from the various orthostatic trials was generated for 60 s after tilt for each participant. Systolic and diastolic blood pressure (DBP), as well as peak CBFv and minimum CBFv were then extracted. From these values, mean arterial pressure as (2*DBP+systolic blood pressure)/3 and mean CBF (CBFvmean) as (2*CBFv minimum+CBFv maximum)/3 were calculated. This also allowed for the calculation of cerebrovascular conductance (CVC) as CBFvmean/mean arterial pressure. Average and peak changes in CBFv and conductance of the MCA and PCA were recorded over the first 60 s of tilt. The 60 s was separated in to two 30 s long bins. The 30 s period immediately after tilt was also divided into six - five second long averages for analysis. It was not possible to insonate the PCA in one SCI participant. As such, steady-state and perturbed dynamic pressure-flow metrics for the PCA are limited to nine SCI individuals.  Transfer function analysis Three minutes of steady-state blood pressure and CBFv were extracted from each stage of tilt after two minutes of acclimation to the new tilt stage. Using transfer function analysis, the dynamic relationships between spontaneous oscillations in blood pressure and MCAv/PCAv were evaluated. The blood pressure and blood flow velocity waveforms were simultaneously sampled and analyzed at 200 Hz, while beat-to-beat MAP and mean CBFv were obtained within 95  each cardiac cycle. The beat-to-beat signals were then linearly interpolated and re-sampled at 2 Hz for spectral analysis as described elsewhere (Wilson et al., 2010). Briefly, the transfer function H(f) between the signals was calculated by H(f) = Sxy(f)/Sxx(f) where Sxy(f) denotes the cross-spectrum between the two signals, and Sxx(f) is the autospectrum of the input signal (i.e., blood pressure). The relationship between blood pressure and CBFv with regard to amplitude and time were denoted as transfer function gain and phase shifts in the low (0.07 ? 0.20 Hz) frequency. The low frequency range was chosen as it is the most established range for examining transfer function analysis in humans (Bernardi et al., 1994). The fraction of output power that can be linearly related to input power at each frequency is denoted by the coherence function. The coherence function is similar to a correlation coefficient in that values approximating 0 may indicate a nonlinear relationship, severe extraneous noise in the signals or no relationship between signals, whereas a coherence value approaching one reflects a strong influence of blood pressure to CBFv. Gain describes the magnitude of change in blood pressure that is reflected by CBFv. A reduction in phase suggests that blood pressure is driving CBFv, or that changes in CBFv rapidly occur after changes in blood pressure. According to previous applications of this methodology, an intact pressure-flow relationship would be associated with reductions in gain, and increases in phase. Conversely, the absence of an intact pressure-flow response would manifest as increases in gain, with reductions in phase. Transfer function metrics were conservatively analyzed using only band points with coherence greater than 0.5 au as per convention (Pinna et al., 2002; Tzeng et al., 2012). Finally, to provide insight into systemic regulation of perfusion pressure, spontaneous baroreflex sensitivity in the low frequency (0.04 ? 0.15 Hz) -index was also calculated (Tzeng et al., 2010). Also, transfer function metrics for the PCA were not possible to calculate in an additional SCI file and two SCImido files.   Statistical analysis Following confirmation of normal distribution, SCI and AB were compared using independent-samples t-tests. Also, paired-sample t-tests were used to compared SCI with and without administration of midodrine. Bivariate correlations were also performed. A P-value less than 0.05 was considered significant unless otherwise reported. Data are reported as mean ? standard deviation. Previous data indicated a required sample size of 4-10 per group when comparing transfer function analysis between SCI and AB (Wilson et al., 2010). Spontaneous TFA metrics 96  of dynamic pressure-flow relationships have been shown to have moderate to strong repeatability (i.e., intraclass correlation of 0.46-0.47) (Wilson et al., 20010).  RESULTS  Able-bodied versus spinal cord injured - homeostatic response to progressive tilt Steady-state changes in systemic and cerebral haemodynamics are presented in Figure 18. Briefly, although blood pressure was significantly lower in SCI during both supine and upright positions, MCAv and PCAv were similar indicating effective static CA. Also, blood pressure increased significantly in AB, but decreased in SCI in response to tilt. Both MCAv and PCAv decreased similarly in SCI as compared to AB (Figure 18). Conductance in the MCA and PCA decreased significantly in response to tilt in AB, but was maintained at supine levels in SCI. Heart rate and PETCO2 responded similarly in both SCI and AB.  Acute haemodynamic responses to orthostatic challenge The acute response to tilt is presented in Figure 19. Those with SCI had significantly lower MAP during the first 60s of tilt. MCAv was not different between AB and SCI. PCAv was reduced in response to orthostatic challenge, but this was similar between AB and SCI. Conductance of the MCA was significantly higher in SCI consistently throughout the 60 s after tilt. In SCI, MCA conductance was maintained at or above baseline levels in the 60 s after tilt, while in AB it consistently decreased over the time period. PCA conductance was also similar between SCI and AB throughout the first 30 s after tilt; however it was maintained in SCI and decreased in AB from 30-60 s after tilt. The heart rate response to tilt was similar in SCI as compared to AB. PETCO2 decreased significantly to a greater extent in AB as compared to SCI over the first 30 s but was similar from 30-60 s.   Transfer function metrics of dynamic cerebral autoregulation Transfer function metrics of the dynamic cerebral pressure-flow relationship in the MCA and PCA are presented in Table 8. Power spectra for MAP and MCAv were lower in SCI (p < 0.05). Also, upright coherence for the blood pressure-MCAv and blood pressure PCAv were lower in SCI as compared to AB (both p < 0.07).  Gain and phase were not different for blood pressure-MCAv or blood pressure PCAv between SCI and AB occurred (Table 8).   97  Spinal cord injured individuals with and without midodrine Homeostatic response to progressive tilt Steady-state changes in SCI in response to midodrine are presented in Figure 18. Supine and upright blood pressures were higher in SCI following midodrine; however, heart rate, PETCO2, MCAv and PCAv values were unaltered.   Acute haemodynamic responses to tilt Midodrine did not influence the MAP response to tilt from 0-30 s. In contrast, from 0-60 s the decline in MAP was mitigated with midodrine (Figure 19). MCAv was similar with and without midodrine. Average and peak declines in PCAv from 30-60 s were less following midodrine administration. Conductance of the MCA and PCA as well as PETCO2 were not influenced following midodrine administration (Figure 19).  Transfer function metrics of cerebral pressure-flow relationships  Midodrine did not substantially alter the dynamic cerebral pressure-flow relationships in the MCA and PCA (Table 8). High frequency MCA phase was significantly reduced in the upright position with midodrine in SCI, while PCA metrics were unchanged. Orthostatic tolerance Symptoms of presyncope were significantly improved at 45? and 60? tilt following midodrine (Figure 20). Those with SCI had an average 59% improvement in OTi (p = 0.003) during the midodrine tilt trial.  There was no relationships between OTi with any transfer function metrics or static haemodynamic variables, (i.e., blood pressure, CBF; mean values, absolute changes from supine, percent changes from supine) related to OTi.      Transfer function metrics and blood pressure In AB individuals only, there was a significant relationship between ?LF and MCA low frequency gain (r = 0.75, p = 0.01) and normalized gain (r = 0.68, p = 0.03). In those with SCI, ?LF was significantly higher than AB (30.0 ? 15.6 vs. 15.7 ? 10.7 ms?mmHg-1; p = 0.03). Midodrine did not influence ?LF in the supine (p = 0.47) or upright position (p = 0.60). In the SCI group, there was a positive relationship between upright mean blood pressure with the MCA low frequency gain at the last stage of fully tolerated tilts (r = 0.64, p = 0.049). Vasomotor control and transfer function metrics 98  In SCI individuals, there was a negative relationship between upright low frequency power of systolic blood pressure with MCA low frequency normalized gain (r = -0.67, p = 0.032), as well as a positive relationship between upright low frequency power of systolic blood pressure and MCA low-frequency phase (r = 0.65, p = 0.005). There were no relationships evident in the AB group.  99          Figure 18. Steady-state haemodynamic responses to tilt. Values are expressed as mean ?SE change in mean arterial pressure (MAP), systolic blood pressure (SBP), heart rate (HR), partial pressure of end-tidal carbon dioxide (PETCO2),  mean cerebral blood flow velocity of the middle cerebral artery (MCAvmean) and posterior cerebral artery (PCAvmean), cerebrovascular conductance of the middle cerebral artery (MCAcvc) and posterior cerebral artery (PCAcvc), pulsatility ratio of the middle cerebral artery (MCA PR) and posterior cerebral artery (PCA PR), resistance-area product of the middle cerebral artery (MCA RAP) and posterior cerebral artery (PCA RAP) in supine and upright position in spinal cord injury  (SCI; thick black line) and SCI after midodrine (SCImido; thick black line), as well as able-bodied individuals (AB; thick grey line). Markers denote significant differences between groups (p < 0.07). a, SCI supine vs. SCI upright; b, SCImido supine vs. SCImido upright;  c, AB supine vs. AB upright; *, SCI vs. SCImido;  ?, AB vs. SCI; #, AB vs. SCImido. 100       101    Figure 19. Time-domain dynamic responses to tilt. Values are expressed as mean ?SE change in mean arterial pressure (MAP), heart rate (HR), partial pressure of end-tidal carbon dioxide (PETCO2),  mean cerebral blood flow velocity of the middle cerebral artery (MCAvmean) and posterior cerebral artery (PCAvmean), cerebrovascular conductance of the middle cerebral artery (MCAcvc) and posterior cerebral artery (PCAcvc), pulsatility ratio of the middle cerebral artery (MCA PR) and posterior cerebral artery (PCA PR), resistance-area product of the middle cerebral artery (MCA RAP) and posterior cerebral artery (PCA RAP) for 15 s before and 60 s after tilt in (SCI; thin black line) and SCI after midodrine (SCImido; thick black line), as well as able-bodied individuals (AB; thick grey line). Markers denote significant differences between groups (p < 0.07). *, SCI vs. SCImido;  ?, AB vs. SCI; #, AB vs. SCImido. Note: for statistics the 60 s period was binned into two 30 seconds periods, and the initial 30 s was binned in six periods of five s duration. For clarity, on the largest SE is presented at the point of occurrence.        102  Table 8. Transfer function metrics of mean arterial pressure and cerebral blood flow velocity in the supine and upright position.  SCI SCImido AB  Supine Max Tilt Supine Max Tilt Supine Max Tilt MAP power spectrum (mmHg2?Hz-1)  MCA power spectrum (cm2s-2?Hz-1) MCA coherence (au)  MCA gain (cms-1?mmHg-1) MCA n-gain (%?mmHg-1) MCA phase (radians)  PCA power spectrum (cm2s-2?Hz-1) PCA coherence (au)  PCA gain (cm-1s?mmHg-1) PCA n-gain (%?mmHg-1) PCA phase (Hz?radians) 0.88 ? 0.61*  2.51 ? 1.60 0.64 ? 0.17 1.76 ? 0.96 2.23 ? 0.91 0.46 ? 0.30  1.01 ? 0.42 0.64 ? 0.15 1.01 ? 0.51 2.17 ? 1.19 0.40 ? 0.27 0.89 ? 0.57*  0.98 ? 0.83* 0.55 ? 0.19* 1.16 ? 0.82 3.93 ? 4.36 0.47 ? 0.39  0.87 ? 0.46 0.59 ? 0.17* 1.00 ? 0.51 2.57 ? 1.56 0.55 ? 0.50 0.64 ? 0.62  1.66 ? 1.13 0.54 ? 0.21 1.87 ? 0.68 2.43 ? 0.88 0.74 ? 0.46  1.20 ? 0.76 0.51 ? 0.21 1.32 ? 0.75 2.94 ? 1.27 0.48 ? 0.19 0.79 ? 0.98  1.28 ? 0.97 0.58 ? 0.21 1.40 ? 0.68? 2.22 ? 1.28 0.36 ? 0.28?  0.68 ? 0.66 0.52 ? 0.20 0.84 ? 0.49? 1.94 ? 1.02? 0.26 ? 0.38 2.34 ? 1.77  3.45 ? 3.28 0.65 ? 0.17 1.34 ? 0.63 1.66 ? 0.53 0.67 ? 0.24  1.27 ? 0.97 0.55 ? 0.14 0.87 ? 0.43 1.98 ? 0.80 0.78 ? 0.34 10.65 ? 10.94?  9.26 ? 9.51 0.84 ? 0. 09? 1.16 ? 0.60 1.71 ? 0.59 0.50 ? 0.13?  3.87 ? 5.36 0.76 ? 0.17? 0.68 ? 0.37? 1.73 ? 0.66? 0.50 ? 0.19? *; significantly different from AB (p < 0 .07), ?; significant different from in SCI after midodrine (p < 0.07), ?; significantly different from supine (p < 0.07). Significant relationships are highlighted with bold text.       103        Figure 20. Effect of midodrine on orthostatic tolerance. Orthostatic tolerance in spinal cord injured individuals (SCI) and SCI with midodrine (SCImido). Left panel: self-reported symptoms of presyncope (0-10 scale), au; arbitrary units. Note: people who developed presyncope were given a 10/10 for symptom severity at subsequent stages. Right panel: Calculated orthostatic tolerance index (OTi). *; significantly different pre-post midodrine, p < 0.01. Data presented as mean ?standard error.  104  DISCUSSION This is the first study to examine systemic and regional cerebral haemodynamics in those with SCI, as well the influence of orthostatic tolerance and midodrine administration. The main findings were: 1) the steady-state and dynamic CBF response to tilt is similar in SCI compared to AB, indicating effective CA; 2) although midodrine does not influence the steady-state CBF response to tilt, it improved time-domain dynamic cerebral pressure-flow relationships; 3) midodrine resulted in an improved orthostatic tolerance in SCI, changes that were generally unrelated to systemic or cerebrovascular metrics; and 4) AB showed the expected inverse relationship between dynamic pressure-flow relationships and baroreflex sensitivity, however those with SCI had an uncoupling of this association.  Able bodied vs. spinal cord injured  Static cerebral autoregulation in able-bodied and spinal cord injured individuals The haemodynamic determinants of average blood flow (Q) through an organ is typically understood through Poiseuille?s law, or the haemodynamic equivalent of Ohm?s law, known as Darcy?s law: Q= ?P/R.  In the context of the brain, ?P is the cerebral perfusion pressure calculated from the difference between mean arterial pressure and the effective downstream pressure of the cerebral circulation, and R is the cerebrovascular resistance (i.e., the inverse of conductance). This study showed that although the blood pressure response to tilt was divergent (i.e., increasing in AB and decreasing in SCI), the MCA and PCA responses were well-maintained. These results are largely consistent with several studies showing that static CA is preserved in those with SCI (Phillips et al., 2013a) and preganglionic autonomic failure (Hetzel et al., 2003). In order to maintain CBF, conductance during tilt was maintained at supine levels in SCI, while decreasing significantly in AB (Figure 19). In support of this notion, early observations of surface vessels of the brain have confirmed that both large and small pre-capillary cerebral arteries undergo such ?auto-regulatory? calibre adjustments in response to steady-state (and dynamic) increases and decreases in arterial blood pressure (Fog 1937; Fog 1939; Kontos et al., 1978).  Such reflex adjustments in vascular resistance or conductance to steady-state alterations in blood pressure are referred to as static CA.  Dynamic cerebral pressure-flow relationships in able-bodied and spinal cord injured individuals 105  The dynamic cerebral pressure-flow relationships in the time domain showed that the acute (i.e., 0 - 25 s after tilt) blood pressure and CBF response after haemodynamic challenge in SCI is similar to AB, while orthostatic exposure >30 s results in reduced blood pressure and PCAv. Blood pressure dropped similarly between AB and SCI over the first 20 seconds after tilt, however only in AB does it return back to baseline levels before 30 s. In SCI, blood pressure continues to gradually drop in the 60 s after tilt (Figure 19). This failure of sympathetic vasomotor control to maintain blood pressure and hence cerebral perfusion pressure in SCI appears to be partially mitigated by increased cerebrovascular conductance in both the MCA and PCA.  