{"Affiliation":[{"label":"Affiliation","value":"Science, Faculty of","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","classmap":"vivo:EducationalProcess","property":"vivo:departmentOrSchool"},"iri":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","explain":"VIVO-ISF Ontology V1.6 Property; The department or school name within institution; Not intended to be an institution name."},{"label":"Affiliation","value":"Zoology, Department of","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","classmap":"vivo:EducationalProcess","property":"vivo:departmentOrSchool"},"iri":"http:\/\/vivoweb.org\/ontology\/core#departmentOrSchool","explain":"VIVO-ISF Ontology V1.6 Property; The department or school name within institution; Not intended to be an institution name."}],"AggregatedSourceRepository":[{"label":"AggregatedSourceRepository","value":"DSpace","attrs":{"lang":"en","ns":"http:\/\/www.europeana.eu\/schemas\/edm\/dataProvider","classmap":"ore:Aggregation","property":"edm:dataProvider"},"iri":"http:\/\/www.europeana.eu\/schemas\/edm\/dataProvider","explain":"A Europeana Data Model Property; The name or identifier of the organization who contributes data indirectly to an aggregation service (e.g. Europeana)"}],"Campus":[{"label":"Campus","value":"UBCV","attrs":{"lang":"en","ns":"https:\/\/open.library.ubc.ca\/terms#degreeCampus","classmap":"oc:ThesisDescription","property":"oc:degreeCampus"},"iri":"https:\/\/open.library.ubc.ca\/terms#degreeCampus","explain":"UBC Open Collections Metadata Components; Local Field; Identifies the name of the campus from which the graduate completed their degree."}],"Creator":[{"label":"Creator","value":"Osborne, Salma (Sally)","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/creator","classmap":"dpla:SourceResource","property":"dcterms:creator"},"iri":"http:\/\/purl.org\/dc\/terms\/creator","explain":"A Dublin Core Terms Property; An entity primarily responsible for making the resource.; Examples of a Contributor include a person, an organization, or a service."}],"DateAvailable":[{"label":"DateAvailable","value":"2010-09-09T17:14:15Z","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/issued","classmap":"edm:WebResource","property":"dcterms:issued"},"iri":"http:\/\/purl.org\/dc\/terms\/issued","explain":"A Dublin Core Terms Property; Date of formal issuance (e.g., publication) of the resource."}],"DateIssued":[{"label":"DateIssued","value":"1988","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/issued","classmap":"oc:SourceResource","property":"dcterms:issued"},"iri":"http:\/\/purl.org\/dc\/terms\/issued","explain":"A Dublin Core Terms Property; Date of formal issuance (e.g., publication) of the resource."}],"Degree":[{"label":"Degree","value":"Master of Science - MSc","attrs":{"lang":"en","ns":"http:\/\/vivoweb.org\/ontology\/core#relatedDegree","classmap":"vivo:ThesisDegree","property":"vivo:relatedDegree"},"iri":"http:\/\/vivoweb.org\/ontology\/core#relatedDegree","explain":"VIVO-ISF Ontology V1.6 Property; The thesis degree; Extended Property specified by UBC, as per https:\/\/wiki.duraspace.org\/display\/VIVO\/Ontology+Editor%27s+Guide"}],"DegreeGrantor":[{"label":"DegreeGrantor","value":"University of British Columbia","attrs":{"lang":"en","ns":"https:\/\/open.library.ubc.ca\/terms#degreeGrantor","classmap":"oc:ThesisDescription","property":"oc:degreeGrantor"},"iri":"https:\/\/open.library.ubc.ca\/terms#degreeGrantor","explain":"UBC Open Collections Metadata Components; Local Field; Indicates the institution where thesis was granted."