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Clinical pathophysiology of hypoxic ischemic brain injury after cardiac arrest: a “two-hit” model Sekhon, Mypinder S; Ainslie, Philip N; Griesdale, Donald E Apr 13, 2017

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REVIEW Open AccessClinical pathophysiology of hypoxicischemic brain injury after cardiac arrest:a “two-hit” modelMypinder S. Sekhon1,2*, Philip N. Ainslie2 and Donald E. Griesdale1,3,4AbstractHypoxic ischemic brain injury (HIBI) after cardiac arrest (CA) is a leading cause of mortality and long-term neurologicdisability in survivors. The pathophysiology of HIBI encompasses a heterogeneous cascade that culminates insecondary brain injury and neuronal cell death. This begins with primary injury to the brain caused by the immediatecessation of cerebral blood flow following CA. Thereafter, the secondary injury of HIBI takes place in the hours anddays following the initial CA and reperfusion. Among factors that may be implicated in this secondary injury includereperfusion injury, microcirculatory dysfunction, impaired cerebral autoregulation, hypoxemia, hyperoxia, hyperthermia,fluctuations in arterial carbon dioxide, and concomitant anemia.Clarifying the underlying pathophysiology of HIBI is imperative and has been the focus of considerable research toidentify therapeutic targets. Most notably, targeted temperature management has been studied rigorously in preventingsecondary injury after HIBI and is associated with improved outcome compared with hyperthermia. Recent advancespoint to important roles of anemia, carbon dioxide perturbations, hypoxemia, hyperoxia, and cerebral edema ascontributing to secondary injury after HIBI and adverse outcomes. Furthermore, breakthroughs in the individualization ofperfusion targets for patients with HIBI using cerebral autoregulation monitoring represent an attractive area of futurework with therapeutic implications.We provide an in-depth review of the pathophysiology of HIBI to critically evaluate current approaches for the earlytreatment of HIBI secondary to CA. Potential therapeutic targets and future research directions are summarized.Keywords: Hypoxic ischemic brain injury, Cardiac arrest, Cerebral oxygen delivery, Targeted temperature management,Cerebral edema, Carbon dioxide, Anemia, Hypothermia, Normobaric hyperoxiaBackgroundCardiac arrest (CA) is a major cause of mortality andneurologic disability. The incidence of out-of-hospitalCA is approximately 80 patients per 100,000 personsannually [1]. Despite advances in resuscitation, outcomesremain dismal, with 10% of patients surviving untilhospital discharge and 5% experiencing full neurologicrecovery [1].The primary determinant of outcome after CA ishypoxic ischemic brain injury (HIBI). HIBI is theprimary cause of death in 68% of inpatient CA and in23% of out-of-hospital CA [2]. HIBI is associated withsignificant neurologic disability, ranging from mildcognitive deficits to minimally conscious and persistentvegetative states [2, 3]. Consequently, considerable effectson quality of life and incidence of psychiatric comorbidi-ties, such as depression, anxiety, and posttraumatic stressdisorder, are highly prevalent in HIBI survivors [4, 5]. Thevast spectrum of acute and chronic HIBI phenotypesrequires detailed understanding of cerebral physiologicperturbations that occur after CA and make clarifying thepathophysiology essential.Management of HIBI is focused on limiting secondaryinjury [3] by optimizing the balance between cerebral* Correspondence: mypindersekhon@gmail.com1Division of Critical Care Medicine, Department of Medicine, VancouverGeneral Hospital, University of British Columbia, Room 2438, Jim PattisonPavilion, 2nd Floor, 855 West 12th Avenue, Vancouver, BC V5Z 1M9, Canada2Centre for Heart, Lung and Vascular Health, School of Health and ExerciseSciences, University of British Columbia Okanagan, Kelowna, BC, CanadaFull list of author information is available at the end of the article© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.Sekhon et al. Critical Care  (2017) 21:90 DOI 10.1186/s13054-017-1670-9oxygen delivery (CDO2) and use. Despite rigorousresearch, HIBI outcomes have not appreciably changedover 20 years [6, 7]. This stagnation is in contrast withimproved outcomes in other critical care diseases [8].Considerable opportunities remain to delineate thepathophysiology of HIBI. HIBI pathophysiology is a“two-hit” model, being determined by primary injuryfrom immediate cessation of CDO2 during CA and sec-ondary injury occurring after resuscitation. We present anarrative review of a two-hit model of HIBI pathophysi-ology as it pertains