As a result of the increased conductance, CBF velocity is preserved in the MCA and PCA. It appears that time-domain cerebral pressure-flow relationships in SCI allow for maintained MCAv and PCAv during orthostatic challenges. Maintained cerebral perfusion in response to acute orthostatic challenge in SCI is remarkable considering recent work showing impaired MCAv in the 30 s after standing up with alpha-1 adrenoreceptor blockade (Lewis et al., 2013). Although the blood pressure response was blunted after alpha-1 adrenoreceptor blockade, Lewis et al. showed that MCAv was also reduced. As such, it may be that SCI-induced chronic sympathetic vasomotor decentralization leads to enhanced capacity to dynamically alter cerebrovascular conductance, as compared to acute sympathetic vasomotor antagonism. Together, the steady-state and dynamic cerebral perfusion-pressure findings show that both MCAv and PCAv regulation is effective in those with SCI. In spite of altered cerebrovascular function, CBF was well-maintained under all conditions, highlighting the remarkable regulatory capacity of the cerebral blood vessels. Increased function, possibly through increased recruitment of myogenic cerebrovascular responses are a potential explanation for maintain CBF regulation in SCI in spite of reduced perfusion pressure (Phillips et al., 2013).   Frequency domain metrics, derived through transfer function analysis, of cerebral pressure-flow relationships are different in those with SCI in both the MCA and PCA (Table 8). Specifically, reduced MCA and PCA coherence was found in the SCI group in the upright position. This finding is broadly consistent with a similar report in tetraplegic SCI (Wilson et al., 2010). Based on the present data and prior studies, it is apparent that frequency domain metrics of cerebral pressure-flow relationships in the large cerebral vessels are altered in those with SCI. Potential mechanisms and technological reasons for these findings are considered below.  106   Transfer function analysis can be interpreted in two distinct manners. The traditional interpretation is to assume that these metrics reflect dynamic CA (see methods for specific interpretation of coherence, phase and gain) (Zhang et al., 1998; Wilson et al., 2010). Interpreted in this way, reduced coherence in the MCA/PCA would indicate increased dynamic CA in those with SCI. Recently, however, transfer function metrics of cerebral dynamic pressure-flow relationships (spontaneous and induced oscillations) have been shown to relate poorly to more established pressure-flow metrics such as rate of regulation, or autoregulation index; they also relate poorly to between metrics (i.e., phase and gain do not correlate strongly) (Tzeng et al., 2012). Further, reduced coherence in SCI, as shown in two other studies (Wilson et al., 2010; Sahota et al., 2012), may not indicate enhanced dynamic CA as altered input power secondary to varying oscillation amplitude in given frequencies, as noted in mean blood pressure power in SCI (Table 8) is sufficient to alter coherence (Claassen et al., 2009). As such, the reported impairment in the dynamic cerebral pressure-flow relationship may be due simply to a poor signal-to-noise ratio, which is a limitation of using TFA under spontaneous conditions (Panerai et al., 2010; Katsogridakis et al., 2012). The effect of midodrine on cerebral blood flow in spinal cord injured individuals The effect of midodrine on static cerebral autoregulation In agreement with two previous studies, the present study found that midodrine did not alter steady-state MCAv in those with SCI (Wecht et al., 2010; Wecht et al., 2011). For the first time, this study has also illustrated that steady-state PCAv is not altered by midodrine administration in SCI. Although the steady-state blood pressure response to the last tolerable stage of tilt was improved by midodrine, MCAv, heart rate, and PETCO2 responses were not different. Interestingly, midodrine effectively mitigated the decline in PCAv that occurs in SCI after tilt (Figure 19). Maintenance of posterior cerebral perfusion after midodrine may help explain why two prior studies that showed marked improvements in orthostatic tolerance in SCI after midodrine, but limited influence on MCAv (Wecht et al., 2010; Wecht et al., 2011). Indeed, prior work examining the CBF response to orthostatic hypotension in autonomic failure patients after ephedrine administration led to comparable improvements in the blood pressure but not CBF response, highlighting intact static CA (Brooks et al., 1989). Clearly, static CA of the MCA is well-maintained in those with SCI. Midodrine, by increasing perfusion pressure, also helps maintain PCAv selectively in those with SCI during orthostasis.  107     The effect of midodrine on dynamic cerebral pressure-flow relationships   The dynamic time-domain haemodynamic responses to orthostasis in SCI were altered by midodrine administration, and further suggest a key role for posterior cerebral circulation in the development of syncope. For example, similar to the steady-state findings, mitigation in the decline of blood pressure from 30-60 s after tilt did not lead to altered MCA blood flow velocity, but did significantly reduce the decline in PCA velocity over the course of the tilt procedure (Figure 19). Certainly midodrine increases steady-state blood pressure through vascular constriction, and it also appears to prevent the orthostatically-mediated downward drift of blood pressure in SCI, allowing for preserved CBF in the posterior cortex. The current study largely supports recent work showing that posterior regional CBF control plays a crucial role in the development of syncope by showing that vertebral artery blood flow was better maintained as compared the internal carotid artery during orthostatic challenge (Sato et al., 2012).  Transfer function analysis derived metrics of dynamic cerebral pressure-flow velocity relationships in the MCA and PCA were not heavily influenced by midodrine in SCI (Table 8). Therefore it seems that PCA dynamic time-domain pressure-flow relationships in SCI became more similar to AB with midodrine in SCI, while MCA regulation was unchanged (Figure 19).  As mentioned earlier, it is reasonable to suggest a greater relative importance of CBF maintenance in the posterior cortex, as reductions in blood flow in the posterior region may cause an interruption of the blood supply to the medulla oblongata, which contains autonomic control centers, and discrete regions responsible for consciousness (Shin et al., 1999).  Indeed, early work using the 133Xenon inhalation technique showed a relative redistribution of cerebral blood flow away from the frontal lobe and toward the occipital lobe in response to orthostatic challenge (Passant et al., 1992). In those with SCI, and orthostatic hypotension in general, improved PCA blood flow disruption likely plays a key role in the development of syncope, and appears to benefit from midodrine administration.  Orthostatic tolerance This study clearly showed for the first time, using both self-reported symptoms of presyncope and a calculated OTi, that orthostatic tolerance is markedly improved by midodrine in those with 108  high level SCI. One double-blind placebo study in AB individuals with neurally-mediated syncope convincingly showed a reduction in episodes of syncope when using midodrine (Ward et al., 1998). Two studies, albeit using only self-reported symptoms of presyncope, and not reporting any statistical analysis, also reported reductions in symptoms during head-up tilt with midodrine (Wecht et al., 2010; Wecht et al., 2011). The current data indicate that in those with SCI, steady state metrics do not relate to the development of presyncopal symptoms, which is not surprising given the marked heterogeneity with regard to the human haemodynamic response to orthostatic challenges (O'Leary et al., 2007).  Cerebral pressure-flow relationships and baroreflex function An inverse relationship between metrics of dynamic cerebral pressure-flow relationship and cardiac vagal baroreflex sensitivity, suggesting that individuals with the lowest capacity to autoregulate CBF were also the ones that mounted the greatest heart rate response to sudden changes in blood pressure (and vice versa) (Tzeng et al., 2010). Although these findings seem to occur in young AB there seems to be a differential response in the elderly (Aengevaeren et al., 2013). The present study replicated this relationship in AB, but not SCI. Such relationships between cerebral pressure-flow metrics and baroreflex sensitivity are thought to be intrinsic, as supported by the intimately coupled cerebral regulatory centres for both cerebral autoregulation and BRS and the clear evolutionary advantage of redundant haemodynamic control systems for the brain (Nakai 1985; Ishitsuka et al., 1986). An un-coupling of these functions in those with SCI may be due to a variety of issues in SCI, including the drastically reduced blood pressure (Claydon et al., 2006), decentralization of sympathetic control cerebrovasculature (Claydon and Krassioukov 2006; Krassioukov 2009) as well as the upregulation of vagal tone (Claydon and Krassioukov 2008); all of which could alter transfer function metrics (Ainslie et al., 2012; Hamner et al., 2012; Purkayastha et al., 2012) (Table 8). As baroreflex sensitivity in the SCI group was elevated as compared to AB, uncoupling of these parameters in SCI may be beneficial, allowing for preservation of dynamic pressure-flow regulation in SCI, in spite of improved cardiovagal control (i.e., instead of reduced dynamic pressure-flow regulation as would be expected from the inverse relationship in AB).    In SCI, those with the lowest blood pressure during tilt had the highest transfer function derived metrics of cerebrovascular control, and the greatest regulatory capacity (i.e., lowest gain) was found in those with blood pressure below the theorized autoregulatory threshold (i.e., mean 109  blood pressure <60 mmHg) (Panerai 2008). This unexpected finding of the highest ?autoregulation? occurring in those with the lowest perfusion pressure generates additional skepticism that transfer function metrics of cerebral pressure and flow velocity validly measure dynamic CA (Tzeng et al., 2012). Limitations Transcranial Doppler was used for the assessment of cerebral blood flow in this study, which required constant diameter in order to accurately reflect changes in flow. Indeed, two studies have shown that administration of similar alpha-1 agonists have not led to reductions in intracranial vessel diameter (Greenfield Jr and Tindall 1968; Johnston et al., 1994), allowing the fair assumption of maintained MCA and PCA diameter with midodrine. Also, if vasoconstriction did occur with midodrine administration, it would be expected that MCAv and PCAv would increase greatly, which did not occur. With respect to vessels downstream to the MCA and PCA, the blood-brain barrier usually prevents intravascular catecholamines from binding to adrenergic receptors located in cerebral arterioles (MacKenzie et al., 1976).   This was not a blinded, placebo-controlled trial. Although subjects were randomized with respect to the order they had the medication vs. control trial, participants were aware of the medications being provided. The participants were however completely blinded to the purpose of the study.  Rate of regulation, or autoregulation index metrics of dynamic CA were not measured in this study. These techniques both require a thigh-cuff deflation, which is not as clinically relevant to the present population as compared to an orthostatic challenge. Orthostatically-mediated changes in blood pressure may activate different CA regulatory factors as compared to thigh-cuff deflation (Panerai 2008). Specifically, it is likely that intracranial pressure and critical closing pressure are altered during tilt but not thigh-cuff deflations, which may limit the comparability of tilt-based dynamic CA to thigh-cuff deflation (Panerai 2008). CONCLUSION This study had a number of primary conclusions. First, static CA was similar in SCI in both the MCA and PCA as compared to AB. Second, the time-domain PCA dynamic response to tilt was not impaired in SCI, but was significantly increased with midodrine. On the other hand, frequency domain metrics of the cerebral dynamic pressure-flow relationship were impaired in 110  SCI and not changed with midodrine. Third, midodrine improved both self-reported and objectively measured orthostatic tolerance Orthostatic tolerance was unrelated to any cerebrovascular metrics. Finally, cardiovagal baroreflex sensitivity was uncoupled from spontaneous dynamic cerebral pressure-flow relationships in SCI. These findings provide important information on cerebrovascular control during orthostasis in SCI, and the importance of regionally evaluating cerebral blood flow regulation. Future studies should examine the changes in cerebrovascular parameters under a graded orthostatic challenge capable of producing pre-syncopal symptoms before and after midodrine (i.e., tilt combined with lower body negative pressure). Also, any future examination dynamic pressure-flow relationships in SCI should consider the use of oscillatory lower body negative pressure to increase coherence between input and output (Zhang et al., 2002).                     111  CHAPTER FIVE ? ASSOCIATION BETWEEN INTEGRATED CARDIOVAGAL BAROREFLEX SENSITIVITY AND CAROTID ARTERY MECHANICS IN SPINAL CORD INJURY: EFFECT OF MIDODRINE HYDROCHLORIDE INTRODUCTION Spinal cord injury (SCI) is a devastating chronic condition that results in not only motor and sensory deficits but also autonomic dysfunction (Krassioukov and Claydon 2006). As a consequence of decentralization of sympathetic fibres, those with SCI suffer from low resting blood pressure and episodes of severe hypotension when moving upright (orthostatic hypotension) (Claydon et al., 2006). Orthostatic hypotension, which is a rapid reduction in arterial blood pressure as a result of sitting-up from the supine position (Scott et al., 2011),  is thought to be as prevalent as 74% in the spinal cord injured (SCI) population, and is most severe in those with higher level injuries (Illman et al., 2000). As resting blood pressure is generally already low in this population, transient hypotension is a critical issue in those with SCI (Claydon et al., 2006). Orthostatic hypotension in SCI is multi-factorial, and factors other than decentralization of the vasculature below the spinal cord lesion, such as cardiac deconditioning and baroreflex dysfunction have also been implicated (Claydon et al., 2006; Phillips et al., 2012c).  The high pressure baroreflex system consists of stretch-receptors located in the tunica adventitia of the aortic arch and carotid bulbs (Fadel et al., 2003). These spray-like nerve endings generate a more rapid rate of depolarization, and hence increase the frequency of action potentials in afferent nerves during periods of increased wall distension (Stanfield and Germann 2008). The signal is transmitted from the carotid bulb via the glossopharangeal nerve (cranial nerve IX) and the aortic arch via the vagal nerve (cranial nerve X) to the nucleus of the solitary tract in the medulla oblongata (Krassioukov and Weaver 1996). This transmission, which provides surrogate information on systemic blood pressure, is integrated with other afferent information in order to modulate efferent nervous activity transmitted through the vagal nerve and sympathetic chain, to target organs, with the aim of rapidly maintaining blood pressure at a set-point (Stanfield and Germann 2008). Specifically, increases in blood pressure lead to increased vagal tone, and sympathetic inhibition, which consequently results in decreased 112  vascular tone, venous return, cardiac contractility and heart rate (while the reverse actions occur in response to reductions in blood pressure) (Pang 2001).  