}],"Description":[{"label":"Description","value":"In this study I examined the effects of progressive hypothermia on minute ventilation, metabolic rate and the ventilatory responses to hypercapnia and hypoxia in the golden-mantled ground squirrel (Spermoohilus lateral is) and the laboratory rat (Rattus norveqicus). These experiments were designed to test the hypothesis that reductions in minute ventilation with progressive body cooling in this species are independant of seasonal changes associated with hibernation and are the result of and therefore parallel the changes in metabolic rate. Similar experiments were carried on the laboratory rat to test the scope of this hypothesis in a non-hibernating mammalian species. Minute ventilation was measured by pneumotachography and carbon dioxide production was measured as an index of metabolic rate. The \"helox-cold\" method was used to induce progressive hypothermia from 36 to 27\u00b0C body temperature under a constant functional plane of halothane anesthesia chosen to suppress shivering. Progressive hypothermia was studied in the ground squirrels during the non-hibernating season and in the laboratory rat throughout the year.\r\nDuring normothermia, breathing frequency and metabolic rate were approximatley 60% lower in the golden-mantled ground squirrel compared to the rat. In both species. however, hypothermia resulted in proportional decreases in minute ventilation, breathing frequency and metabolic rate. The inspiratory flow rate, an index of respiratory drive was also reduced with decreasing body temperature and showed a similar linear relationship with the ventilatory requirement of each species at any given body temperature. A gradual decrease in duty cycle was observed in both species which was significant only at lower levels of minute ventilation. Breathing remained rhythmic throughout hypothermia although apneic periods occured between breaths at body temperatures below 31\u00b0C. The slopes of the ventilatory responses to hypoxia and hypercapnla in the ground squirrel were decreased in proportion to the decreases in minute ventilation and metabolic rate. Ventilatory sensitivity in the rat, however, was not altered.\r\nThese results demonstrate that ventilation and metabolic rate are tightly coupled during hypothermia. In addition both species decrease their minute ventilation to match reduced metabolic demands by decreasing breathing frequency alone. Tidal volume is not altered by decreases in body temperature, presumably to ensure adequate alveolar ventilation. The temperature coefficient (Q\u2081\u2080) determined in the present study for the effect of body temperature on minute ventilation and metabolic rate in the golden-mantled ground squirrel is similar to that obtained during hibernation in the same species (McArthur, 1986 and Webb, 1987). Consequently, the exponential equations defining the drop in minute ventilation and metabolic rate during progressive hypothermia in the present study accurately predict the values observed at 7\u00b0C body temperature during hibernation. It is therefore concluded that ventilation at reduced body temperatures is regulated independantly of the physiological processes that are unique to hibernation and is simply coupled to the metabolic demand of the ground squirrel.","attrs":{"lang":"en","ns":"http:\/\/purl.org\/dc\/terms\/description","classmap":"dpla:SourceResource","property":"dcterms:description"},"iri":"http:\/\/purl.org\/dc\/terms\/description","explain":"A Dublin Core Terms Property; An account of the resource.; Description may include but is not limited to: an abstract, a table of contents, a graphical representation, or a free-text account of the resource."