to physiologic parameters involved inmaintaining the balance of CDO2 and use. We highlightadvances pertaining to cerebral autoregulation, optimalhemoglobin, carbon dioxide, cerebral edema, normoba-ric hyperoxia, and targeted temperature management.Primary injuryDuring CA, cessation of CDO2 occurs with resultantneuron ischemia and cell death within minutes [9](Fig. 1). The cerebrum consumes 20% to 25% of cardiacoutput to maintain function [3]. The brain is devoid ofnutrient stores, and consequently neuroglycopenia andmetabolic crisis occur within minutes after CA [10],leading to cell death.As CDO2 decreases, adenosine triphosphate productionhalts, causing cessation of energy-dependent ion channelfunction [11]. Subsequent intracellular Na+ accumulationresults in cytotoxic edema. Depletion of adenosine tri-phosphate leads to anaerobic metabolism, cerebral lactateaccumulation, and intracellular acidosis [12]. Additionally,cellular ischemia causes intracellular Ca2+ influx throughFig. 1 A schematic demonstrating the various microvascular and cellular pathophysiologic consequences which occur during the primary andsecondary injury in hypoxic ischemic brain injury (HIBI). Decreased cerebral oxygen delivery manifests as reduced neuronal aerobic metabolism,causing reduced cellular adenosine triphosphate (ATP) production. Intracellular calcium accumulation leads to mitochondrial toxicity and furtherreduced ATP production. Inability to sustain cellular respiration results in cell death and apoptosis. Additionally, in the microvasculature, endothelialdysfunction leads to a porous blood-brain barrier, formation of cerebral edema, formation of microthrombi and limitation of cerebral blood flow withexacerbation of cellular ischemia. AQP 4 Aquaporin-4, RBC Red blood cells, WBC White blood cellsSekhon et al. Critical Care  (2017) 21:90 Page 2 of 10N-methyl-D-aspartate channels, which activates lyticenzymes [13] and mitochondrial dysfunction, therebydepleting adenosine triphosphate further [14]. Finally,excitatory neurotransmitter release activates lipases andproteases, which leads to apoptosis [15].Clinically, loss of neurologic function is manifested bya decreased level of consciousness after global cerebralischemia. Historically, Rossen et al. demonstrated thatcessation of cerebral blood flow (CBF) by neck cuff in-sufflation to 600 mmHg in humans precipitated acutedecreased level of consciousness within 10 seconds [16].Decreased level of consciousness after CA occurs within20 seconds after onset of ventricular fibrillation [17].Loss of neurologic function has been demonstrated by iso-electric electroencephalography in observational studies[16]. Pana et al. identified human studies demonstratingisoelectric electroencephalography rhythms within 15 sec-onds and 30 seconds of asystole and ventricular fibrilla-tion, respectively [16]. These findings are corroborated byanimal studies establishing a similar timeline of 10 to30 seconds from the onset of cerebral ischemia to isoelec-tric electroencephalography [18].Although primary injury causes substantial neuronalloss, the ensuing postresuscitation additive cerebralinjury accounts for significant cerebral ischemia andcellular death. The key pathophysiologic factors that areimplicated in secondary injury are physiologic modifiersinvolved in maintaining the balance between CDO2 anduse. We next discuss secondary injury and physiologicdeterminants that are targets of therapeutic interven-tions after HIBI.Secondary injurySecondary injury is the additive cerebral injury charac-terized by an imbalance in postresuscitation CDO2 anduse, ultimately culminating in neuronal death. It beginsimmediately after return of spontaneous circulation(ROSC). Structures especially susceptible include thehippocampi, thalami, cerebral cortex, corpus striatum,and cerebellar vermi [3] (Fig. 2), owing to highly meta-bolically active tissue. Aside from hypothermia, there arelimited studies examining physiologic variables thatexacerbate secondary injury. Table 1 summarizes themechanisms of secondary injury.Microcirculation and reperfusion injuryAfter ROSC, microcirculatory perturbations lead tofurther neuron dysfunction. The cerebrovascular endo-thelium plays a critical role in maintaining blood-brainbarrier integrity, regulation of microcirculatory bloodflow, and release of autoanticoagulant mediators [19].Endothelial functions are compromised, and biomarkersof cerebrovascular endothelial injury are associated withadverse outcomes in HIBI [20].Following ROSC, reperfusion injury causes neuronaldysfunction despite restoration of CDO2 [21]. An initialperiod of cerebral hyperemia is followed by hypoperfu-sion, resulting in a “no-reflow” [22] state that exacer-bates secondary injury. Mechanisms implicated in theno-reflow state include impaired vasomotor regulation,decreased nitric oxide production, and resultant vaso-constriction [3, 19, 20]. Extravasation of intravascularwater through a porous blood-brain barrier withFig. 