The cardiovagal baroreflex plays an important role in the acute regulation of blood pressure. For example, in AB individuals, blood pressure changes in the first 2-3 seconds after an orthostatic challenge are 100% mediated by cardiovagal heart rate changes, and ? of blood pressure thereafter (Ogoh et al., 2003). Further, reductions cardiovagal baroreflex sensitivity (BRS) are associated with the onset of presyncope during a progressive orthostatic challenge (Lewis et al., 2010). As those with SCI have reduced ability to modulate vasomotor tone in response to baroreflex mediated signals, by deduction the cardiovagal baroreflex plays a larger role in this population (Krassioukov 2009). The cardiovagal baroreflex also plays a role mitigating long term cardiovascular disease rise. For example, a dysfunction in BRS leads to increases in blood pressure variability (Mancia et al., 1986; Ogoh et al., 2006), which is associated with accelerated end-organ damage (Tatasciore et al., 2007), increased stroke incidence (Tatasciore et al., 2008), and post-stroke complications (Shimbo et al., 2012).  As recently reviewed, cardiovagal BRS is commonly reported as being reduced in those with SCI (Phillips et al., 2012c). The associations between low BRS and several health conditions suggest reduced cardiovagal BRS in SCI may play a role in the high prevalence of orthostatic hypotension and increased stroke risk in this population (Illman et al., 2000; Wu et al., 2012).  It is not known however if the alterations in baroreflex function in those with SCI is due to the mechanical (i.e., distension pattern/stiffness of the carotid) or neural component (i.e., afferent-efferent coupling in brainstem, transmission of efferent signal sinoatrial node), or both. Recently, the combination of common carotid artery (CCA) stiffness, as well as blood pressure and heart rate has allowed for independent analysis of mechanical and neural components in able-bodied individuals (Steinback et al., 2005; Studinger et al., 2007; Saeed et al., 2009; Sugawara et al., 2013; Taylor et al., 2013). To date, the separate neural and mechanical components of the baroreflex have not been quantified in those with SCI. It is known that aging impairs both the neural and mechanical components of the baroreflex (Kornet et al., 2002; Kornet et al., 2005), and that SCI leads to accelerated cardiovascular aging (Groah et al., 2001; Phillips et al., 2012b). Also, a number of cardiac electrophysiological abnormalities (e.g., bradyarrhythmias, ST elevation) have been reported after SCI (Lehmann et al., 1987; Marcus et al., 2002; Weaver and Polosa 2005). Understanding where along the baroreflex loop dysfunction 113  occurs may highlight therapeutic targets for improving baroreflex function in those with SCI. In order to provide further insight into this area of research, it is clear that an examination of both the neural and mechanical arcs of the cardiovagal baroreflex in SCI is required.   Midodrine hydrochloride is an alpha1-agonist which serves to increase blood pressure by constricting arterioles and veins to increase total peripheral resistance (McTavish and Goa 1989). It is uncertain how this pharmaceutical therapy affects the central arteries, and baroreflex function. One study involving AB showed that a combination of increased transmural pressure (blood pressure +30mmHg) and sympathetic vasomotor activity (similar to the effect of midodrine), induced by cold pressor test led to increased systolic and diastolic carotid artery diameter, and increased arterial stiffness (i.e., compliance, local pulse wave velocity, Peterson?s pressure modulus), but no effect on arterial elasticity (i.e., ? stiffness index) (Liu et al., 2011a). Indeed, phenylephrine induced increases in blood pressure led to increased carotid artery dimensions (Bonyhay et al., 1997). Increased distension and systolic diameter of the central stretch receptors through phenylephrine administration led to increased vagal tone at the sino-atrial node, as well as increased cardiovagal BRS in AB (Steinback et al., 2005). Therefore, increased distension caused by midodrine-mediated increases in blood pressure may also enhance cardiovagal baroreflex function.   The current study has three purposes. The first purpose was to evaluate the relationship between arterial stiffness and cardiovagal BRS in those with SCI and AB.  The second purpose was to evaluate the influence of orthostatic challenge on arterial stiffness and cardiovagal BRS in those with SCI as compared to AB. The third purpose was to evaluate the influence of midodrine on both the influence of orthostatic challenge on arterial stiffness and cardiovagal BRS and the relationship between arterial stiffness and BRS. It was hypothesized that arterial stiffness will be increased in those with SCI and that increased arterial stiffness will be related to reduced cardiovagal BRS. Also, it was hypothesized that midodrine will not influence arterial stiffness or cardiovagal BRS in those with SCI.   METHODS Eight individuals with SCI participated in this study (C4-7), AIS A and B; Table 9). Seven participants were <1 year post injury while one was >1 year post injury (Table 10). The control group was composed of eight age- and gender-matched individuals (AB). SCI participants with 114  SCI were approached by a research coordinator after being notified from an attending physician that the patient met study inclusion criteria. Control participants were recruited through word of mouth around the University of British Columbia campus. Participant characteristics are presented in Table 10.    All participants were instructed to abstain from exercise and alcohol for 24 h before testing. No caffeine was permitted the day of testing. Additionally, participants were instructed to abstain from all other medications on the day of testing, and to only have a small meal (i.e., yogurt) approximately one hour before testing. Those who were smokers or had any history of cardiovascular disease were excluded from participation. All participants provided written informed consent in accord with the Clinical Research Ethics Board at the University of British Columbia, who approved this study.  In the SCI group, testing took place over two days. Each testing day was separated by at least 48 hours and took place between 10am-12pm. The two testing days were identical except for the administration of 10 mg midodrine on one of the days. The order of days (i.e., whether baseline or midodrine trial went first) was randomized. On the midodrine day, a 10 mg oral dose was administered. The set-up for post-midodrine testing was initiated precisely 45 minutes after midodrine administration (Figure 17A) in order test at the time of peak effects, which is approximately one hour after ingestion (Wright et al., 1998). A 10 mg dose was chosen as this has been shown to lead to the greatest improvements in orthostatic hypotension and symptoms of orthostatic intolerance, with no more side-effects than a five mg dose (Wright et al., 1998). Each participant was tested in the seated position after 15 minutes of quiet rest.  Protocol As described in Figure 17B, after being transferred to the tilt table, participants rested supine for 15 minutes whilst baseline haemodynamic data was recorded. Participants were progressively tilted from supine to 30?, 45?, and 60? collecting five minutes of continuous data at each tilt level. Transition between tilt levels was achieved in less than five seconds.     115  Table 9. Selected cardiovascular variables for SCI and AB participants. Variable SCI  (n = 8) AB (n = 8) p value Age (years) Weight (kg) BMI (kg?m-2) Gender (# female) 30 ? 11 69.7 ? 14.8 22.3 ? 4.2 1 26 ? 7 72.3 ? 13.8 23.9 ? 4.5 1 0.49 0.73 0.46 N/A AB, able-bodied controls; SCI, high level spinal cord injury; BMI, body mass index; N/A, not applicable.   Table 10. Individual demographic information for SCI individuals. Participant No. (SCI) SCI Level DOI (weeks) AIS Grade Age (yr) Stature (cm) Mass (kg) Education (Years Post Secondary) Sex 1 C4 6.5 A 47 175.5 79.0 2 M 2 C5 144 A 36 180.5 70.0 5 M 3 C5 7 A 17 175.0 54.0 0 M 3 C5 5 A 19 189.0 70.5 1 M 5 C5 7 B 42 175.0 71.0 0 M 6 C5 10 B 28 178.0 94.0 0 M 7 C6 8 A 22 162.0 45.5 1 F 8 C7 11 A 26 177.0 74.0 0 M DOI, duration of injury; AIS grade, American Spinal Cord Injury Association Impairment Scale.  Data acquisition Brachial blood pressure (BpTRU-BPM-100, Coquitlam; VSM Medical, Vancouver, BC, Canada) on the right brachial artery, beat-by-beat blood pressure via finger photoplethysmography (Finometer PRO, Finapres Medicine Systems, Amsterdam, Netherlands), and electrocardiogram (ML 132; ADInstruments) was measured in each participant. All data were collected at 1000 Hz using an analog-to-digital converter (Powerlab/16SP ML 795; ADInstruments, CO Springs, CO) interfaced with data acquisition software on a laptop computer (LabChart 7 ADInstruments). Finometer blood pressure was corrected to the brachial level.  116   In the supine position, and during the homeostatic portion of each tilt (i.e., after the initial decrease in blood pressure) up to three minutes of beat-to-beat CCA diameter using a 10 Mhz linear array transducer (9L-RS, GE Medical Systems, Horten, Norway) was recorded approximately two mm proximal to the carotid bifurcation using a Logiq*e ultrasound (GE Medical Systems), which had a second electrocardiogram attached. After synchronizing the ultrasound with the Chart recording, audio video interleave images at 30 frames/s using a frame grabber (VGA2USB-LR Epiphan Systems, Ottawa, Canada) were recorded. Arterial diameter was then measured on a frame by frame basis, where maximum and minimum diameters (systolic diameter, diastolic diameter) were interpolated and extracted for subsequent analysis (Medical Imaging Applications-LLC, Coralville, Iowa).     Data analysis Thirty successive heart beats with matched beat-by-beat pressure, and CCA diameters to calculate arterial distensibilty, arterial compliance, and ? stiffness index to evaluate CCA stiffness. Distensibility was calculated = ?D/?P/Dd, compliance was calculated as  ?D/?P, and ? stiffness index was calculated as ln(SBP/DBP)/(Ds-Dd)/Dd; where ?D denotes changes in CCA diameter over a cardiac cycle, ?P denotes pulse pressure, Dd denotes diastolic diameter, SBP denotes systolic blood pressure, DBP denotes diastolic blood pressure, and Ds denotes systolic CCA diameter. Spontaneous cardiovagal BRS was analyzed using specialized software (NKFP 8.7.0 and BRS 5.7.0, Nevrokard, Izola, Slovenia). Three to five minute long samples were analyzed to calculate BRS using the sequence method, as obtained via calculation of the slope between changes in RR interval and changes in systolic blood pressure. Inclusion criteria were as follows: a RR interval variation of greater than five ms, blood pressure changes greater than 0.5 mmHg, minimum sequence duration of four beats, sequence correlation coefficient greater than 0.85, and a one beat delay between systolic blood pressure and RR interval (see Figure 5 for example). Sequence method derived BRS is the most commonly reported BRS metric in those with SCI, and as such was chosen the outcome measure of interest (Phillips et al., 2012c).  Statistical analysis Following confirmation of normal distribution using the Shapiro-Wilks test, midodrine-free and midodrine values were compared using paired-sample t-tests while midodrine free and midodrine data were compared to the control group using independent-sample t-tests. Bivariate correlations were also performed. Significance was considered for P < 0.05. Data are reported as mean ? 117  standard deviation. Previous data indicated a required sample size of 8-10 per group when comparing baroreflex sensitivity between SCI and AB (Claydon and Krassioukov 2008). Spontaneous metrics of cardiovagal BRS have been shown to have poor repeatability (i.e., coeffiecnt of variation of 50%) (Iellamo et al., 1996), however the validity as compared to gold-standard methods (i.e., modified Oxford technique) have ranged from weak (Lipman et al., 2003) to strong (Parlow et al., 1995).      RESULTS  Able-bodied versus spinal cord injured  In the supine position, with the exception of reduced systolic blood pressure in SCI as compared to AB, no differences between groups existed for any haemodynamic, CCA, or BRS parameters (Table 11). When upright, blood pressure and CCA diameter was smaller in SCI (all p < 0.05). The diameter difference was also lower in SCI when upright as compared to AB. In SCI, ? stiffness was elevated upright (12%) compared to AB. Although BRS was reduced in the upright posture in both AB and SCI , the relative change in BRS was greater in SCI (p < 0.05).  Relationship between common carotid artery parameters and baroreflex sensitivity As shown in Table 12 the SCI group reported a significant negative relationship between BRS and ? stiffness index in the upright position (R2 = 0.55, p = 0.03), while the AB group did not show a significant relationship (p = 0.15). This relationship was not evident in the supine position in either group (Table 12).  Spinal cord injured individuals with and without midodrine  Midodrine led to increased blood pressure, and decreased heart rate in the supine and upright position in SCI as compared to AB (Table 11). No changes in any BRS or CCA parameters occurred with midodrine in SCI, although diameter difference trended to increase in the supine position (p = 0.06). There were no differences in the response to tilt in any measured parameters with and without midodrine in SCI.  118  Table 11. Homeostatic haemodynamic variables for spinal cord injury before and after midodrine and able-bodied participants. Variable SCI SCImido AB  Supine Max Tilt Supine Max Tilt Supine Max Tilt SBP (mmHg) DBP (mmHg) MAP (mmHg) PP (mmHg) HR (mmHg) Systolic CCA diameter (mm) Diastolic CCA diameter (mm) CCA ? diameter (mm) ? index  Compliance (mm/mmHg) Distensibility (mmHg-1) BRStotal (ms?mmHg-1) BRSup (ms?mmHg-1) BRSdown (ms?mmHg-1) 101 ? 4* 58 ? 6 73 ? 4 43 ? 7 65 ? 13 6.5 ? 0.4 6.0 ? 0.5 0.5 ? 0.2 6.7 ? 2.0 0.013 ? 0.004 0.0021 ? 0.0008 28.6 ? 29.2 28.8 ? 27.4 28.1 ? 29.8 70 ? 12*? 44 ? 9*? 53 ? 9*? 26 ? 8*? 85 ? 17*? 5.9 ? 0.5*? 5.5 ? 0.6? 0.4 ? 0.1*? 7.5 ? 3.1* 0.016 ? 0.0.008 0.0029 ? 0.0018 9.6 ? 7.1* 11.2 ? 7.9* 8.8 ? 7.9* 120 ? 8? 65 ? 31? 83 ? 5? 55 ? 9? 56 ? 6? 6.5 ? 0.5 5.9 ? 0.5 0.6 ? 0.1 6.2 ? 1.5 0.011 ? 0.003 0.0019 ? 0.0005 30.4 ? 28.8 24.4 ? 14.5 42.6 ? 41.1 94 ? 15?? 59 ? 13? 85 ? 17?? 35 ? 7?? 76 ? 19? 5.9 ? 0.5 5.5 ? 0.6 0.4 ? 0.2 6.7 ? 3.3 0.013 ? 0.005 0.0024 ? 0.0008 18.8 ? 27.0? 20.4 ? 27.7 25.7 ? 33.9? 111 ? 13 65 ? 14 81 ? 13 47 ? 10 60 ? 7 6.8 ? 0.6 6.1 ? 0.7 0.7 ? 0.3 5.4 ? 3.2 0.016 ? 0.008 0.0027 ? 0.0015 25.2 ? 11.1 24.7 ? 11.5 26.4 ? 11.8 121 ? 13? 76 ? 7? 73 ? 4? 91 ? 8 61 ? 9 6.4 ? 0.64? 5.8 ? 0.6? 0.6 ? 0.1 4.5 ? 0.8 0.014 ? 0.003 0.0024 ? 0.0005 19.2 ? 10? 20.1 ? 9.0? 19.2 ? 10.7? AB, able-bodied controls; SCI, high level spinal cord injury before midodrine; SCImido, high level spinal cord injury after midodrine; SBP, systolic blood pressure; DBP, diastolic blood pressure; MAP, mean arterial blood pressure; HR, heart rate; CCA, common carotid artery. *; CCA ? diameter, diameter difference of the common carotid artery. *Significantly different from AB (p < 0 .05), ?; significant different from in SCI after midodrine (p < 0 .05), ?; significantly different from supine (p < 0 .05). Significant relationships are highlighted with bold text.    119                       Table 12. Relationships between cardiovagal baroreflex sensitivity and common carotid artery stiffness in able-bodied and spinal cord injured individuals in the upright position. Variable ? index Compliance Distensibility Diameter difference  r P Value r P Value r P Value r P Value BRStotal -0.67 0.005 0.39 0.14 0.34 0.18 0.46 0.08 BRSup -0.59 0.02 0.2 0.47 0.19 0.49 0.29 0.30 BRSdown -0.69 0.004 0.49 0.07 0.41 0.132 0.47 0.08 BRStotal, combined up and down baroreflex gain; BRSup, baroreflex gain during increasing blood pressure, BRSdown, baroreflex gain during decreasing blood pressure.   