}],"DigitalResourceOriginalRecord":[{"label":"DigitalResourceOriginalRecord","value":"https:\/\/circle.library.ubc.ca\/rest\/handle\/2429\/28318?expand=metadata","attrs":{"lang":"en","ns":"http:\/\/www.europeana.eu\/schemas\/edm\/aggregatedCHO","classmap":"ore:Aggregation","property":"edm:aggregatedCHO"},"iri":"http:\/\/www.europeana.eu\/schemas\/edm\/aggregatedCHO","explain":"A Europeana Data Model Property; The identifier of the source object, e.g. the Mona Lisa itself. This could be a full linked open date URI or an internal identifier"}],"FullText":[{"label":"FullText","value":"EFFECTS OF HYPOTHERMIA ON VENTILATION AND VENTILATORY RESPONSES TO HYPERCAPNIA AND HYPOXIA IN THE GOLDEN-MANTLED GROUND SQUIRREL AND THE MISTAR RAT By SALMA (SALLY) OSBORNE nee VESSAL B.Sc, The University of B r i t i s h Columbia, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology) We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER 1988 \u00a9 Salma Osborne, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of \/ ^ r f f ^ y The University of Ornish Columbia Vancouver, Canada Date M*- fhtheA I .ft? DE-6 (2788) i i ABSTRACT I n t h i s s tudy I examined the e f f e c t s o f p r o g r e s s i v e hypothermia on minute v e n t i l a t i o n , m e t a b o l i c r a t e and the v e n t i l a t o r y r e s p o n s e s to h y p e r c a p n i a and h y p o x i a i n the g o l d e n - m a n t l e d ground s q u i r r e l ( S p e r m o o h i l u s l a t e r a l i s ) and the l a b o r a t o r y r a t ( R a t t u s n o r v e q i c u s ) . These ex p e r i m e n t s were d e s i g n e d to t e s t the h y p o t h e s i s t h a t r e d u c t i o n s i n minute v e n t i l a t i o n w i t h p r o g r e s s i v e body c o o l i n g i n t h i s s p e c i e s are independant o f s e a s o n a l changes a s s o c i a t e d w i t h h i b e r n a t i o n and are the r e s u l t o f and t h e r e f o r e p a r a l l e l the changes i n m e t a b o l i c r a t e . S i m i l a r e x p e r i m e n t s were c a r r i e d on the l a b o r a t o r y r a t to t e s t the scope o f t h i s h y p o t h e s i s i n a n o n - h i b e r n a t i n g mammalian s p e c i e s . Minute v e n t i l a t i o n was measured by pneumotachography and c a r b o n d i o x i d e p r o d u c t i o n was measured as an index o f m e t a b o l i c r a t e . The \" h e l o x - c o l d \" method was used to induce p r o g r e s s i v e hypothermia from 36 to 27 \u00b0C body temperature under a c o n s t a n t f u n c t i o n a l p l a n e o f h a l o t h a n e a n e s t h e s i a chosen to s u p p r e s s s h i v e r i n g . P r o g r e s s i v e h y p o thermia was s t u d i e d i n the ground s q u i r r e l s d u r i n g the n o n - h i b e r n a t i n g season and i n the l a b o r a t o r y r a t throughout the y e a r . D u r i n g normothermia, b r e a t h i n g f r e q u e n c y and m e t a b o l i c r a t e were a p p r o x i m a t l e y 60% lower i n the g o l d e n - m a n t l e d ground s q u i r r e l compared t o the r a t . I n both s p e c i e s . i i i however, hypothermia r e s u l t e d i n p r o p o r t i o n a l d e c r e a s e s i n minute v e n t i l a t i o n , b r e a t h i n g frequency and m e t a b o l i c r a t e . The i n s p i r a t o r y f l o w r a t e , an index o f r e s p i r a t o r y d r i v e was a l s o reduced w i t h d e c r e a s i n g body temperature and showed a s i m i l a r l i n e a r r e l a t i o n s h i p w i t h the v e n t i l a t o r y r e q u i r e m e n t o f each s p e c i e s a t any g i v e n body temperature. A g r a d u a l d e c r e a s e i n duty c y c l e was obser v e d i n both s p e c i e s which was s i g n i f i c a n t o n l y a t lower l e v e l s o f minute v e n t i l a t i o n . B r e a t h i n g remained r h y t h m i c t h r o u g h o u t hypothermia a l t h o u g h a p n e i c p e r i o d s o c c u r e d between b r e a t h s a t body temperatures below 31 \u00b0C. The s l o p e s o f the v e n t i l a t o r y r e s p o n s e s to h