2 Magnetic resonance imaging sequences show focal hypoxic ischemic brain injury (HIBI) within the hippocampi and basal gangliabilaterally. The images shown represent the acute changes after HIBI within the first week after resuscitation. In the top row, T2-weightedsequences reveal abnormal signaling in the hippocampi and basal ganglia as highlighted by the red arrows. In the bottom row, restricteddiffusion-weighted imaging confirms HIBI in the affected regions of the hippocampi and basal ganglia as highlighted by the red arrowsSekhon et al. Critical Care  (2017) 21:90 Page 3 of 10perivascular edema leads to increased intravascularviscosity and cerebrovascular resistance [22]. Othermechanisms implicated in reperfusion injury include freeradical release, glutamate production, and intracellularCa2+ accumulation [23].Endothelial autoanticoagulant dysfunction causes diffusemicrothrombi in the cerebrovasculature [24]. Concomi-tant impaired vasodilation causes increased cerebrovas-cular resistance and reduces CBF [3, 22]. Interventionalstudies demonstrate that heparin and tissue plasminogenactivator improve microcirculatory flow [25]. These find-ings have not translated into improved outcomes whenevaluated prospectively, however [24, 26]. Finally, intra-venous prostacyclin is suggested to promote endothelialfunction through vasodilatory and antiplatelet effects [19],but clinical studies are not yet available. Table 2 summa-rizes mechanisms involved in reperfusion injury.HemoglobinHemoglobin is a major determinant of arterial oxygencontent. In animal studies of traumatic brain injury,concomitant anemia exacerbates secondary injury fromapoptosis [27]. However, physiologic benefits of im-proved CDO2 from transfusion must be balanced byrisks associated with exogenous red blood cells. Al-though hemoglobin <70 g/L is the accepted transfusionthreshold for nonbleeding critical care patients [28], itremains unclear if a liberal threshold is appropriate forpatients with brain injury, who are susceptible to sec-ondary injury from anemia [29].Evidence of anemia in contributing to secondary injuryin HIBI is limited to observational studies. Nakao et al.conducted a retrospective study of 137 subjects with wit-nessed CA and established that higher admissionhemoglobin was an independent predictor of a 28-day fa-vorable neurologic outcome (OR 1.26, 95% CI 1.00–1.58)[30]. These findings were corroborated by Wang et al.,who demonstrated an association with adverse outcomeand lower admission hemoglobin [31]. Recently, Johnsonet al. conducted a multicenter observational study of 598patients and found that favorable outcome patients hadsignificantly higher hemoglobin (126 g/L versus 106 g/L,p < 0.001), a finding that persisted after adjustment [32].Despite regression adjustment, admission anemia maybe subject to strong residual or unmeasured confounding.It is unclear if admission hemoglobin captures the magni-tude of effect that anemia has on secondary injury.Wormsbecker et al. accounted for this by investigating therelationship between mean hemoglobin over 7 days andneurologic outcome. They established that patients with afavorable outcome had significantly higher 7-day meanhemoglobin (115 g/L versus 107 g/L, p = 0.05) [33].Table 1 Summary of mechanisms of secondary brain injury after hypoxic ischemic brain injuryPathophysiology Mechanisms ConsequencesMicrovascular dysfunction Microthrombi, cerebral vasoconstriction, blood-brainbarrier disruptionIncreased cerebrovascular resistance, decreased CBF,decreased cerebral O2 delivery, vasogenic cerebral edemaCerebral edema Vasogenic cerebral edema, cytotoxic cerebral edema Increased ICP and decreased CPP, decreased CBF,herniation, brain deathAnemia Decreased arterial oxygen content Cerebral ischemiaImpaired autoregulation Narrowed and right-shifted autoregulation Pressure passive cerebral hemodynamics, cerebralischemia and hyperemiaCarbon dioxide Hypocapnia-induced vasoconstriction, hypercapnia-induced vasodilationDecreased CBF, cerebral ischemia, increased ICP,decreased CPP, decreased CBFHyperoxia Increased O2 free radicals Neuronal cell dysfunction and cell deathHyperthermia Increased CMRO2, decreased seizure threshold,induction of apoptosisNeuronal cell metabolic crisis, cell death, nonconvulsiveseizures, increased CMRO2, neuronal cell deathAbbreviations: CBF