120  DISCUSSION This study has two main new findings. First, this study showed that cardiovagal BRS is lower in the upright position in SCI as compared to AB, and is related to increased CCA stiffness. Second, midodrine did not impact on either supine or upright CCA stiffness or cardiovagal BRS.  Able-bodied versus spinal cord injured  This study for the time shows that arterial stiffness increases in those with SCI during orthostatic challenge. Previous work examining AB have reported an increase (Steinback et al., 2005; Saeed et al., 2009; Sugawara et al., 2013) or no change (Phillips et al., 2012a; Taylor et al., 2013) in central arterial stiffness (i.e., carotid or aorta) during mild orthostatic challenges (e.g., low level lower body negative pressure (LBNP) or upright tilt).  The reason for the divergent findings is unclear, and in the current study may be due to: 1) the well maintained blood pressure in the upright position, 2) the high proportion of males, 3) different arterial stiffness parameters, and 4) the relative youth and health in the AB group (Laitinen et al., 1998; Legramante et al., 2001). It appears that ? stiffness index decreased in the AB group from the present study however this was not statistically significant (p = 0.4). Also, the present study reported a reduction in BRS during orthostatic challenge in SCI and AB; a finding that has been reported previously (Iellamo et al., 2001; Aslan et al., 2007). At least in AB, reductions in BRS with tilt are thought to result from a combination of vagal withdrawal, sympathetic activation and potentially altered stretch receptor mechanics (Pickering et al., 1972; O'Leary et al., 2003; Steinback et al., 2005). It was not observed that greater reductions in diastolic CCA diameter in the present SCI group in response to orthostatic challenge, which is in contrast to prior findings (Wecht et al., 2004). This discrepancy could possibly be due to different orthostatic stimuli (e.g., 45 degrees (Wecht et al., 2004) compared with the last tolerated stage of tilt in the present study). Alternatively, this may be due to improved CCA imaging, which was detected over 10x more cardiac cycles in the present study and recorded at 30 frames/second (whereas frame-rate was unclear in the prior work (Wecht et al., 2004)).  Relationship between common carotid artery parameters and baroreflex sensitivity The cardiovagal baroreflex is 100% responsible for mediating changes in blood pressure during the first 2-3 s after orthostatic challenge, and reductions in BRS are associated with the onset of preyncope during orthostatic challenge (Ogoh et al., 2003; Ogoh et al., 2006). This is the first study to examine the role arterial stiffness plays in cardiovagal BRS within the SCI population. 121  Within the AB group, the current data showed no change in CCA stiffness, but a reduction in BRS when moving upright, as well as no relationship between arterial stiffness and BRS. Collectively, these findings deductively suggest a lack of mechanical influence on the changes in BRS in AB (if CCA mechanisms were influencing BRS in AB, stiffness should have increased when moving upright, and BRS should have correlated with stiffness). Similarly, Taylor et al. (2013) recently showed no change in the mechanical component of the cardiovagal baroreflex when moving upright and concluded that in AB the mechanical component plays no role in the reduction in BRS (Taylor et al., 2013). In contrast, in the SCI group, increases in CCA stiffness during orthostatic stress were found to play a significant role exacerbating the decline in BRS. In fact, 55% of the variation in BRS in the upright position was explained by carotid stiffness in SCI, with the other variability in BRS potentially being explained by some downstream neural mechanism (Komine et al., 2009; Willie et al., 2011a). It would seem possible that in those with SCI, the drastically decreased blood pressure when assuming the upright posture leads to structural changes in the elastic properties of the central arteries. One potential mechanism, because of the marked blood pressure reduction, is that drastically increased sympathetic outflow above the site of injury led to increased carotid stiffness and reduced cardiovagal BRS in SCI. Indeed, it has been shown that central arterial stiffening does occur during severe orthostatic stress (Phillips et al., 2012a). The SCI group in this study however were all injured above the sympathetic chain; therefore, unless aberrant sympathetic preganglionic neurons preserved some central influence over the superior cervical ganglion, central sympathetic modulation of CCA stiffness is unlikely (Krassioukov and Weaver 1996). Alternatively, according to the non-linear relationship between transmural pressure and stiffness, large reduction in transmural pressure may induce unexpected changes in central arterial mechanics, leading to decreased elasticity (Lacolley et al., 1992; Taylor et al., 1995; Nichols and O'Rourke 2005). Although the underlying cause of increased CCA stiffness during orthostatic challenge in SCI remains to be elucidated, increased stiffness likely plays an important role in blunting cardiovagal BRS in this population.  Spinal cord injured with and without midodrine This study also showed no difference in CCA diameter after acute midodrine administration in SCI. This finding is consistent with previous experiments showing no change in central arterial stiffness during alpha1-adrenoreceptor agonist administration in AB (Krog 1964; Bonyhay et al., 1997; Studinger et al., 2007). Also, midodrine failed to mitigate the absolute decreases in BRS, 122  blood pressure or CCA stiffness in SCI. These findings indicate that improvement in orthostatic tolerance through midodrine, as noted elsewhere in this thesis, is not improved through alterations in CCA mechanics, or cardiovagal BRS. Also, as CCA diameter parameters did not change with acute elevation in blood pressure via alpha1-adrenoreceptor agonist administration, it would be expected that cardiovagal BRS would not improve. It is possible however that the sympathetic arc of the baroreflex was improved by midodrine administration, however this was not measured.  Limitations Some limitations existed in this study. The use of spontaneous cardiovagal BRS may measure different components of the baroreflex as compared to techniques that open the closed-loop system by externally perturbing blood pressure (Kamiya et al., 2011). Also, the sample size in the current study is quite small, which is a widely recognized limitation in SCI research due to the limited mobility of participants as well as the low prevalence of the condition (Phillips et al., 2012c).  CONCLUSIONS This study showed that cardiovagal BRS is decreased in the upright position in SCI as compared to AB, and is related to increased CCA stiffness. Midodrine does not influence either the baroreflex or CCA mechanical changes occurring in response to tilt in SCI. It appears that midodrine does not improve orthostatic tolerance through improvements in cardiovagal BRS. Future studies should examine the relationship between CCA stiffness and baroreflex function using techniques that perturb blood pressure and more accurately measure both cardiac-vagal and vascular BRS.        123   CHAPTER SIX ? GENERAL SUMMARY AND CONCLUSIONS INTEGRATION AND INTERPRETATION OF MAJOR FINDINGS  Cardiovascular disease in the number one killer of those with SCI (Garshick et al., 2005; Myers et al., 2007). Research has demonstrated those with SCI have reduced cognitive function, encounter severe orthostatic hypotension, and are at elevated risk of stroke (Davidoff et al., 1992; Claydon et al., 2006; Wu et al., 2012). In those with SCI, conditions such as poor cognitive function, orthostatic intolerance, and increased stroke risk are thought to be related to dysfunction in CBF (Gonzalez et al., 1991; Silvestrini et al., 2000; Wecht et al., 2012). To date, as highlighted in the present thesis, research examining CBF regulation in those with SCI is limited.   In this dissertation, Chapter two provided a narrative and quantitative review of the literature pertaining to CBF control and baroreflex function in those with SCI. From the available evidence, several major gaps in the literature emerged: 1) static cerebral autoregulation of global CBF is maintained in those with SCI (Nanda et al., 1974; Nanda et al., 1976; Handrakis et al., 2009); however, metrics of dynamic cerebral autoregulation and cerebrovascular reactivity may be altered (Wilson et al., 2010; Sahota et al., 2012), 2) no literature exists on CBF regulation in the posterior cerebral region, which is of relevance given the critical role the brainstem plays in cardiovascular regulation and consciousness (Shin et al., 1999), 3) although cardiovagal baroreflex function appears to be reduced in those with SCI  (Convertino et al., 1991; Koh et al., 1994; Claydon and Krassioukov 2008), it is not known if the fundamental relationship between baroreflex sensitivity and CBF regulation is intact in this population (Tzeng et al., 2010), 4) it is unknown where the pathway of baroreflex function  becomes dysfunctional in SCI (i.e., due to increased arterial stiffness (Phillips et al., 2012b), or an upstream neural component (Taylor et al., 2013); and 5) it is not clear if dysfunction of CBF and reduced BRS are due simply to low blood pressure in SCI, or cardiovascular/cerebrovascular regulatory dysfunction. Consequently, the series of studies contained in this dissertation examined CBF regulation from both a global perspective, as well as regional comparisons (i.e., anterior vs. posterior cerebral regions).  The relationship between CBF regulation and baroreflex function was also examined in SCI, and the role increased arterial stiffness plays in this response was evaluated. In those with SCI, blood 124  pressure was also pharmaceutically normalized to levels found in able-bodied individuals to examine if low blood pressure (and hence perfusion pressure) was partially responsible for dysfunction.    In Chapter three, it was found that the metabolic component of cerebrovascular regulation (i.e., neurovascular coupling) was severely impaired and essentially absent in those with SCI. Cognitive function (i.e., verbal fluency) was also reduced in SCI. After using midodrine to normalize blood pressure to able-bodied levels, both neurovascular coupling as well as cognitive function was significantly improved. A relationship was found between increases in cerebrovascular conductance and cognitive function (R2=0.44, p<0.05) in those with SCI.  These data for the first time reveal a direct link between blood pressure, CBF regulation, and cognition. Also, these findings indicate that improving low blood pressure can lead to increased cognitive function in those with SCI.   Another component of cerebrovascular control was examined in SCI for Chapter four in the context of regional CBF regulation. Static and time-domain dynamic cerebral regulation were found to be preserved in SCI. Frequency-domain metrics of cerebral pressure-flow relationships were increased (i.e., reduced coherence between blood pressure and MCAv/PCAv) in SCI, consistent with a previous report (Wilson et al., 2010). Orthostatic tolerance was improved with midodrine by 59% in SCI. In addition, midodrine resulted in a selective increase in the time-domain dynamic CBF response in the posterior region. This study showed that static cerebral regulation is preserved in SCI, while time-domain dynamic CBF regulation and orthostatic tolerance are improved in SCI with midodrine. These findings also support the contention that improving posterior CBF regulation may be beneficial to tolerating orthostasis (Shin et al., 1999). Finally, although this study verified an inverse relationship between baroreflex sensitivity and CBF regulation in able-bodied individuals, this relationship was absent in SCI. An uncoupling of the fundamental inverse relationship between BRS and dynamic CBF regulation certainly indicates an alteration in haemodynamic regulation in SCI, however whether this adaption is beneficial or harmful to cerebrovascular regulation remains to be elucidated.   To build upon present understanding of baroreflex dysfunction in SCI (Convertino et al., 1991; Claydon and Krassioukov 2008), the investigation presented in Chapter five examined the relationship between common carotid artery stiffness and baroreflex sensitivity in SCI. It was found that reduced baroreflex sensitivity was related to increased common carotid artery stiffness 125  in those with SCI. Increasing blood pressure up to AB levels in SCI with midodrine however, did not alter parameters of arterial stiffness or baroreflex function. The absence of this intervention to improve BRS indicates that low blood pressure is not the underlying cause of reduced BRS in those with SCI. Findings also indicate that reduced mechanical function (i.e., increased arterial stiffness) is related to reduced baroreflex sensitivity in those with SCI. As midodrine did not influence arterial stiffness parameters, it is not surprising that midodrine did not influence baroreflex sensitivity. Clinically relevant doses of midodrine (i.e., 10mg orally administered) likely do not improve orthostatic tolerance through improvements in cardiovagal baroreflex sensitivity. Other mechanisms such as cerebrovascular regulation, and time-domain haemodynamic responses seem a more likely means by which midodrine would improve orthostatic tolerance.   In summary, it appears that CBF regulation in the posterior region is more effected after SCI, as shown by an absence of neurovascular coupling. Cognitive function is also reduced in SCI, and related to the capacity of the brain to modulate CBF during cognition. Normalizing blood pressure improves both neurovascular coupling, and cognitive function. Regulation of CBF in response to steady-state and rapid changes in blood pressure appears intact in SCI. Effective buffering of sustained reductions in blood pressure highlight the remarkable capacity of the cerebrovasculature to maintain flow. Orthostatic tolerance is certainly reduced in SCI; however, such reductions did not appear to relate to any of the included measures of CBF control. The fundamental relationship between baroreflex function and CBF regulation was lost in SCI indicating an uncoupling between these fundamentally related haemodynamic control mechanisms. Reductions in baroreflex function appear to be strongly influenced by increased arterial stiffness in SCI. Normalizing blood pressure increased CBF in the posterior region and increased orthostatic tolerance, but exerted no effect on baroreflex function or common carotid artery stiffness. It appears that midodrine may influence orthostatic tolerance by preserving CBF in the posterior region and not through cardiovagal baroreflex function.   