y p o x i a and h y p e r c a p n l a i n the ground s q u i r r e l were d e c r e a s e d i n p r o p o r t i o n to the d e c r e a s e s i n minute v e n t i l a t i o n and m e t a b o l i c r a t e . V e n t i l a t o r y s e n s i t i v i t y i n the r a t , however, was not a l t e r e d . These r e s u l t s demonstrate t h a t v e n t i l a t i o n and m e t a b o l i c r a t e a re t i g h t l y c o u p l e d d u r i n g hypothermia. I n a d d i t i o n b o t h s p e c i e s d e c r e a s e t h e i r minute v e n t i l a t i o n t o match reduced m e t a b o l i c demands by d e c r e a s i n g b r e a t h i n g f r e q u e n c y a l o n e . T i d a l volume i s not a l t e r e d by d e c r e a s e s i n body t e m p e r a t u r e , presumably to ensure adequate a l v e o l a r v e n t i l a t i o n . The temperature c o e f f i c i e n t (Qio.) d e t e r m i n e d i n the p r e s e n t s t u d y f o r the e f f e c t o f body temperature on minute v e n t i l a t i o n and m e t a b o l i c r a t e i n the go l d e n - m a n t l e d ground s q u i r r e l i s s i m i l a r to t h a t o b t a i n e d d u r i n g h i b e r n a t i o n i n the same s p e c i e s ( M c A r t h u r , 1986 and Webb, i v 1987). C o n s e q u e n t l y , the e x p o n e n t i a l e q u a t i o n s d e f i n i n g the drop i n minute v e n t i l a t i o n and m e t a b o l i c r a t e d u r i n g p r o g r e s s i v e hypothermia i n the p r e s e n t s t u d y a c c u r a t e l y p r e d i c t the v a l u e s o b s e r v e d a t 7\u00b0C body temperature d u r i n g h i b e r n a t i o n . I t i s t h e r e f o r e c o n c l u d e d t h a t v e n t i l a t i o n a t reduced body temp e r a t u r e s i s r e g u l a t e d i n d e p e n d a n t l y o f the p h y s i o l o g i c a l p r o c e s s e s t h a t are unique t o h i b e r n a t i o n and i s s i m p l y c o u p l e d to the m e t a b o l i c demand o f the ground s q u i r r e l . V TABLE OF CONTENTS page A b s t r a c t i i Table o f c o n t e n t s v L i s t o f T a b l e s v i L i s t o f F i g u r e s v i i Acknowledgements x i Chapter One: G e n e r a l I n t r o d u c t i o n 1 Chapter Two: V e n t i l a t i o n d u r i n g p r o g r e s s i v e h y p o t hermia I n t r o d u c t i o n 12 M a t e r i a l s and Methods 17 R e s u l t s 27 D i s c u s s i o n 63 Chapter Three: E f f e c t o f hypothermia on the v e n t i l a t o r y r e s p o n s e s to changes i n i n s p i r e d C O 2 and O 2 c o n c e n t r a t i o n s . I n t r o d u c t i o n 74 M a t e r i a l s and Methods 76 R e s u l t s 80 D i s c u s s i o n 105 Chapter Four: G e n e r a l D i s c u s s i o n 112 L i t e r a t u r e C i t e d 125 Appendix I 133 v i LIST OF TABLES page Table I : E f f e c t o f l o w e r i n g l e v e l s o f h a l o t h a n e 19 a n e s t h e s i a on v e n t i l a t i o n and C O 2 p r o d u c t i o n a t 36 and 27\u00b0C body temperature i n the r a t and ground s q u i r r e l . Table I I : R e s p i r a t o r y t i m e s , duty c y c l e and r e s p i r a t o r y 36 d r i v e i n normothermic r a t s and s q u i r r e l s . Table I I I : E f f e c t s o f changes i n body temperature on the 69 average v a l u e s o f v e n t i l a t o r y and m e t a b o l i c v a r i a b l e s i n ground s q u i r r e l s and r a t s . Table IV: L i n e a r r e g r e s s i o n a n a l y s i s o f the r e l a t i o n - 89 s h i p between minute v e n t i l a t i o n and the i n s p i r e d O 2 and C O 2 f r a c t i o n i n r a t s and gold e n - m a n t l e d ground s q u i r r e l s a t 36 and 27\u00b0C body temperature. Table V: E f f e c t s o f changes i n body temperature on the 115 average v a l u e s o f v e n t i l a t o r y and m e t a b o l i c v a r i a b l e s i