Cerebral blood flow, ICP Intracranial pressure, CPP Cerebral perfusion pressure, CMRO2 Cerebral metabolic rate of oxygen uptakeTable 2 Pathophysiologic summary of cerebral reperfusion injury after cardiac arrestPathophysiology Mechanisms ConsequencesEndothelial dysfunction Impaired vasomotor control of blood flow,microthrombi formation, blood-brain barrierdisruptionImpaired blood flow in microcirculation and limitedoxygen delivery, cerebral edemaFree radical formation Activation of lytic cellular enzymes Neuronal apoptosis and cell deathIntracellular Ca2+ accumulation, Mitochondrial toxicity, activation of cellular lyticenzymesReduced adenosine triphosphate production, celldeath, apoptosisImpaired nitric oxide, Vasoconstriction, “no reflow” Reduced cerebral blood flow, cerebral ischemiaExcitatory neurotransmitter release Glutamate release Excitotoxicity, seizures, apoptosis, cell deathSekhon et al. Critical Care  (2017) 21:90 Page 4 of 10Furthermore, multivariable regression demonstrated thatlower 7-day mean hemoglobin was associated with ad-verse outcome (OR 0.75 per 10 g/L change in hemoglobin,95% CI 0.57–0.97) [33]. Importantly, Ameloot et al. estab-lished a link between hemoglobin and a measure of brainoxygenation in an observational study of 82 patients. Theyfound a linear association between hemoglobin and brain re-gional saturation of oxygen (rSO2) using near-infrared spec-troscopy [34], with hemoglobin <100 g/L being identified asa cutoff for lower rSO2 [34]. Additionally, they demonstratedthat mean hemoglobin concentration <123 g/L was associ-ated with worse neurologic outcome, particularly in pa-tients with rSO2 < 62.5% (OR 2.88, 95% CI 1.02–8.16)[34]. Further research is required to establish an associ-ation between anemia with simultaneous brain hypoxiaand investigate the effect of transfusion thresholds on out-come in HIBI.Carbon dioxidePartial pressure of arterial carbon dioxide (PaCO2) modu-lates cerebrovascular resistance and CBF via its effects onvascular smooth muscle [35]. Specifically, hypocapnia(PaCO2 < 35 mmHg) induces cerebrovascular vasocon-striction and decreases CBF by about 2% to 3% for every1 mmHg of PaCO2 [35]. Clinically, hypocapnia reducesintracranial pressure (ICP) by reducing cerebrovascularvolume [35]. However, sustained hypocapnia can decreaseCBF, increase cerebral oxygen extraction, and induce ische-mia [36, 37]. Conversely, hypercapnia (PaCO2 > 45 mmHg)is a cerebrovascular vasodilator that causes hyperemia, ex-acerbates ICP [38], and reduces CBF [38]. Hypercapnia isalso associated with excitotoxicity and increased cerebraloxygen demand [39]. Importantly, PaCO2 vascular reactiv-ity is preserved after HIBI, making regulation of PaCO2clinically significant and a crucial determinant of CDO2[40]. The optimal PaCO2 in individual patients is notknown but presents a unique opportunity for advancedneurophysiologic monitoring using transcranial Dopplerultrasonography to evaluate CBF, ICP, and cerebrovascularresistance with varying PaCO2 levels in HIBI.Perturbations in PaCO2 in HIBI have been evaluatedin observational studies of HIBI. Roberts et al.conducted a retrospective study of 193 patients and in-vestigated the effects of hypocapnia and hypercapniacompared with normocapnia (PaCO2 35–45 mmHg) onoutcome. They demonstrated a relationship between ad-verse neurologic outcome and both hypocapnia (OR2.43, 95% CI 1.04–5.65) and hypercapnia (OR 2.20, 95%CI 1.03–4.71) [35]. Exposure of hypocapnia and hyper-capnia occurred 36% and 42% of the time after CA [35],respectively, making the exposure of CO2 fluctuationsignificant. The authors followed that study with an ana-lysis of a prospective registry of patients with HIBI andfound a significant association between normocapnia andgood neurologic outcome (OR 4.44, 95% CI 1.33–14.85)[41]. Schneider et al. conducted a large multicenter data-base study of 16,542 patients with HIBI and investigatedthe effects of hypocapnia in HIBI, and they demonstrateda significant association between hospital mortality andhypocapnia (OR 1.12, 95% CI 1.00–1.24) compared withnormocapnia [42]. Given the sound biological plausibilityand available clinical data, regulation of PaCO2 warrantsfurther systematic study to determine the precise optimaltherapeutic strategy after HIBI. Critical links with intracra-nial physiologic parameters pertaining to ICP, CBF, andbrain oxygenation and fluctuations in PaCO2 are logicalfuture goals in this field.Cerebral edemaAfter HIBI, cerebral edema is a recognized complicationthat causes secondary injury. Because of a fixed overallintracranial volume, an increase in the parenchymal bulkfrom cerebral edema in HIBI can cause