FUTURE STUDIES  Volumetric Flow: Future examinations of CBF regulation, in both basic science and clinical research, should examine volumetric flow (combining diameter and flow velocity). Recent studies have shown that transcranial Doppler derived metrics of CBF differ from those obtained 126  through volumetric assessment (Willie et al., 2012). As 4% changes in diameter have been shown during vasoactive infusions, (Giller et al., 1993) and changes in diameter are weighted to the fourth power in relationship to blood flow according to Darcy?s law, such alterations in blood flow are likely not minor as has been suggested. Volumetric assessments are most commonly completed by extra-cranial evaluation of the internal carotid and vertebral arteries, and have highlighted that CBF regulation certainly occurs through dynamic alteration of conductance of large neck vessels (Sato et al. 2012). This approach also helps to limit the assumption that cerebral arterial diameters are constant and can provide insight into regional CBF (Sato et al., 2012). Externally perturbing blood pressure: The use of spontaneous cardiovagal baroreflex sensitivity may measure different components of the cardiovagal baroreflex as compared to techniques that open the closed-loop system by externally perturbing blood pressure (Kamiya et al., 2011). With no external perturbation of blood pressure, it is debated whether resting (i.e., closed-loop) measures of arterial blood pressure are accurately detecting baroreflex function, or are the result of oscillatory influences from other factors independently influencing blood pressure and heart rate (Diaz and Taylor 2006). This study would have been improved by examining baroreflex sensitivity using techniques that externally perturb blood pressure (as opposed to spontaneous markers of cardiovagal baroreflex sensitivity). Externally perturbing blood pressure using the squat-stand maneuver or oscillatory lower body negative pressure increases both linearity between input and output and signal-to-noise ratio (Katsogridakis et al., 2012). Increasing linearity and signal-to-noise ratio increases low coherence values common in spontaneous baroreflex sensitivity/dynamic cerebral autoregulation measures. As dynamic baroreflex function and dynamic cerebral autoregulation are inherently non-linear systems, transfer function analysis (which assumes a linear relationship between input and output) is far from an ideal marker of efficacious haemodynamic regulatory capacity (Chacon et al., 2011). Future studies should examine the relationship between common carotid artery stiffness and baroreflex function using techniques that perturb blood pressure and more accurately measure both cardiac-vagal and vascular BRS. Hysteresis of baroreflex sensitivity: The baroreflex has been shown have greater responsiveness to increasing as opposed to decreasing blood pressure (Eckberg 1980). This hysteresis, which is due to both mechanical and neural components should certainly be examined in future studies (Studinger et al., 2007). It is possible that chronic 127  exposure to rapid alterations in blood pressure (i.e., orthostatic hypotension, autonomic dysreflexia) leads to differential adaptation of the baroreflex to increasing or decreasing pressure.                    128  REFERENCES Aaslid R. (1987). Visually evoked dynamic blood flow response of the human cerebral circulation. Stroke. (18), 771-775. Aaslid R., Blaha M., Sviri G., Douville C.M., Newell D.W. (2007). Asymmetric dynamic cerebral autoregulatory response to cyclic stimuli. Stroke. (38), 1465-1469. Aaslid R., Lindegaard K.F., Sorteberg W., Nornes H. (1989). Cerebral autoregulation dynamics in humans. Stroke. (20), 45-52. Aengevaeren V.L., Claassen J.A.H.R., Levine B.D., Zhang R. (2013). Cardiac baroreflex function and dynamic cerebral autoregulation in elderly Masters athletes. Journal of Applied Physiology. (114), 195-202. Ainslie P.N., Burgess K., Subedi P., Burgess K.R. (2007). Alterations in cerebral dynamics at high altitude following partial acclimatization in humans: wakefulness and sleep. Journal of Applied Physiology. (102), 658-664. Ainslie P.N., Duffin J. (2009). Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation. Am J Physiol Regul Integr Comp Physiol. (296), R1473-95. Ainslie P.N., Lucas S.J.E., Fan J.L., Thomas K.N., Cotter J.D., Tzeng Y.-C., Burgess K.R. (2012). Influence of sympathoexcitation at high altitude on cerebrovascular function and ventilatory control in humans. Journal of Applied Physiology. (113), 1058-1067. Alan N., Ramer L.M., Inskip J.A., Golbidi S., Ramer M.S., Laher I., Krassioukov A.V. (2010). Recurrent autonomic dysreflexia exacerbates vascular dysfunction after spinal cord injury. Spine J. (10), 1108-17. Anthonisen N.R., Milic-Emili J. (1966). Distribution of pulmonary perfusion in erect man. Journal of Applied Physiology. (21), 760-766. Aries M.J.H., Elting J.W., De Keyser J., Kremer B.P.H., Vroomen P.C.A.J. (2010). Cerebral Autoregulation in Stroke A Review of Transcranial Doppler Studies. Stroke. (41), 2697-2704. Aslan S.C., Randall D.C., Donohue K.D., Knapp C.F., Patwardhan A.R., McDowell S.M., Taylor R.F., Evans J.M. (2007). Blood pressure regulation in neurally intact human vs. acutely injured paraplegic and tetraplegic patients during passive tilt. Am J Physiol Regul Integr Comp Physiol. (292), R1146-57. 129  Azevedo E., Castro P., Santos R., Freitas J., Coelho T., Rosengarten B., Panerai R. (2011). Autonomic dysfunction affects cerebral neurovascular coupling. Clinical Autonomic Research. (21), 395-403. Azevedo E., Rosengarten B., Santos R., Freitas J.o., Kaps M. (2007). Interplay of cerebral autoregulation and neurovascular coupling evaluated by functional TCD in different orthostatic conditions. Journal of neurology. (254), 236-241. Bailey D.M., Jones D.W., Sinnott A., Brugniaux J.V., New K.J., Hodson D., Marley C.J., Smirl J.D., Ogoh S., Ainslie P.N. (2013). Impaired cerebral haemodynamic function associated with chronic traumatic brain injury in professional boxers. Clinical Science. (124), 177. Barber D.B., Rogers S.J., Fredrickson M.D., Able A.C. (2000). Midodrine hydrochloride and the treatment of orthostatic hypotension in tetraplegia: two cases and a review of the literature. Spinal Cord. (38), 109-11. Bauman W.A., Spungen A.M. (2000). Metabolic changes in persons after spinal cord injury. Physical medicine and rehabilitation clinics of North America. (11), 109. Bernardi L., Leuzzi S., Radaelli A., Passino C., Johnston J.A., Sleight P. (1994). Low-frequency spontaneous fluctuations of RR interval and blood pressure in conscious humans: a baroreceptor or central phenomenon? Clinical Science. (87), 649-654. Blaber A.P., Bondar R.L., Moradshahi P., Serrador J.M., Hughson R.L. (2001). Inspiratory CO2 increases orthostatic tolerance during repeated tilt. Aviation, space, and environmental medicine. (72), 985. Blaber A.P., Bondar R.L., Stein F., Dunphy P.T., Moradshahi P., Kassam M.S., Freeman R. (1997). Transfer function analysis of cerebral autoregulation dynamics in autonomic failure patients. Stroke. (28), 1686-1692. Blanco M., Nombela F., Castellanos M., Rodriguez-Yanez M., Garcia-Gil M., Leira R., Lizasoain I., Serena J., Vivancos J., Moro M.A. (2007). Statin treatment withdrawal in ischemic stroke A controlled randomized study. Neurology. (69), 904-910. Bluvshtein V., Korczyn A.D., Akselrod S., Pinhas I., Gelernter I., Catz A. (2011). Hemodynamic responses to head-up tilt after spinal cord injury support a role for the mid-thoracic spinal cord in cardiovascular regulation. Spinal Cord. (49), 251-6. 130  Bonyhay I., Jokkel G., Karlocai K., Reneman R., Kollai M. (1997). Effect of vasoactive drugs on carotid diameter in humans. American Journal of Physiology-Heart and Circulatory Physiology. (273), H1629-H1636. Brooks D.J., Redmond S., Mathias C.J., Bannister R., Symon L. (1989). The effect of orthostatic hypotension on cerebral blood flow and middle cerebral artery velocity in autonomic failure, with observations on the action of ephedrine. Journal of Neurology, Neurosurgery & Psychiatry. (52), 962-966. Burgess K.R., Fan J.L., Peebles K., Thomas K., Lucas S., Lucas R., Dawson A., Swart M., Shepherd K., Ainslie P. (2010). Exacerbation of obstructive sleep apnea by oral indomethacin. CHEST Journal. (137), 707-710. Cariga P., Ahmed S., Mathias C.J., Gardner B.P. (2002). The prevalence and association of neck (coat-hanger) pain and orthostatic (postural) hypotension in human spinal cord injury. Spinal Cord. (40), 77-82. Cassaglia P.A., Griffiths R.I., Walker A.M. (2008). Sympathetic nerve activity in the superior cervical ganglia increases in response to imposed increases in arterial pressure. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (294), R1255-R1261. Cassaglia P.A., Griffiths R.I., Walker A.M. (2009). Cerebral sympathetic nerve activity has a major regulatory role in the cerebral circulation in REM sleep. Journal of Applied Physiology. (106), 1050-1056. Catz A., Bluvshtein V., Korczyn A.D., Pinhas I., Gelernter I., Nissel T., Vered Y., Bornstein N.M., Akselrod S. (2007a). Modified cold pressor test by cold application to the foot after spinal cord injury: suggestion of hemodynamic control by the spinal cord. American journal of physical medicine & rehabilitation. (86), 875-882. Catz A., Bluvshtein V., Pinhas I., Akselrod S., Gelernter I., Nissel T., Vered Y., Bornstein N., Korczyn A.D. (2007b). Cold pressor test in tetraplegia and paraplegia suggests an independent role of the thoracic spinal cord in the hemodynamic responses to cold. Spinal Cord. (46), 33-38. Catz A., Bluvshtein V., Pinhas I., Akselrod S., Gelernter I., Nissel T., Vered Y., Bornstein N.M., Korczyn A.D. (2006). Hemodynamic effects of liquid food ingestion in mid-thoracic 131  paraplegia: is supine postprandial hypotension related to thoracic spinal cord damage? Spinal Cord. (45), 96-103. Chacon M., Araya C., Panerai R.B. (2011). Non-linear multivariate modeling of cerebral hemodynamics with autoregressive Support Vector Machines. Medical engineering & physics. (33), 180-187. Chiu W.T., Lin H.C., Lam C., Chu S.F., Chiang Y.H., Tsai S.H. (2010). Review paper: epidemiology of traumatic spinal cord injury: comparisons between developed and developing countries. Asia Pac J Public Health. (22), 9-18. Claassen J.A.H.R., Levine B.D., Zhang R. (2009). Dynamic cerebral autoregulation during repeated squat-stand maneuvers. Journal of Applied Physiology. (106), 153-160. Claydon V.E., Krassioukov A.V. (2006). Orthostatic hypotension and autonomic pathways after spinal cord injury. Journal of Neurotrauma. (23), 1713-1725. Claydon V.E., Krassioukov A.V. (2008). Clinical correlates of frequency analyses of cardiovascular control after spinal cord injury. Am J Physiol Heart Circ Physiol. (294), H668-78. Claydon V.E., Steeves J.D., Krassioukov A. (2006). Orthostatic hypotension following spinal cord injury: understanding clinical pathophysiology. Spinal Cord. (44), 341-51. Compton J.S., Redmond S., Symon L. (1987). Cerebral blood velocity in subarachnoid haemorrhage: a transcranial Doppler study. Journal of Neurology, Neurosurgery & Psychiatry. (50), 1499-1503. Convertino V.A., Adams W.C., Shea J.D., Thompson C.A., Hoffler G.W. (1991). Impairment of carotid-cardiac vagal baroreflex in wheelchair-dependent quadriplegics. Am J Physiol. (260), R576-80. Cooke W.H., Rickards C.A., Ryan K.L., Convertino V.A. (2008). Autonomic compensation to simulated hemorrhage monitored with heart period variability. Critical care medicine. (36), 1892. D'Esposito M., Deouell L.Y., Gazzaley A. (2003). Alterations in the BOLD fMRI signal with ageing and disease: a challenge for neuroimaging. Nature Reviews Neuroscience. (4), 863-872. 132  Davidoff G., Morris J., Roth E., Bleiberg J. (1985). Cognitive dysfunction and mild closed head injury in traumatic spinal cord injury. Archives of physical medicine and rehabilitation. (66), 489. Davidoff G.N., Roth E.J., Richards J.S. (1992). Cognitive deficits in spinal cord injury: epidemiology and outcome. Archives of physical medicine and rehabilitation. (73), 275. Dawson S.L., Blake M.J., Panerai R.B., Potter J.F. (2000). Dynamic but not static cerebral autoregulation is impaired in acute ischaemic stroke. Cerebrovascular Diseases. (10), 126-132. DeBoer R.W., Karemaker J.M., Strackee J. (1987). Hemodynamic fluctuations and baroreflex sensitivity in humans: a beat-to-beat model. American Journal of Physiology-Heart and Circulatory Physiology. (253), H680-H689. DeVivo M.J., Krause J.S., Lammertse D.P. (1999). Recent trends in mortality and causes of death among persons with spinal cord injury. Arch Phys Med Rehabil. (80), 1411-9. Diaz T., Taylor J.A. (2006). Probing the arterial baroreflex: is there a 'spontaneous' baroreflex? Clin Auton Res. (16), 256-61. Diomedi M., Placidi F., Cupini L.M., Bernardi G., Silvestrini M. (1998). Cerebral hemodynamic changes in sleep apnea syndrome and effect of continuous positive airway pressure treatment. Neurology. (51), 1051-1056. Dowler R.N., Harrington D.L., Haaland K.Y., Swanda R.M., Fee F., Fiedler K. (1997). Profiles of cognitive functioning in chronic spinal cord injury and the role of moderating variables. JOURNAL-INTERNATIONAL NEUROPSYCHOLOGICAL SOCIETY. (3), 464-472. Dowler R.N., O'Brien S.A., Haaland K.Y., Harrington D.L., Feel F., Fiedler K. (1995). Neuropsychological functioning following a spinal cord injury. Applied neuropsychology. (2), 124-129. Duschek S., Hadjamu M., Schandry R. (2007). Enhancement of cerebral blood flow and cognitive performance following pharmacological blood pressure elevation in chronic hypotension. Psychophysiology. (44), 145-53. Duschek S., Matthia E., Schandry R. (2005). Essential hypotension is accompanied by deficits in attention and working memory. Behavioral Medicine. (30), 149-160. 133  Duschek S., Schandry R. (2004). Cognitive performance and cerebral blood flow in essential hypotension. Psychophysiology. (41), 905-13. Duschek S., Schandry R. (2007). Reduced brain perfusion and cognitive performance due to constitutional hypotension. Clinical Autonomic Research. (17), 69-76. Duschek S., Weisz N., Schandry R. (2003). Reduced cognitive performance and prolonged reaction time accompany moderate hypotension. Clinical Autonomic Research. (13), 427-432. Eames P.J., Blake M.J., Dawson S.L., Panerai R.B., Potter J.F. (2002). Dynamic cerebral autoregulation and beat to beat blood pressure control are impaired in acute ischaemic stroke. Journal of Neurology, Neurosurgery & Psychiatry. (72), 467-472. Eckberg D.L. (1980). Nonlinearities of the human carotid baroreceptor-cardiac reflex. Circulation research. (47), 208-216. Eckberg D.L., Orshan C.R. (1977). Respiratory and baroreceptor reflex interactions in man. J Clin Invest. (59), 780-5. Eckberg D.L., Sleight P. Human Baroreflexes in Health and Disease. Oxford, Clarendon Press1992.  Edvinsson L., Krause D.N. Cerebral blood flow and metabolism, vol 27. Lippincott Williams & Wilkins Philadelphia2002.  Eidelman B.H., Debarge O., Corbett J.L., Frankel H. (1972). Absence of cerebral vasoconstriction with hyperventilation in tetraplegic man: evidence for neurogenic control of cerebral circulation. The Lancet. (300), 457-460. Eigenbrodt M.L., Rose K.M., Couper D.J., Arnett D.K., Smith R., Jones D. (2000). Orthostatic hypotension as a risk factor for stroke: the atherosclerosis risk in communities (ARIC) study, 1987-1996. Stroke. (31), 2307-13. Elias M.F., Dore G.A., Davey A., Robbins M.A., Elias P.K. (2010). From Blood Pressure to Physical Disability The Role of Cognition. Hypertension. (55), 1360-1365. Endres M., Gertz K., Lindauer U., Katchanov J., Schultze J., Schr?ck H., Nickenig G., Kuschinsky W., Dirnagl U., Laufs U. (2003). Mechanisms of stroke protection by physical activity. Annals of Neurology. (54), 582-590. Ernsting J., Parry D.J. (1957). Some observations on the effects of stimulating the stretch receptors in the carotid artery of man. J Physiol. (137), 45P-46P. 134  Fadel P.J., Ogoh S., Keller D.M., Raven P.B. (2003). Recent insights into carotid baroreflex function in humans using the variable pressure neck chamber. Exp Physiol. (88), 671-80. Faraci F.M., Heistad D.D. (1990). Regulation of large cerebral arteries and cerebral microvascular pressure. Circulation research. (66), 8-17. Feletou M., Kohler R., Vanhoutte P.M. (2011). Nitric oxide: Orchestrator of endothelium-dependent responses. Annals of Medicine. (44), 694-716. Filosa J.A. (2010). Vascular tone and neurovascular coupling: considerations toward an improved in vitro model. Frontiers in neuroenergetics. (2),  Fog M. (1937). Cerebral circulation: the reaction of the pial arteries to a fall in blood pressure. Archives of Neurology and Psychiatry. (37), 351. Fog M. (1939). Cerebral circulation: II. Reaction of pial arteries to increase in blood pressure. Archives of Neurology and Psychiatry. (41), 260. Fouad-Tarazi F.M., Okabe M., Goren H. (1995). Alpha sympathomimetic treatment of autonomic insufficiency with orthostatic hypotension. The American journal of medicine. (99), 604-610. Fritsch J.M., Eckberg D.L., Graves L.D., Wallin B.G. (1986). Arterial pressure ramps provoke linear increases of heart period in humans. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (251), R1086-R1090. Fronek K., Bloor C.M., Amiel D., Chvapil M. (1978). Effect of long-term sympathectomy on the arterial wall in rabbits and rats. Exp Mol Pathol. (28), 279-89. Fu Q., Shibata S., Hastings J.L., Prasad A., Palmer M.D., Levine B.D. (2009). Evidence for unloading arterial baroreceptors during low levels of lower body negative pressure in humans. American Journal of Physiology-Heart and Circulatory Physiology. (296), H480-H488. Furlan J.C., Fehlings M.G., Shannon P., Norenberg M.D., Krassioukov A.V. (2003). Descending vasomotor pathways in humans: correlation between axonal preservation and cardiovascular dysfunction after spinal cord injury. Journal of neurotrauma. (20), 1351-1363. Gandhi G.K., Ball K.K., Cruz N.F., Dienel G.A. (2010). Hyperglycaemia and diabetes impair gap junctional communication among astrocytes. ASN neuro. (2),  135  Garshick E., Kelley A., Cohen S.A., Garrison A., Tun C.G., Gagnon D., Brown R. (2005). A prospective assessment of mortality in chronic spinal cord injury. Spinal Cord. (43), 408-16. Geary G.G., Krause D.N., Purdy R.E., Duckles S.P. (1998). Simulated microgravity increases myogenic tone in rat cerebral arteries. Journal of Applied Physiology. (85), 1615-1621. Giller C.A., Bowman G., Dyer H., Mootz L., Krippner W. (1993). Cerebral arterial diameters during changes in blood pressure and carbon dioxide during craniotomy. Neurosurgery. (32), 737-742. Girouard H., Iadecola C. (2006). Neurovascular coupling in the normal brain and in hypertension, stroke, and Alzheimer disease. Journal of Applied Physiology. (100), 328-335. Gisolf J., Wilders R., Immink R.V., Van Lieshout J.J., Karemaker J.M. (2004). Tidal volume, cardiac output and functional residual capacity determine end???tidal CO2 transient during standing up in humans. The Journal of physiology. (554), 579-590. Gonzalez F., Chang J.Y., Banovac K., Messina D., Martinez-Arizala A., Kelley R.E. (1991). Autoregulation of cerebral blood flow in patients with orthostatic hypotension after spinal cord injury. Paraplegia. (29), 1-7. Gosling R.G., King D.H. (1974). Arterial assessment by Doppler-shift ultrasound. Proceedings of the Royal Society of Medicine. (67), 447. Greenfield Jr J.C., Tindall G.T. (1968). Effect of norepinephrine, epinephrine, and angiotensin on blood flow in the internal carotid artery of man. Journal of Clinical Investigation. (47), 1672. Grimm D.R., Almenoff P.L., Bauman W.A., De Meersman R.E. (1998). Baroreceptor sensitivity response to phase IV of the Valsalva maneuver in spinal cord injury. Clin Auton Res. (8), 111-8. Groah S.L., Weitzenkamp D., Sett P., Soni B., Savic G. (2001). The relationship between neurological level of injury and symptomatic cardiovascular disease risk in the aging spinal injured. Spinal Cord. (39), 310-317. Grobecker H., Kees F.K., Linden M., Schrader E., Welte S. (1987). Studies on the bioavailability of midodrine and alpha-2, 5-dimethoxyphenyl-beta-aminoethanol-hydrochloride. Arzneimittel-Forschung. (37), 447-450. 136  Grobecker H.F., Kees F.K. (1993). Pharmacokinetic parameters and haemodynamic actions of midodrine in young volunteers. International angiology: a journal of the International Union of Angiology. (12), 119-124. Groothuis J.T., Hopman M.T.E. (2008). Hemodynamic responses to the cold pressor test in spinal cord-injured individuals; control of the splanchnic vascular bed is the key factor. Spinal Cord. (47), 95-95. Grubb B.P., Karas B., Kosinski D., Boehm K. (1999). Preliminary observations on the use of midodrine hydrochloride in the treatment of refractory neurocardiogenic syncope. Journal of interventional cardiac electrophysiology. (3), 139-143. Grubb Jr R.L., Raichle M.E., Eichling J.O., Ter-Pogossian M.M. (1974). The effects of changes in PaCO2 cerebral blood volume, blood flow, and vascular mean transit time. Stroke. (5), 630-639. Guttmann L. (1953). The treatment and rehabilitation of patients with injuries of the spinal cord. Guttman, L British Medical History of World War II: Surgery. 431-516. Guttmann L., Munro A.F., Robinson R., Walsh J.J. (1963). Effect of tilting on the cardiovascular responses and plasma catecholamine levels in spinal man. Spinal Cord. (1), 4-18. Hamner J.W., Tan C.O., Tzeng Y.C., Taylor J.A. (2012). Cholinergic control of the cerebral vasculature in humans. The Journal of physiology.  Handrakis J.P., DeMeersman R.E., Rosado-Rivera D., LaFountaine M.F., Spungen A.M., Bauman W.A., Wecht J.M. (2009). Effect of hypotensive challenge on systemic hemodynamics and cerebral blood flow in persons with tetraplegia. Clinical Autonomic Research. (19), 39-45. Harper A.M., Glass H.I. (1965). Effect of alterations in the arterial carbon dioxide tension on the blood flow through the cerebral cortex at normal and low arterial blood pressures. Journal of neurology, neurosurgery, and psychiatry. (28), 449. Hartman R.E., Kamper J.E., Goyal R., Stewart J.M., Longo L.D. (2012). Motor and cognitive deficits in mice bred to have low or high blood pressure. Physiology & behavior. (105), 1092-1097. Hendrikse J., van der Grond J., Lu H., van Zijl P.C.M., Golay X. (2004). Flow territory mapping of the cerebral arteries with regional perfusion MRI. Stroke. (35), 882-887. 137  Hetzel A., Reinhard M., Guschlbauer B., Braune S. (2003). Challenging cerebral autoregulation in patients with preganglionic autonomic failure. Clinical Autonomic Research. (13), 27-35. Houtman S., Colier W.N., Oeseburg B., Hopman M.T. (2000). Systemic circulation and cerebral oxygenation during head-up tilt in spinal cord injured individuals. Spinal Cord. (38), 158. Houtman S., Oeseburg B., Hopman M.T. (1999). Non-invasive assessment of autonomic nervous system integrity in able-bodied and spinal cord-injured individuals. Clin Auton Res. (9), 115-22. Houtman S., Serrador J.M., Colier W.N., Strijbos D.W., Shoemaker K., Hopman M.T. (2001). Changes in cerebral oxygenation and blood flow during LBNP in spinal cord-injured individuals. Journal of Applied Physiology. (91), 2199-204. Howden R., Lightfoot J.T., Brown S.J., Swaine I.L. (2004). The effects of breathing 5% CO2 on human cardiovascular responses and tolerance to orthostatic stress. Experimental Physiology. (89), 465-471. Hunt B.E., Fahy L., Farquhar W.B., Taylor J.A. (2001). Quantification of mechanical and neural components of vagal baroreflex in humans. Hypertension. (37), 1362-8. Iadecola C. (2004). Neurovascular regulation in the normal brain and in Alzheimer's disease. Nature Reviews Neuroscience. (5), 347-360. Iellamo F., Legramante J.M., Massaro M., Galante A., Pigozzi F., Nardozi C., Santilli V. (2001). Spontaneous baroreflex modulation of heart rate and heart rate variability during orthostatic stress in tetraplegics and healthy subjects. J Hypertens. (19), 2231-40. Iellamo F., Legramante J.M., Raimondi G., Castrucci F., Massaro M., Peruzzi G. (1996). Evaluation of reproducibility of spontaneous baroreflex sensitivity at rest and during laboratory tests. J Hypertens. (14), 1099-1104. Illman A., Stiller K., Williams M. (2000). The prevalence of orthostatic hypotension during physiotherapy treatment in patients with an acute spinal cord injury. Spinal Cord. (38), 741-7. Ishitsuka T., Iadecola C., Underwood M.D., Reis D.J. (1986). Lesions of nucleus tractus solitarii globally impair cerebrovascular autoregulation. American Journal of Physiology-Heart and Circulatory Physiology. (251), H269-H281. 138  Jankovic J., Gilden J.L., Hiner B.C., Kaufmann H., Brown D.C., Coghlan C.H., Rubin M., Fouad-Tarazi F.M. (1993). Neurogenic orthostatic hypotension: a double-blind, placebo-controlled study with midodrine. The American journal of medicine. (95), 38-48. Jegede A.B., Rosado-Rivera D., Bauman W.A., Cardozo C.P., Sano M., Moyer J.M., Brooks M., Wecht J.M. (2010). Cognitive performance in hypotensive persons with spinal cord injury. Clinical Autonomic Research. (20), 3-9. Joannides R., Haefeli W.E., Linder L., Richard V., Thuillez C., L??scher T.F. (1995). Nitric oxide is responsible for flow-dependent dilatation of human peripheral conduit arteries in vivo. Circulation. (91), 1314-1319. Johnson R.H., Spalding J.M.K. (1974). Disorders of the autonomic nervous system.  Johnston W.E., DeWitt D.S., Vinten-Johansen J., Stump D.A., Prough D.S. (1994). Phenylephrine does not reduce cerebral perfusion during canine cardiopulmonary bypass. Anesthesia & Analgesia. (79), 14-18. Kamiya A., Kawada T., Shimizu S., Sugimachi M. (2011). Closed-loop spontaneous baroreflex transfer function is inappropriate for system identification of neural arc but partly accurate for peripheral arc: predictability analysis. The Journal of Physiology. (589), 1769-1790. Karemaker J.M., Wesseling K.H. (2008). Variability in cardiovascular control: the baroreflex reconsidered. Cardiovasc Eng. (8), 23-9. Katsogridakis E., Bush G., Fan L., Birch A.A., Simpson D.M., Allen R., Potter J.F., Panerai R.B. (2012). Detection of impaired cerebral autoregulation improves by increasing arterial blood pressure variability. Journal of Cerebral Blood Flow & Metabolism. EPUB Ahead of Print. Kemna L.J., Posse S., Tellmann L., Schmitz T., Herzog H. (2001). Interdependence of Regional and Global Cerebral Blood Flow During Visual Stimulation&colon; An O-15-Butanol Positron Emission Tomography Study. Journal of Cerebral Blood Flow & Metabolism. (21), 664-670. Knowles R.G., Moncada S. (1994). Nitric oxide synthases in mammals. Biochemical Journal. (298), 249. 139  Koh J., Brown T.E., Beightol L.A., Ha C.Y., Eckberg D.L. (1994). Human autonomic rhythms: vagal cardiac mechanisms in tetraplegic subjects. The Journal of physiology. (474), 483-495. Komine H., Sugawara J., Hayashi K., Yoshizawa M., Yokoi T. (2009). Regular endurance exercise in young men increases arterial baroreflex sensitivity through neural alteration of baroreflex arc. J Appl Physiol. (106), 1499-505. Kontos H.A., Wei E.P., Navari R.M., Levasseur J.E., Rosenblum W.I., Patterson J.L. (1978). Responses of cerebral arteries and arterioles to acute hypotension and hypertension. American Journal of Physiology-Heart and Circulatory Physiology. (234), H371-H383. Korner P.I., Tonkin A.M., Uther J.B. (1976). Reflex and mechanical circulatory effects of graded Valsalva maneuvers in normal man. Journal of Applied Physiology. (40), 434-440. Kornet L., Hoeks A.P., Janssen B.J., Willigers J.M., Reneman R.S. (2002). Carotid diameter variations as a non-invasive tool to examine cardiac baroreceptor sensitivity. Journal of hypertension. (20), 1165-1173. Kornet L., Hoeks A.P.G., Janssen B.J.A., Houben A.J., De Leeuw P.W., Reneman R.S. (2005). Neural activity of the cardiac baroreflex decreases with age in normotensive and hypertensive subjects. Journal of hypertension. (23), 815-823. Krassioukov A. (2009). Autonomic function following cervical spinal cord injury. Respiratory Physiology and Neurobiology. (169), 157-64. Krassioukov A., Claydon V.E. (2006). The clinical problems in cardiovascular control following spinal cord injury: an overview. Progress in Brain Research. (152), 223-9. Krassioukov A., Eng J.J., Warburton D.E., Teasell R. (2009). A systematic review of the management of orthostatic hypotension after spinal cord injury. Arch Phys Med Rehabil. (90), 876-85. Krassioukov A., Weaver L.C. (1996). Anatomy of the autonomic nervous system. Phys Med Rehabil. (10), 1-14. Krog J. (1964). Autonomic nervous control of the cerebral blood flow in man. Journal of the Oslo city hospitals. (14), 25. Krum H., Louis W.J., Brown D.J., Howes L.G. (1992). Pressor dose responses and baroreflex sensitivity in quadriplegic spinal cord injury patients. J Hypertens. (10), 245-50. 140  La Rovere M.T., Bigger J.T., Jr., Marcus F.I., Mortara A., Schwartz P.J. (1998). Baroreflex sensitivity and heart-rate variability in prediction of total cardiac mortality after myocardial infarction. ATRAMI (Autonomic Tone and Reflexes After Myocardial Infarction) Investigators. Lancet. (351), 478-84. La Rovere M.T., Maestri R., Robbi E., Caporotondi A., Guazzotti G., Febo O., Pinna G.D. (2011). Comparison of the prognostic values of invasive and noninvasive assessments of baroreflex sensitivity in heart failure. J Hypertens. (29), 1546-52. Lacolley P.J., Pannier B.M., Slama J.L., Hoeks P.G., London G.M., Safar M.E. (1992). Carotid arterial haemodynamics after mild degrees of lower-body negative pressure in man. Clinical Science. (83), 535-540. Laitinen T., Hartikainen J., Vanninen E., Niskanen L., Geelen G., L??nsimies E. (1998). Age and gender dependency of baroreflex sensitivity in healthy subjects. Journal of Applied Physiology. (84), 576-583. Lanfranchi P.A., Somers V.K. (2002). Arterial baroreflex function and cardiovascular variability: interactions and implications. Am J Physiol Regul Integr Comp Physiol. (283), R815-26. Larsen F.S., Olsen K.S., Hansen B.A., Paulson O.B., Knudsen G.M. (1994). Transcranial Doppler is valid for determination of the lower limit of cerebral blood flow autoregulation. Stroke. (25), 1985-1988. Lassen N.A. (1959). Cerebral blood flow and oxygen consumption in man. Physiological reviews. (39), 183. Laude D., Elghozi J.L., Girard A., Bellard E., Bouhaddi M., Castiglioni P., Cerutti C., Cividjian A., Di Rienzo M., Fortrat J.O. (2004). Comparison of various techniques used to estimate spontaneous baroreflex sensitivity (the EuroBaVar study). American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (286), R226-R231. Legramante J.M., Raimondi G., Massaro M., Iellamo F. (2001). Positive and negative feedback mechanisms in the neural regulation of cardiovascular function in healthy and spinal cord-injured humans. Circulation. (103), 1250-5. Lehmann K.G., Lane J.G., Piepmeier J.M., Batsford W.P. (1987). Cardiovascular abnormalities accompanying acute spinal cord injury in humans: incidence, time course and severity. Journal of the American College of Cardiology. (10), 46-52. 141  Levine B.D., Giller C.A., Lane L.D., Buckey J.C., Blomqvist C.G. (1994). Cerebral versus systemic hemodynamics during graded orthostatic stress in humans. Circulation. (90), 298-306. Lewis N.C.S., Ainslie P.N., Atkinson G., Jones H., Grant E.J.M., Lucas S.J.E. (2013). Initial orthostatic hypotension and cerebral blood flow regulation: effect of alpha1-adrenoreceptor activity. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (304), R147-R154. Lewis N.C.S., Atkinson G., Lucas S.J.E., Grant E.J.M., Jones H., Tzeng Y., Horsman H., Ainslie P.N. (2010). Diurnal variation in time to presyncope and associated circulatory changes during a controlled orthostatic challenge. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (299), R55-R61. Lindegaard K.-F., Lundar T., Wiberg J., Sjoberg D., Aaslid R., Nornes H. (1987). Variations in middle cerebral artery blood flow investigated with noninvasive transcranial blood velocity measurements. Stroke. (18), 1025-1030. Lipman R.D., Salisbury J.K., Taylor J.A. (2003) Spontaneous indices are inconsistent with arterial baroreflex gain. Hypertension. (4), 481-487. Liu J., Cao T.S., Duan Y.Y., Yang Y.L., Yuan L.J. (2011a). Effects of cold pressor-induced sympathetic stimulation on the mechanical properties of common carotid and femoral arteries in healthy males. Heart Vessels. (26), 214-21. Liu P., Yao Y., Liu M.Y., Fan W.L., Chao R., Wang Z.G., Liu Y.C., Zhou J.H., Zhao J. (2011b). Spinal Trauma in Mainland China from 2001 to 2007: An Epidemiological Study Based on a Nationwide Database. Spine (Phila Pa 1976).  Logan I.C., Witham M.D. (2012). Efficacy of treatments for orthostatic hypotension: a systematic review. Age and ageing. (41), 587-594. Low P.A. (2003). Testing the autonomic nervous system. Semin Neurol. (23), 407-21. Low P.A., Gilden J.L., Freeman R., Sheng K.-N., McElligott M.A. (1997). Efficacy of midodrine vs placebo in neurogenic orthostatic hypotension. JAMA: the journal of the American Medical Association. (277), 1046-1051. Lucas S.J., Tzeng Y.C., Galvin S.D., Thomas K.N., Ogoh S., Ainslie P.N. (2010). Influence of changes in blood pressure on cerebral perfusion and oxygenation. Hypertension. (55), 698-705. 142  MacKenzie E.T., McCulloch J., O'Kean M., Pickard J.D., Harper A.M. (1976). Cerebral circulation and norepinephrine: relevance of the blood-brain barrier. American Journal of Physiology--Legacy Content. (231), 483-488. Maiorana A., O'Driscoll G., Taylor R., Green D. (2003). Exercise and the nitric oxide vasodilator system. Sports Med. (33), 1013-35. Mancia G., Parati G., Pomidossi G., Casadei R., Di Rienzo M., Zanchetti A. (1986). Arterial baroreflexes and blood pressure and heart rate variabilities in humans. Hypertension. (8), 147-153. Marcus R.R., Kalisetti D., Raxwal V., Kiratli B.J., Myers J., Perkash I., Froelicher V.F. (2002). Early repolarization in patients with spinal cord injury: prevalence and clinical significance. The journal of spinal cord medicine. (25), 33. Marini U., Cecchi A., Venturini M. (1984). Controlled clinical investigation of dimetophrine versus midodrine in the management of moderately decreased arterial blood pressure. Current Medical Research and Opinion. (9), 265-274. Markus H., Cullinane M. (2001). Severely impaired cerebrovascular reactivity predicts stroke and TIA risk in patients with carotid artery stenosis and occlusion. Brain. (124), 457-67. Mathias C.J., Bannister R. Autonomic disturbances in spinal cord lesions. In: Autonomic Failure: A Textbook of Clinical Disorders of the Autonomic Nervous System, 4th edn. Oxford University Press, New York 2002.  Mathias C.J., Christensen N.J., Frankel H.L., Peart W.S. (1980). Renin release during head-up tilt occurs independently of sympathetic nervous activity in tetraplegic man. Clin Sci (Lond). (59), 251-6. Mattace-Raso F.U., van den Meiracker A.H., Bos W.J., van der Cammen T.J., Westerhof B.E., Elias-Smale S., Reneman R.S., Hoeks A.P., Hofman A., Witteman J.C. (2007). Arterial stiffness, cardiovagal baroreflex sensitivity and postural blood pressure changes in older adults: the Rotterdam Study. J Hypertens. (25), 1421-6. McEvoy R.D., Mykytyn I., Sajkov D., Flavell H., Marshall R., Antic R., Thornton A.T. (1995). Sleep apnoea in patients with quadriplegia. Thorax. (50), 613-619. McTavish D., Goa K.L. (1989). Midodrine. Drugs. (38), 757-777. Mitro P., Trejbal D., Rybar R. (1999). Midodrine hydrochloride in the treatment of vasovagal syncope. Pacing and Clinical Electrophysiology. (22), 1620-1624. 143  Miyatani M., Masani K., Oh P.I., Miyachi M., Popovic M.R., Craven B.C. (2009). Pulse wave velocity for assessment of arterial stiffness among people with spinal cord injury: a pilot study. J Spinal Cord Med. (32), 72-8. Monahan K.D., Tanaka H., Dinenno F.A., Seals D.R. (2001). Central arterial compliance is associated with age- and habitual exercise-related differences in cardiovagal baroreflex sensitivity. Circulation. (104), 1627-32. Moody M., Panerai R.B., Eames P.J., Potter J.F. (2005). Cerebral and systemic hemodynamic changes during cognitive and motor activation paradigms. Am J Physiol Regul Integr Comp Physiol. (288), R1581-8. Mukand J., Karlin L., Barrs K., Lublin P. (2001). Midodrine for the management of orthostatic hypotension in patients with spinal cord injury: A case report. Arch Phys Med Rehabil. (82), 694-6. Munakata M., Kameyama J., Nunokawa T., Ito N., Yoshinaga K. (2001). Altered Mayer wave and baroreflex profiles in high spinal cord injury. Am J Hypertens. (14), 141-8. Myers J., Lee M., Kiratli J. (2007). Cardiovascular disease in spinal cord injury: an overview of prevalence, risk, evaluation, and management. American Journal of Physical Medicine and Rehabilitation. (86), 142-52. Nakai M. (1985). An increase in cerebral blood flow elicited by electrical stimulation of the solitary nucleus in rats with cervical cordotomy and vagotomy. The Japanese journal of physiology. (35), 57. Nanda R.N., Wyper D.J., Harper A.M., Johnson R.H. (1974). Cerebral blood flow in paraplegia. Spinal Cord. (12), 212-218. Nanda R.N., Wyper D.J., Johnson R.H., Harper A.M. (1976). The effect of hypocapnia and change of blood pressure on cerebral blood flow in men with cervical spinal cord transection. Journal of the neurological sciences. (30), 129-135. Nichols W.W., O'Rourke M.F. McDonald's Blood Flow in Arteries: Theoretic, Experimental and Clinical Principles, Fifth edn. Hodder Arnold, Philadelphia, PA 2005.  Noreau L., Proulx P., Gagnon L., Drolet M., Laramee M.-T. (2000). Secondary impairments after spinal cord injury: a population-based study. American journal of physical medicine & rehabilitation. (79), 526-535. 144  Novak V., Hajjar I. (2010). The relationship between blood pressure and cognitive function. Nature Reviews Cardiology. (7), 686-698. Novak V., Spies J.M., Novak P., McPhee B.R., Rummans T.A., Low P.A. (1998). Hypocapnia and cerebral hypoperfusion in orthostatic intolerance. Stroke. (29), 1876-1881. O'Leary D.D., Hughson R.L., Shoemaker J.K., Greaves D.K., Watenpaugh D.E., Macias B.R., Hargens A.R. (2007). Heterogeneity of responses to orthostatic stress in homozygous twins. J Appl Physiol. (102), 249-54. O'Leary D.D., Kimmerly D.S., Cechetto A.D., Shoemaker J.K. (2003). Differential effect of head-up tilt on cardiovagal and sympathetic baroreflex sensitivity in humans. Experimental Physiology. (88), 769-774. Ogoh S., Brothers R.M., Eubank W.L., Raven P.B. (2008). Autonomic neural control of the cerebral vasculature: acute hypotension. Stroke. (39), 1979-87. Ogoh S., Fadel P.J., Nissen P., Jans O., Selmer C., Secher N.H., Raven P.B. (2003). Baroreflex-mediated changes in cardiac output and vascular conductance in response to alterations in carotid sinus pressure during exercise in humans. J Physiol. (550), 317-24. Ogoh S., Sato K., Fisher J.P., Seifert T., Overgaard M., Secher N.H. (2011). The effect of phenylephrine on arterial and venous cerebral blood flow in healthy subjects. Clin Physiol Funct Imaging. (31), 445-51. Ogoh S., Tzeng Y.C., Lucas S.J., Galvin S.D., Ainslie P.N. (2010). Influence of baroreflex-mediated tachycardia on the regulation of dynamic cerebral perfusion during acute hypotension in humans. J Physiol. (588), 365-71. Ogoh S., Yoshiga C.C., Secher N.H., Raven P.B. (2006). Carotid-cardiac baroreflex function does not influence blood pressure regulation during head-up tilt in humans. J Physiol Sci. (56), 227-33. Ormezzano O., Cracowski J.L., Quesada J.L., Pierre H., Mallion J.M., Baguet J.P. (2008). EVAluation of the prognostic value of BARoreflex sensitivity in hypertensive patients: the EVABAR study. J Hypertens. (26), 1373-8. Pagani M., Montano N., Porta A., Malliani A., Abboud F.M., Birkett C., Somers V.K. (1997). Relationship between spectral components of cardiovascular variabilities and direct measures of muscle sympathetic nerve activity in humans. Circulation. (95), 1441-1448. 145  Panerai R.B. (2008). Cerebral autoregulation: from models to clinical applications. Cardiovascular Engineering. (8), 42-59. Panerai R.B., Dineen N.E., Brodie F.G., Robinson T.G. (2010). Spontaneous fluctuations in cerebral blood flow regulation: contribution of PaCO2. Journal of Applied Physiology. (109), 1860-1868. Pang C.C. (2001). Autonomic control of the venous system in health and disease: effects of drugs. Pharmacol Ther. (90), 179-230. Parati G., Di Rienzo M., Mancia G. (2000). How to measure baroreflex sensitivity: from the cardiovascular laboratory to daily life. J Hypertens. (18), 7-19. Parlow J., Viale J.P., Annat G., Hughson R., Quintin L. (1995). Spontaneous cardiac baroreflex in humans. Comparison with drug-induced responses. Hypertension. (25), 1058-68. Passant U., Minthon L., Eng S.V., Edvinsson L., Faldt R., Gustafson L. (1992). Redistribution of blood flow in the cerebral cortex of normal subjects during head-up postural change. Clinical Autonomic Research. (2), 119-124. Paulson O.B., Strandgaard S., Edvinsson L. (1990). Cerebral autoregulation. Cerebrovascular and brain metabolism reviews. (2), 161. Payne S.J. (2006). The estimation of cerebral metabolic rate of O2 from Near Infra-Red Spectroscopy. In. IEEE, pp 3974-3977. Peebles K.C., Ball O.G., MacRae B.A., Horsman H.M., Tzeng Y.C. (2012). Sympathetic regulation of the human cerebrovascular response to carbon dioxide. Journal of Applied Physiology. (113), 700-706. Peebles K.C., Richards A.M., Celi L., McGrattan K., Murrell C.J., Ainslie P.N. (2008). Human cerebral arteriovenous vasoactive exchange during alterations in arterial blood gases. Journal of Applied Physiology. (105), 1060-1068. Perez-Lugones A., Schweikert R., Pavia S., Sra J., Akhtar M., Jaeger F., Tomassoni G.F., Saliba W., Leonelli F.M., Bash D. (2001). Usefulness of midodrine in patients with severely symptomatic neurocardiogenic syncope: a randomized control study. Journal of cardiovascular electrophysiology. (12), 935-938. Perlmuter L.C., Sarda G., Casavant V., O'Hara K., Hindes M., Knott P.T., Mosnaim A.D. (2012). A review of orthostatic blood pressure regulation and its association with mood and cognition. Clinical Autonomic Research. (22), 99-107. 146  Phillips A.A., Ainslie P.N., Krassioukov A.V., Warburton D.E.R. (2013a). Regulation of Cerebral Blood Flow after Spinal Cord Injury. Journal of Neurotrauma. In Press. Phillips A.A., Bredin S.S.D., Cote A.T., Drury C.T., Warburton D.E.R. (2012a). Aortic distensibility is reduced during intense lower body negative pressure and is related to low frequency power of systolic blood pressure. European Journal of Applied Physiology. Epub ahead of print. Phillips A.A., Cote A.T., Bredin S.S., Krassioukov A.V., Warburton D.E. (2012b). Aortic Stiffness Increased in Spinal Cord Injury when Matched for Physical Activity. Medicine and science in sports and exercise. (44), 2065-2070. Phillips A.A., Krassioukov A.V., Ainslie P., Warburton D.E.R. (2012c). Baroreflex function following spinal cord injury. Journal of Neurotrauma. (29), 2431-2445. Phillips A.A., Krassioukov A.V., Ainslie P.N., Tzeng Y.C., Warburton D.E. (2013b). Dynamic cerebral autoregulation of the posterior cerebral artery after high level spinal cord injury. FASEB J (Accepted). Phillips A.A., Krassioukov A.V., Zheng M.M.Z., Warburton D.E.R. (2013c). Neurovascular Coupling of the Posterior Cerebral Artery in Spinal Cord Injury: A Pilot Study. Brain Sciences. (3), 781-789. Pick J. (1970). Central autonomic connections. In: The Autonomic Nervous System. Lippincott, Philadelphia, PA Pickering T.G., Gribbin B., Petersen E.S., Cunningham D.J.C., Sleight P. (1972). Effects of autonomic blockade on the baroreflex in man at rest and during exercise. Circulation research. (30), 177-185. Piknova B., Kocharyan A., Schechter A.N., Silva A.C. (2011). The role of nitrite in neurovascular coupling. Brain research. (1407), 62-68. Pinna G.D., Maestri R., Raczak G., La Rovere M.T. (2002). Measuring baroreflex sensitivity from the gain function between arterial pressure and heart period. Clin Sci (Lond). (103), 81-8. Pittner H., Stormann H., Enzenhofer R. (1976). Pharmacodynamic actions of midodrine, a new alpha-adrenergic stimulating agent, and its main metabolite, ST 1059. Arzneimittel-Forschung. (26), 2145. 147  Popa C., Popa F., Grigorean V.T., Onose G., Sandu A.M., Popescu M., Burnei G., Strambu V., Sinescu C. (2010). Vascular dysfunctions following spinal cord injury. J Med Life. (3), 275-85. Previnaire J.G., Soler J.M. (2010). Cardiovascular control during head-up tilt test in spinal cord injury patients. Spinal Cord. (49), 673-673. Przybylowski T., Bangash M.F., Reichmuth K., Morgan B.J., Skatrud J.B., Dempsey J.A. (2003). Mechanisms of the cerebrovascular response to apnoea in humans. Journal of Physiology-London. (548), 323-332. Purkayastha S., Saxena A., Eubank W., Hoxha B., Raven P.B. (2012). Alpha-1 adrenergic receptor control of the cerebral vasculature in humans at rest and during exercise. Experimental Physiology. EPUB Ahead of Print. Raj S.R., Faris P.D., McRae M., Sheldon R.S. (2012). Rationale for the prevention of syncope trial IV: assessment of midodrine. Clinical Autonomic Research. (22), 275-280. Raven P.B. (2008). Recent advances in baroreflex control of blood pressure during exercise in humans: an overview. Med Sci Sports Exerc. (40), 2033-6. Reichmuth K.J., Dopp J.M., Barczi S.R., Skatrud J.B., Wojdyla P., Hayes D., Morgan B.J. (2009). Impaired Vascular Regulation in Patients with Obstructive Sleep Apnea Effects of Continuous Positive Airway Pressure Treatment. American journal of respiratory and critical care medicine. (180), 1143-1150. Reivich M. (1964). Arterial PCO2 and cerebral hemodynamics. American Journal of Physiology--Legacy Content. (206), 25-35. Rickards C.A., Ryan K.L., Cooke W.H., Convertino V.A. (2011). Tolerance to central hypovolemia: the influence of oscillations in arterial pressure and cerebral blood velocity. Journal of Applied Physiology.  Rickards C.A., Ryan K.L., Cooke W.H., Lurie K.G., Convertino V.A. (2007). Inspiratory resistance delays the reporting of symptoms with central hypovolemia: association with cerebral blood flow. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (293), R243-R250. Robertson D., Davis T.L. (1995). Recent advances in the treatment of orthostatic hypotension. Neurology. (45), S26-S32. 148  Romme J.J.C.M., van Dijk N., Go-Schon I.K., Reitsma J.B., Wieling W. (2011). Effectiveness of Midodrine treatment in patients with recurrent vasovagal syncope not responding to non-pharmacological treatment (STAND-trial). Europace. (13), 1639-1647. Rose K.M., Couper D., Eigenbrodt M.L., Mosley T.H., Sharrett A.R., Gottesman R.F. (2010). Orthostatic hypotension and cognitive function: the Atherosclerosis Risk in Communities Study. Neuroepidemiology. (34), 1-7. Rosengarten B., Auch D., Kaps M. (2007a). Effects of initiation and acute withdrawal of statins on the neurovascular coupling mechanism in healthy, normocholesterolemic humans. Stroke. (38), 3193-3197. Rosengarten B., Dost A., Kaufmann A., Gortner L., Kaps M. (2002). Impaired cerebrovascular reactivity in type 1 diabetic children. Diabetes Care. (25), 408-410. Rosengarten B., Paulsen S., Molnar S., Kaschel R., Gallhofer B., Kaps M. (2007b). Activation-flow coupling differentiates between vascular and Alzheimer type of dementia. Journal of the neurological sciences. (257), 149-154. Rowley N.J., Dawson E.A., Hopman M.T., George K., Whyte G.P., Thijssen D.H., Green D.J. (2012). Conduit Diameter and Wall Remodelling In Elite Athletes and Spinal Cord Injury. Med Sci Sports Exerc.  Ryan K.L., Rickards C.A., Hinojosa???Laborde C., Cooke W.H., Convertino V.A. (2011). Arterial pressure oscillations are not associated with muscle sympathetic nerve activity in individuals exposed to central hypovolaemia. The Journal of Physiology. (589), 5311-5322. Saeed N.P., Reneman R.S., Hoeks A.P. (2009). Contribution of vascular and neural segments to baroreflex sensitivity in response to postural stress. J Vasc Res. (46), 469-77. Sahota I.S., Ravensbergen H.R.J.C., McGrath M.S., Claydon V.E. (2012). Cerebrovascular responses to orthostatic stress after spinal cord injury. Journal of Neurotrauma. (29), 2446-2456. Samniah N., Sakaguchi S., Lurie K.G., Iskos D., Benditt D.G. (2001). Efficacy and safety of midodrine hydrochloride in patients with refractory vasovagal syncope. American Journal of Cardiology. (88), 80-83. 