intracranialhypertension [43] with resultant decreases in cerebralperfusion pressure, CBF, and CDO2 [3]. This viciouscycle of cerebral edema precipitating increased ICPcauses transtentorial herniation and brain death.The origin of cerebral edema occurs as a result ofeither vasogenic or cytotoxic mechanisms. In the earlystages, vasogenic edema emanates from fluid shifts fromthe intravascular to the cerebral interstitial space. Key tothis process, aquaporin-4 is a membrane protein thattransports water across cell membranes in the centralnervous system. Aquaporin-4 proteins are located inperivascular astrocytic endfeet, processes, and ependyma[44]. The aquaporin-4 perivascular pool is identified asthe predominant cluster involved in the pathophysiologyof cerebral edema after HIBI, with increased aquaporin-4 expression occurring within 48 h after the onset ofcerebral ischemia [44]. Interestingly, Nakayama et al.showed that 7.5% hypertonic saline attenuated cerebraledema in a wild-type mouse model of HIBI but had noeffect in an aquaporin-4-knockout model, thereby demon-strating the importance of aquaporin-4 in the patho-physiology of cerebral edema and highlighting itstherapeutic potential [44]. Hypertonic saline administra-tion also restores blood-brain barrier integrity mediatedby aquaporin-4 in the hippocampi, cerebellum, cortex, andbasal ganglia [44]. Furthermore, Nakayama et al. establishedthat achieving serum osmolality >350 mOsm/L with con-tinuous infusion of conivaptan, a V1 and V2 antagonist, at-tenuated cerebral edema [45], thereby demonstrating thatthe effect of aquaporin-4 to decrease cerebral edema occursthrough osmotic gradients, as opposed to a specific intra-venous osmotic agent itself (e.g., 7.5% hypertonic saline).Alternatively, cytotoxic edema originates from cellularmetabolic crisis and intracellular energy depletion.Decreased adenosine triphosphate (Fig. 1) leads toSekhon et al. Critical Care  (2017) 21:90 Page 5 of 10energy-dependent ion channel failure and intracellularsodium and water retention. Rungta et al. establishedthat the Na+Cl− receptor SLC26A11 is a critical modula-tor of intracellular transport of chloride and subsequentcerebral edema after ischemia [46]. The authors showedthat blockade of this receptor attenuated cytotoxic cere-bral edema [46] after HIBI. The role of Na+Cl− receptorantagonism after HIBI is yet to be clarified but repre-sents a future therapeutic target.Furthermore, sulfonylurea receptors are also impli-cated in the pathophysiology of cerebral edema afterischemia. Glyburide, a sulfonylurea receptor inhibitor, at-tenuates malignant cerebral edema after acute middlecerebral infarction [47]. These findings are corroboratedby animal studies that demonstrate sulfonylurea receptorantagonism decreases cerebral edema after neuronalischemia [48].Cerebral autoregulationThe brain has an innate ability to regulate blood flow tomatch metabolic demands. This phenomenon, termedcerebral autoregulation, allows the cerebrovasculature toundergo vasoconstriction and vasodilation over a rangeof mean arterial pressure (MAP) to maintain stable CBF[49]. Cerebral autoregulation mitigates the effects of hy-poperfusion (ischemia) and hyperperfusion [49].The identification of individualized MAP targets afterHIBI using cerebral autoregulation monitoring is anattractive concept that has garnered significant interest.Initially, Nishizawa et al. demonstrated a linear relation-ship between MAP and CBF (as indexed by jugular ven-ous oximetry) [50], suggesting complete dysfunctionalcerebral autoregulation after HIBI. Thereafter, Sundgreenet al. constructed cerebral autoregulation curves forpatients with HIBI by performing stepwise increases inMAP with norepinephrine and simultaneously estimatingCBF with middle cerebral artery velocity on the basis oftranscranial Doppler ultrasonography [51]. Of the 18 pa-tients studied by Sundgreen et al., cerebral autoregulationwas absent in 8 and present in 10 patients. In five of tenpatients with preserved cerebral autoregulation, the lowerlimit of autoregulation was right-shifted with a medianMAP 114 mmHg (range 80–120 mmHg) [51]. This senti-nel study demonstrated the heterogeneous nature of cere-bral autoregulation in patients with HIBI and suggestedthat the lower limit of autoregulation may be significantlyhigher than traditional MAP targets after HIBI.Recently, monitoring with near-infrared spectroscopyhas garnered significant interest as a noninvasivemethod of optimal MAP identification and assessmentof cerebral autoregulation after HIBI. Near-infraredspectroscopy measures the rSO2 in the outermost 2 cmof the frontal lobe, represents the state of oxygenatedhemoglobin in the microvasculature, and approximatesCBF [52]. Therefore, continually integrating fluctuationsbetween MAP and rSO2, a Pearson’s product-momentcorrelation coefficient is generated. This correlationcoefficient (COx) varies between −1 and +1. Positive COxvalues, where there is a positive and linear correlationbetween MAP and rSO2, indicate dysfunctional autoregu-lation [53]. Near-zero and negative COx values indicateintact autoregulation (i.e., rSO2 remains relatively constantdespite varying MAP). The optimal MAP is identified asthe MAP with the lowest value of COx, as shown in Fig. 3.Lee et al. demonstrated that COx identified the lower limitof autoregulation in a swine model of pediatric HIBI [53].Recently, Ameloot et al. retrospectively calculated COxusing MAP and rSO2 to indicate that autoregulation wasintact in 33 of 51 subjects with HIBI. Thereafter, Pham etal. showed that COx was significantly higher in nonsurvi-vors of HIBI than in survivors [54]. Although higher COxwas associated with nonsurvivors, there was noassociation between rSO2 and mortality. Recently, ourresearch team demonstrated feasibility of monitoring COxin real time and identification of optimal MAP prospect-ively in 20 patients after CA [55]. Subjects spent approxi-mately 50% of time outside a ±5 mmHg range from theoptimal MAP, and, importantly, the optimal MAP wasconsistently identified in 19 of 20 subjects. The concept ofindividualized perfusion pressures is emerging as an at-tractive therapeutic target and improved clinical outcomeis associated if actual MAP is maintained within proximityof the identified optimal MAP. It is imperative torecognize the downsides of targeting significantly right-shifted optimal MAP, particularly in patients with com-promised left ventricular function after CA. Increasingafterload on a decompensated left ventricle can dramatic-ally reduce stroke volume and cardiac output, placing theinjured brain at increased risk of ischemia. Therefore,increased MAP targets in HIBI should be weighed againstconcurrent myocardial function. Considerable work re-mains to further delineate if individualized perfusiontargets decrease brain hypoxia and secondary injury andare associated with improved neurologic outcome.TemperatureTargeted temperature management has historically beenthe focus of considerable HIBI research. It is a mainstayin the management of HIBI by mitigating secondaryinjury after CA [56]. At the cellular level, the beneficialeffects of hypothermia are well documented. Cerebralmetabolism is reduced by 5% to 10% per 1 °C decreasein core body temperature. In addition, global carbondioxide production and oxygen consumption are de-creased proportionally to reductions in core bodytemperature [57]. By decreasing cerebral metabolism,hypothermia avoids excessive intracellular anaerobicmetabolism, which leads to increased lactate production.Sekhon et al. Critical Care  (2017) 21:90 Page 6 of 10Hypothermia also improves cerebral glucose use andallows available cellular energy stores to be used fornecessary cellular functions in keeping with neuronalsurvival [56]. Additional benefits of hypothermia includeprevention of apoptosis by decreasing proapoptoticmediators such as p53, tumor necrosis factor α, andcaspase enzymes while increasing expression of antia-poptotic proteins such as Bcl-2 [56, 57]. Hypothermiaalso prevents mitochondrial dysfunction, a key pathwayinvolved in the promotion of apoptosis by release ofcytochrome c oxidase into the cellular cytoplasm [56].Finally, hypothermia decreases inflammatory mediatorssuch as the interleukin-1 family of cytokines [58] as wellas chemotaxis of leukocytes into cerebral interstitialtissue [56], reduces excitotoxic neurotransmitter release(glutamate and glycine) [57], and decreases free radicalproduction after HIBI [57]. Sustained hypothermia alsohas detrimental physiologic effects pertaining to immunesuppression, hemoconcentration, coagulopathy, arrhyth-mias, electrolyte disturbances, and hemodynamic in-stability, which must be weighed against the possiblebenefits [56]. Furthermore, unintentional hypothermiacan occur after CA, indicating possible severe damage tothe key centers of thermoregulation, including the hypo-thalamus [56].Hyperthermia is associated with numerous pathophys-iologic sequelae that are potentially harmful after HIBI.Specifically, hyperthermia may increase blood-brain bar-rier permeability, leading to worsening cerebral edema,ICP,, and cerebral ischemia. Furthermore, hyperthermiaincreases glutamate production, which in turn causesintracellular Ca2+ influx, leading to neuronal cell death,seizures, and further secondary injury [3]. Increasedcerebral metabolism, hyperemic blood flow, and in-creased ICP are additional downstream consequences ofuncontrolled hyperthermia in HIBI [3]. Recently, weshowed that hyperthermia is associated with dysfunc-tional autoregulation in patients with HIBI [55].Clinical studies have established a firm link betweenhypothermia and improved outcome after CA. In 2002,two randomized controlled trials demonstrated markedimprovement in clinical outcomes in patients with CAafter ventricular fibrillation or ventricular tachycardiawho were treated with hypothermia compared withstandard of care [6, 59]. A persistent criticism of bothstudies was that the standard-of-care groups maintainedcore body temperatures >37 °C, thereby exposingpatients to the harmful effects of hyperthermia. Thisprompted a third recent randomized controlled trialcomparing core body temperature control of 36 °C(normothermia) versus 33 °C (hypothermia) after CA[7]. This pragmatic trial included patients with HIBIwith all initial cardiac rhythms and ultimately did notdemonstrate an appreciable benefit of hypothermiaversus normothermia [7]. Importantly, it must be statedthat the maintenance of normothermia at 36 °C after CArequires active cooling. The negative effects of sustainedhyperthermia and adverse outcomes after CA are wellestablished [60, 61], thereby reinforcing the importanceof aggressive core body temperature control in patientsfollowing CA. It is possible that individualized tem-perature targets exist within patients with HIBI, and theinability of current studies to concurrently monitorcerebral metabolism, ICP, and biomarkers of neuronFig. 3 The zone of preserved autoregulation after hypoxic ischemic brain injury appears to be narrowed and right-shifted after cardiac arrest.Within the zone of autoregulation, regional saturation of oxygen (rSO2) is stable owing to the innate vasoconstriction and vasodilation of thecerebral vasculature to maintain stable cerebral blood flow. Outside the zone of autoregulation, a linear relationship exists between rSO2 andmean arterial pressure (MAP). By continually integrating the fluctuations of MAP and rSO2 with one another, a correlation coefficient (COx) canbe generated. The COx approaches negative values or near-zero within the preserved zone of autoregulation, resulting in a U-shaped curve. Thenadir of the U-shaped curve represents the optimal MAP (MAPOPT) for each individual patientSekhon et al. Critical Care  (2017) 21:90 Page 7 of 10degeneration has limited our ability to make thesepatient-specific distinctions.Normobaric hyperoxiaThe dissolved portion of oxygen in plasma is a minorcontributor to overall oxygen content. However, indisease states, this portion may have a pivotal role inensuring adequate hemoglobin saturation for CDO2 andovercome diffusion barriers to restore normal cellularmetabolism. Augmenting arterial oxygen content is tou-ted as a crucial modifiable factor in optimizing CDO2after HIBI, with normobaric hyperoxia being suggestedto achieve this goal.Upon ROSC, reperfusion injury occurs as a result ofoxygen free radical production, which leads to intracellu-lar oxidation [62]. Examples include superoxide (O2−),hydrogen peroxide (H2O2), hydroxyl anion (OH−), andnitrite (NO2−). Endogenous antioxidants balance thegeneration of free radicals and stabilize cellular function.Inadvertent normobaric hyperoxia in HIBI may tip thisbalance in favor of free radical production, cellularoxidation, and neuronal death [62]. Although a system-atic review of animal studies of HIBI suggested thatincreased neuron dysfunction occurs after normobarichyperoxia, there was significant between-study heterogen-eity with respect to ventilation strategies, timing and doseof normobaric hyperoxia, concomitant use of hypothermia,and the chosen primary outcomes [63]. There are alsoseveral reported adverse effects associated with normobarichyperoxia, including increased vascular resistance (cerebral,myocardial, and systemic), decreased CBF, seizures, andincreased release neuronal degeneration biomarkers suchas neuron-specific enolase [57, 62, 64, 65].Researchers in several studies have evaluated normo-baric hyperoxia in HIBI, with conflicting results. Kuismaet al. conducted a randomized study of patients whowere given 21% or 100% inspired oxygen after ROSC[66]. The group that received 21% inspired oxygenexhibited lower serum levels of neuron-specific enolasethan the normobaric hyperoxia group that did notundergo concomitant hypothermia. Kilgannon et al. in-terrogated the Project IMPACT database with more than400,000 patients [67]. They included patients with non-traumatic CA and cardiopulmonary resuscitation