149  Sato K., Fisher J.P., Seifert T., Overgaard M., Secher N.H., Ogoh S. (2012). Blood flow in internal carotid and vertebral arteries during orthostatic stress. Experimental Physiology. (97), 1272-1280. Saul J.P., Rea R.F., Eckberg D.L., Berger R.D., Cohen R.J. (1990). Heart rate and muscle sympathetic nerve variability during reflex changes of autonomic activity. American Journal of Physiology-Heart and Circulatory Physiology. (258), H713-H721. Schatz I.J. (1984). Orthostatic hypotension: II. Clinical diagnosis, testing, and treatment. Archives of internal medicine. (144), 1037. Schmidt B., Klingelhofer J., Perkes I., Czosnyka M. (2009). Cerebral autoregulatory response depends on the direction of change in perfusion pressure. Journal of Neurotrauma. (26), 651-656. Schubert T., Szameitat A.J. (2003). Functional neuroanatomy of interference in overlapping dual tasks: an fMRI study. Cognitive Brain Research. (17), 733-746. Schuepbach D., Goenner F., Staikov I., Mattle H.P., Hell D., Brenner H.D. (2002). Temporal modulation of cerebral hemodynamics under prefrontal challenge in schizophrenia: a transcranial Doppler sonography study. Psychiatry Res. (115), 155-70. Schuepbach D., Weber S., Kawohl W., Hell D. (2007). Impaired rapid modulation of cerebral hemodynamics during a planning task in schizophrenia. Clin Neurophysiol. (118), 1449-59. Scott J.M., Warburton D.E., Williams D., Whelan S., Krassioukov A. (2011). Challenges, concerns and common problems: physiological consequences of spinal cord injury and microgravity. Spinal Cord. (49), 4-16. Seals D.R., Walker A.E., Pierce G.L., Lesniewski L.A. (2009). Habitual exercise and vascular ageing. J Physiol. (587), 5541-9. Senard J.M., Arias A., Berlan M., Tran M.A., Rascol A., Montastruc J.L. (1991). Pharmacological evidence of alpha1-and alpha2-adrenergic supersensitivity in orthostatic hypotension due to spinal cord injury: a case report. European journal of clinical pharmacology. (41), 593-596. Serrador J.M., Picot P.A., Rutt B.K., Shoemaker J.K., Bondar R.L. (2000). MRI measures of middle cerebral artery diameter in conscious humans during simulated orthostasis. Stroke. (31), 1672-1678. 150  Serrador J.M., Sorond F.A., Vyas M., Gagnon M., Iloputaife I.D., Lipsitz L.A. (2005). Cerebral pressure-flow relations in hypertensive elderly humans: transfer gain in different frequency domains. Journal of Applied Physiology. (98), 151-159. Shimbo D., Newman J.D., Aragaki A.K., LaMonte M.J., Bavry A.A., Allison M., Manson J.E., Wassertheil-Smoller S. (2012). Association Between Annual Visit-to-Visit Blood Pressure Variability and Stroke in Postmenopausal WomenNovelty and Significance Data From the Women's Health Initiative. Hypertension. (60), 625-630. Shin H.-K., Yoo K.-M., Chang H.M., Caplan L.R. (1999). Bilateral intracranial vertebral artery disease in the New England medical Center Posterior Circulation Registry. Archives of neurology. (56), 1353-1358. Siesjo B.K. Brain energy metabolism. John Wiley & Sons Chichester:1978.  Silvestrini M., Vernieri F., Pasqualetti P., Matteis M., Passarelli F., Troisi E., Caltagirone C. (2000). Impaired cerebral vasoreactivity and risk of stroke in patients with asymptomatic carotid artery stenosis. Jama. (283), 2122-7. Simmons G.H., Manson J.M., Halliwill J.R. (2007). Mild central chemoreflex activation does not alter arterial baroreflex function in healthy humans. J Physiol. (583), 1155-63. Sloan M.A., Haley E.C., Kassell N.F., Henry M.L., Stewart S.R., Beskin R.R., Sevilla E.A., Tomer J.C. (1989). Sensitivity and specificity of transcranial Doppler ultrasonography in the diagnosis of vasospasm following subarachnoid hemorrhage. Neurology. (39), 1514-1514. Stanfield C.L., Germann W.J. Principles of Human Physiology. Benjamin-Cummings Publishing Company2008.  Steinback C.D., O'Leary D.D., Bakker J., Cechetto A.D., Ladak H.M., Shoemaker J.K. (2005). Carotid distensibility, baroreflex sensitivity, and orthostatic stress. J Appl Physiol. (99), 64-70. Steinback C.D., Salzer D., Medeiros P.J., Kowalchuk J., Shoemaker J.K. (2009). Hypercapnic vs. hypoxic control of cardiovascular, cardiovagal, and sympathetic function. Am J Physiol Regul Integr Comp Physiol. (296), R402-10. Stjernberg L., Blumberg H., Wallin B.G. (1986). Sympathetic activity in man after spinal cord injury. Outflow to muscle below the lesion. Brain. (109 ( Pt 4)), 695-715. 151  Stoner L., Sabatier M., VanhHiel L., Groves D., Ripley D., Palardy G., McCully K. (2006). Upper vs lower extremity arterial function after spinal cord injury. The journal of spinal cord medicine. (29), 138. Studinger P.t., Goldstein R., Taylor J.A. (2007). Mechanical and neural contributions to hysteresis in the cardiac vagal limb of the arterial baroreflex. The Journal of physiology. (583), 1041-1048. Sugawara J., Willie C.K., Miyazawa T., Komine H., Ainsle P.N., Ogoh S. (2013). Effects of transient change in carotid arterial stiffness on arterial baroreflex during mild orthostatic stimulation. Artery Research. (6), 130-135. Tatasciore A., Renda G., Zimarino M., Soccio M., Bilo G., Parati G., Schillaci G., De Caterina R. (2007). Awake systolic blood pressure variability correlates with target-organ damage in hypertensive subjects. Hypertension. (50), 325-332. Tatasciore A., Zimarino M., Renda G., Zurro M., Soccio M., Prontera C., Emdin M., Flacco M., Schillaci G., De Caterina R. (2008). Awake blood pressure variability, inflammatory markers and target organ damage in newly diagnosed hypertension. Hypertension Research. (31), 2137. Tatu L., Moulin T., Bogousslavsky J., Duvernoy H. (1996). Arterial territories of human brain Brainstem and cerebellum. Neurology. (47), 1125-1135. Taylor C.E., Willie C.K., Atkinson G., Jones H., Tzeng Y.C. (2013). Postural Influences On The Mechanical And Neural Components Of The Cardiovagal Baroreflex. Acta Physiologica. (208), 66-73. Taylor J.A., Halliwill J.R., Brown T.E., Hayano J., Eckberg D.L. (1995). 'Non-hypotensive' hypovolaemia reduces ascending aortic dimensions in humans. J Physiol. (483 ( Pt 1)), 289-98. Teasell R.W., Arnold J.M., Krassioukov A., Delaney G.A. (2000). Cardiovascular consequences of loss of supraspinal control of the sympathetic nervous system after spinal cord injury. Archives of physical medicine and rehabilitation. (81), 506. Thijssen D.H., Ellenkamp R., Smits P., Hopman M.T. (2006). Rapid vascular adaptations to training and detraining in persons with spinal cord injury. Archives of physical medicine and rehabilitation. (87), 474-481. 152  Thompson B.G., Pluta R.M., Girton M.E., Oldfield E.H. (1996). Nitric oxide mediation of chemoregulation but not autoregulation of cerebral blood flow in primates. Journal of neurosurgery. (84), 71-78. Thulesius O., Gjores J.E., Berlin E. (1979). Vasoconstrictor effect of midodrine, ST 1059, noradrenaline, etilefrine and dihydroergotamine on isolated human veins. European journal of clinical pharmacology. (16), 423-424. Tiecks F.P., Lam A.M., Aaslid R., Newell D.W. (1995). Comparison of static and dynamic cerebral autoregulation measurements. Stroke. (26), 1014-1019. Timmers H.J., Wieling W., Karemaker J.M., Lenders J.W. (2003). Denervation of carotid baro- and chemoreceptors in humans. J Physiol. (553), 3-11. Tombaugh T.N., Kozak J., Rees L. (1999). Normative data stratified by age and education for two measures of verbal fluency: FAS and animal naming. Archives of Clinical Neuropsychology. (14), 167-177. Tzeng Y.-C., MacRae B.A. (2013). Interindividual relationships between blood pressure and cerebral blood flow variability in the presence of intact and blunted cerebrovascular control. Journal of Applied Physiology. EPUB Ahead of Print. Tzeng Y.C., Ainslie P.N., Cooke W.H., Peebles K.C., Willie C.K., MacRae B.A., Smirl J.D., Horsman H.M., Rickards C.A. (2012). Assessment of cerebral autoregulation: the quandary of quantification. American Journal of Physiology-Heart and Circulatory Physiology. (303), H658-H671. Tzeng Y.C., Chan G.S.H., Willie C.K., Ainslie P.N. (2011). Determinants of human cerebral pressure-flow velocity relationships: new insights from vascular modelling and Ca2+ channel blockade. The Journal of physiology. (589), 3263-3274. Tzeng Y.C., Lucas S.J., Atkinson G., Willie C.K., Ainslie P.N. (2010). Fundamental relationships between arterial baroreflex sensitivity and dynamic cerebral autoregulation in humans. J Appl Physiol. (108), 1162-8. Tzeng Y.C., Sin P.Y.W., Lucas S.J.E., Ainslie P.N. (2009). Respiratory modulation of cardiovagal baroreflex sensitivity. Journal of Applied Physiology. (107), 718-724. Ursino M., Giulioni M., Lodi C.A. (1998). Relationships among cerebral perfusion pressure, autoregulation, and transcranial Doppler waveform: a modeling study. Journal of neurosurgery. (89), 255-266. 153  Van De Borne P., Mezzetti S., Montano N., Narkiewicz K., Degaute J.P., Somers V.K. (2000). Hyperventilation alters arterial baroreflex control of heart rate and muscle sympathetic nerve activity. Am J Physiol Heart Circ Physiol. (279), H536-41. Vernieri F., Pasqualetti P., Matteis M., Passarelli F., Troisi E., Rossini P.M., Caltagirone C., Silvestrini M. (2001). Effect of collateral blood flow and cerebral vasomotor reactivity on the outcome of carotid artery occlusion. Stroke. (32), 1552-8. Vichiansiri R., Saengsuwan J., Manimmanakorn N., Patpiya S., Preeda A., Samerduen K., Poosiripinyo E. (2012). The Prevalence of Dyslipidemia in Patients with Spinal Cord Lesion in Thailand. Cholesterol. (2012),  Vogel E.R., Sandroni P., Low P.A. (2005). Blood pressure recovery from Valsalva maneuver in patients with autonomic failure. Neurology. (65), 1533-1537. Ward C.R., Gray J.C., Gilroy J.J., Kenny R.A. (1998). Midodrine: a role in the management of neurocardiogenic syncope. Heart. (79), 45-49. Weaver L.C., Polosa C. (2005). Spinal cord injury alters cardiac electrophysiology and increases the susceptibility to ventricular arrhythmias. Autonomic Dysfunction After Spinal Cord Injury. (152), 275. Wecht J.M., Radulovic M., Lafountaine M.F., Rosado-Rivera D., Zhang R.L., Bauman W.A. (2009). Orthostatic responses to nitric oxide synthase inhibition in persons with tetraplegia. Arch Phys Med Rehabil. (90), 1428-34. Wecht J.M., Radulovic M., Lessey J., Spungen A.M., Bauman W.A. (2004). Common carotid and common femoral arterial dynamics during head-up tilt in persons with spinal cord injury. Journal of rehabilitation research and development. (41), 89-94. Wecht J.M., Radulovic M., Rosado-Rivera D., Zhang R.L., Lafountaine M.F., Bauman W.A. (2011). Orthostatic Effects of Midodrine Versus L-NAME on Cerebral Blood Flow and the Renin-Angiotensin-Aldosterone System in Tetraplegia. Arch Phys Med Rehabil.  Wecht J.M., Radulovic M., Weir J.P., Lessey J., Spungen A.M., Bauman W.A. (2005). Partial angiotensin-converting enzyme inhibition during acute orthostatic stress in persons with tetraplegia. J Spinal Cord Med. (28), 103-8. Wecht J.M., Rosado-Rivera D., Handrakis J.P., Radulovic M., Bauman W.A. (2010). Effects of midodrine hydrochloride on blood pressure and cerebral blood flow during orthostasis in persons with chronic tetraplegia. Arch Phys Med Rehabil. (91), 1429-35. 154  Wecht J.M., Rosado-Rivera D., Jegede A., Cirnigliaro C.M., Jensen M.A., Kirshblum S., Bauman W.A. (2012). Systemic and cerebral hemodynamics during cognitive testing. Clinical Autonomic Research. (22), 25-33. Wecht J.M., Weir J.P., Bauman W.A. (2006). Blunted heart rate response to vagal withdrawal in persons with tetraplegia. Clin Auton Res. (16), 378-83. Wei E.P., Kontos H.A., Patterson J.L. (1980). Dependence of pial arteriolar response to hypercapnia on vessel size. American Journal of Physiology-Heart and Circulatory Physiology. (238), H697-H702. White R.P., Markus H.S. (1997). Impaired dynamic cerebral autoregulation in carotid artery stenosis. Stroke. (28), 1340-1344. Widder B., Kleiser B., Krapf H. (1994). Course of cerebrovascular reactivity in patients with carotid artery occlusions. Stroke. (25), 1963-1967. Willie C.K., Ainslie P.N., Taylor C.E., Jones H., Sin P.Y., Tzeng Y.C. (2011a). Neuromechanical Features of the Cardiac Baroreflex After Exercise. Hypertension.  Willie C.K., Colino F.L., Bailey D.M., Tzeng Y.C., Binsted G., Jones L.W., Haykowsky M.J., Bellapart J., Ogoh S., Smith K.J., Smirl J.D., Day T.A., Lucas S.J., Eller L.K., Ainslie P.N. (2011b). Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function. J Neurosci Methods. (196), 221-37. Willie C.K., Cowan E.C., Ainslie P.N., Taylor C.E., Smith K.J., Sin P.Y.W., Tzeng Y.C. (2011c). Neurovascular coupling and distribution of cerebral blood flow during exercise. Journal of neuroscience methods. (198), 270-273. Willie C.K., Macleod D.B., Shaw A.D., Smith K.J., Tzeng Y.C., Eves N.D., Ikeda K., Graham J., Lewis N.C., Day T.A. (2012). Regional brain blood flow in man during acute changes in arterial blood gases. The Journal of Physiology.  Wilson L.C., Cotter J.D., Fan J.L., Lucas R.A., Thomas K.N., Ainslie P.N. (2010). Cerebrovascular reactivity and dynamic autoregulation in tetraplegia. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (298), R1035-42. Wong S.C., Eng J.J., Krassioukov A.V., Zbogar D., Scott J.M., Esch B.T., Warburton D.E. (2007). Arterial compliance: effect of training status in able bodied persons and persons with spinal cord injury. Appl Physiol Nutr Metab. (32), S94. 155  Woodcock J.P., Gosling R.G., Fitzgerald D.E. (1972). A new noninvasive technique for assessment of superficial femoral artery obstruction. British Journal of Surgery. (59), 226-231. Wright R.A., Kaufmann H.C., Perera R., Opfer-Gehrking T.L., McElligott M.A., Sheng K.N., Low P.A. (1998). A double-blind, dose-response study of midodrine in neurogenic orthostatic hypotension. Neurology. (51), 120-4. Wu J.C., Chen Y.C., Liu L., Chen T.J., Huang W.C., Cheng H., Tung-Ping S. (2012). Increased risk of stroke after spinal cord injury: a nationwide 4-year follow-up cohort study. Neurology. (78), 1051-7. Xie A., Skatrud J.B., Khayat R., Dempsey J.A., Morgan B., Russell D. (2005). Cerebrovascular response to carbon dioxide in patients with congestive heart failure. American journal of respiratory and critical care medicine. (172), 371-378. Xu T.Y., Staessen J.A., Wei F.F., Xu J., Li F.H., Fan W.X., Gao P.J., Wang J.G., Li Y. (2011). Blood Flow Pattern in the Middle Cerebral Artery in Relation to Indices of Arterial Stiffness in the Systemic Circulation. Am J Hypertens.  Yamamoto M., Meyer J.S., Sakai F., Jakoby R. (1980). Effect of differential spinal cord transection on human cerebral blood flow. Journal of the neurological sciences. (47), 395-406. Yoshida M., Murayama Y., Chishaki A., Sunagawa K. (2008). Noninvasive transcutaneous bionic baroreflex system prevents severe orthostatic hypotension in patients with spinal cord injury. Conf Proc IEEE Eng Med Biol Soc. (2008), 1985-7. Yufu K., Takahashi N., Okada N., Wakisaka O., Shinohara T., Nakagawa M., Hara M., Yoshimatsu H., Saikawa T. (2011). Gender difference in baroreflex sensitivity to predict cardiac and cerebrovascular events in type 2 diabetic patients. Circ J. (75), 1418-23. Zachariah P.K., Bloedow D.C., Moyer T.P., Sheps S.G., Schirger A., Fealey R.D. (1986). Pharmacodynamics of midodrine, an antihypotensive agent. Clinical Pharmacology & Therapeutics. (39), 586-591. Zhang L.N., Zhang L.F., Ma J. (2001). Simulated microgravity enhances vasoconstrictor responsiveness of rat basilar artery. Journal of Applied Physiology. (90), 2296-2305. Zhang R., Claassen J.A.H.R., Shibata S., Kilic S., Martin-Cook K., Diaz-Arrastia R., Levine B.D. (2009a). Arterial-cardiac baroreflex function: insights from repeated squat-stand 156  maneuvers. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. (297), R116-R123. Zhang R., Wilson T.E., Witkowski S., Cui J., Crandall C.G., Levine B.D. (2004). Inhibition of nitric oxide synthase does not alter dynamic cerebral autoregulation in humans. American Journal of Physiology-Heart and Circulatory Physiology. (286), H863-H869. Zhang R., Zuckerman J.H., Giller C.A., Levine B.D. (1998). Transfer function analysis of dynamic cerebral autoregulation in humans. American Journal of Physiology-Heart and Circulatory Physiology. (274), H233-H241. Zhang R., Zuckerman J.H., Iwasaki K., Wilson T.E., Crandall C.G., Levine B.D. (2002). Autonomic neural control of dynamic cerebral autoregulation in humans. Circulation. (106), 1814-1820. Zhang Y., Popovic Z.B., Bibevski S., Fakhry I., Sica D.A., Van Wagoner D.R., Mazgalev T.N. (2009b). Chronic vagus nerve stimulation improves autonomic control and attenuates systemic inflammation and heart failure progression in a canine high-rate pacing model. Circ Heart Fail. (2), 692-9. Zollei E., Paprika D., Rudas L. (2003). Measures of cardiovascular autonomic regulation derived from spontaneous methods and the Valsalva maneuver. Auton Neurosci. (103), 100-5. Zonta M., Angulo M.C., Gobbo S., Rosengarten B., Hossmann K.A., Pozzan T., Carmignoto G. (2003). Neuron-to-astrocyte signaling is central to the dynamic control of brain microcirculation. Nature neuroscience. (6), 43-50.   

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.24.1-0074203/manifest

Comment

Related Items