within24 h prior to intensive care admission. Their objectivewas to examine the association between hyperoxia andmortality. Compared with the subjects in the normoxiagroup, subjects with normobaric hyperoxia (partial pres-sure of arterial oxygen [PaO2] >300 mmHg) had higher as-sociated in-hospital mortality (OR 1.8, 95% CI 1.5–2.2).Compared with normoxia, hypoxia (PaO2 < 60 mmHg)was also associated with increased in-hospital mortality(OR 1.3, 95% CI 1.1–1.5). Spindelboeck et al. studied nor-mobaric hyperoxia and hypoxemia during CA and foundthat both were associated with increased mortality [68],suggesting that the deleterious effects of normobarichyperoxia may occur in early stages of HIBI. Finally,Bellomo et al. conducted a retrospective analysis ofpatients with CA and demonstrated that normobarichyperoxia and hypoxemia were associated with increasedmortality; however, after adjustment, this relationship wasno longer significant [69]. Importantly, significant limita-tions in methodology should be noted, particularly theretrospective nature of these studies, the limitation ofusing mortality as a primary outcome in a brain injurypopulation, and the fact that the definition of normobarichyperoxia with a single PaO2 > 300 mmHg does not cap-ture the true biological exposure of patients to normobarichyperoxia after CA. Furthermore, hypothermia was notroutinely used in the aforementioned studies.Additional retrospective analyses investigating the use ofnormobaric hyperoxia with concomitant hypothermiahave addressed this shortcoming. Janz et al. demonstratedan association between adverse neurologic outcome andnormobaric hyperoxia administration [70]. These resultsare contrasted by those reported by Ihle et al. and Lee etal., who failed to show an association between normobarichyperoxia and adverse neurologic outcome with concomi-tant hypothermia [71, 72]. Thereafter, a prospective studyrevealed an association between favorable neurologic out-come and higher mean PaO2 [73]. Thus, concomitanthypothermia may play a role in modifying the deleteriouseffects of normobaric hyperoxia in HIBI.ConclusionsHIBI pathophysiology is complex, with a significant con-tribution attributable to secondary injury. Researchershave investigated the effects of interventions aimed atpreventing secondary injury, most notably hypothermia.Future targets of research include individualized perfu-sion targets, normobaric hyperoxia, transfusion triggers,and PaCO2 goals.AbbreviationsATP: Adenosine triphosphate; CA: Cardiac arrest; CBF: Cerebral blood flow;CDO2: Cerebral oxygen delivery; CMRO2: Cerebral metabolic rate of oxygenuptake; Cox: Correlation coefficient; CPP: Cerebral perfusion pressure;HIBI: Hypoxic ischemic brain injury; ICP: Intracranial pressure; MAP: Meanarterial pressure; PaCO2: Partial pressure of arterial carbon dioxide; PaO2: Partialpressure of arterial oxygen; RBC: Red blood cells; ROSC: Return of spontaneouscirculation; rSO2: Regional saturation of oxygen; WBC: White blood cellsAcknowledgementsWe acknowledge our colleagues in the intensive care unit at VancouverGeneral Hospital for their insightful guidance.FundingMSS is funded by Vancouver Coastal Health Research Institute. DEG is fundedby the VGH & UBC Hospital Foundation Best of Health Fund.Availability of data and materialsNot applicable.Sekhon et al. Critical Care  (2017) 21:90 Page 8 of 10Authors’ contributionsMSS contributed the majority of the manuscript preparation and backgroundresearch. PNA contributed to the manuscript preparation. DEG contributed tothe manuscript preparation. All authors read and approved the final manuscript.Competing interestsThe authors declare that they have no competing interests.Authors’ informationNot applicable.Consent for publicationNot applicable.Ethics approval and consent to participateNot applicable.Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.Author details1Division of Critical Care Medicine, Department of Medicine, VancouverGeneral Hospital, University of British Columbia, Room 2438, Jim PattisonPavilion, 2nd Floor, 855 West 12th Avenue, Vancouver, BC V5Z 1M9, Canada.2Centre for Heart, Lung and Vascular Health, School of Health and ExerciseSciences, University of British Columbia Okanagan, Kelowna, BC, Canada.3Department of Anaesthesiology, Pharmacology and Therapeutics, VancouverGeneral Hospital, University of British Columbia, West 12th Avenue,Vancouver, BC V5Z 1M9, Canada. 4Centre for Clinical Epidemiology andEvaluation, Vancouver Coastal Health Research Institute, University of BritishColumbia, 899 West 12th Avenue, Vancouver BC V5Z 1M9, Canada.References1. 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