n hy p o t h e r m i c and h i b e r n a t i n g g o l d e n mantled ground s q u i r r e l s . v i i LIST OF FIGURES page F i g u r e 1. Schematic diagram o f the e x p e r i m e n t a l s e t u p used to induce hypothermia and measure v e n t i l a t i o n and m e t a b o l i c r a t e i n S. l a t e r a l i s and n o r v e q i c u s 2 3 F i g u r e 2. R e p r e s e n t a t i v e b r e a t h i n g t r a c e s o b t a i n e d by pneumotachography i n ground s q u i r r e l s and r a t s d u r i n g normothermia and hypothermia 29 F i g u r e 3. Comparison o f v e n t i l a t i o n d u r i n g normothermia and p r o g r e s s i v e hypothermia i n r a t s and ground s q u i r r e l s 31 F i g u r e 4. Comparison o f t i d a l volume d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia i n r a t s and ground s q u i r r e l s 33 F i g u r e 5. Comparison o f b r e a t h i n g f r e q u e n c y d u r i n g normothermia and p r o g r e s s i v e hypothermia i n r a t s and ground s q u i r r e l s 35 F i g u r e 6. Comparison o f CO2 p r o d u c t i o n d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia i n r a t s and ground s q u i r r e l s 39 F i g u r e 7. Comparison o f the a i r c o n v e c t i o n r e q u i r e m e n t ( V E \/ V C O 2 ^ d u r i n g normothermia and p r o g r e s s i v e h y p o t h ermia i n r a t s and ground s q u i r r e l s 41 F i g u r e 8. Comparison o f v e n t i l a t i o n d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia e x p r e s s e d as p e r c e n t o f the normothermic v a l u e a t 36\u00b0C body temperature i n r a t s and ground s q u i r r e l s 43 F i g u r e 9. Comparison o f t i d a l volume d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia e x p r e s s e d as p e r c e n t o f the normothermic v a l u e a t 36\u00b0C body temperature i n r a t s and ground s q u i r r e l s 46 v i i i page F i g u r e 10. Comparison o f b r e a t h i n g f r e q u e n c y d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia e x p r e s s e d as p e r c e n t o f the normothermic v a l u e a t 36\u00b0C body temperature i n r a t s and ground s q u i r r e l s 48 F i g u r e 11. Comparison o f C O 2 p r o d u c t i o n d u r i n g normothermia and p r o g r e s s i v e h y p o t hermia e x p r e s s e d as p e r c e n t o f the normothermic v a l u e a t 36\u00b0C body temperature i n r a t s and ground s q u i r r e l s 50 F i g u r e 12. Comparison o f the a i r c o n v e c t i o n r e q u i r e m e n t ( V E \/ V * C O 2 ^ d u r i n g normothermia and p r o g r e s s i v e h y p o t h ermia e x p r e s s e d as p e r c e n t o f the normothermic v a l u e a t 36\u00b0C body temperature i n r a t s and ground s q u i r r e l s 52 F i g u r e 13. R e l a t i o n s h i p between t i d a l volume, and r e s p i r a t o r y t i m e s a t 3 d i f f e r e n t body t e m p e r a t u r e s i n the r a t and ground s q u i r r e l 55 F i g u r e 14. R e l a t i o n s h i p between duty c y c l e ( T J \/ T T O T ^ and minute v e n t i l a t i o n i n ground s q u i r r e l s and r a t s a t 36, 31 and 27\u00b0 C body temperature 57 F i g u r e 15. Comparison o f r e s p i r a t o r y times as a f r a c t i o n o f the t o t a l b r e a t h d u r a t i o n i n ground s q u i r r e l s and r a t s a t 36, 31, and 27 \u00b0C 59 F i g u r e 16. R e l a t i o n s h i p between c e n t r a l r e s p i r a t o r y d r i v e ( V f \/ T j ) and minute v e n t i l a t i o n i n ground s q u i r r e l s and r a t s a t t h r e e d i f f e r e n t body t e m p e r a t u r e s 62 F i g u r e 17. Hey p l o t r e p r e s e n t i n g the c o n t r i b u t i o n o f t i d a l volume and b r e a t h i n g f r e q u e n c y to minute v e n t i l a t i o n a t 36, 31 and 27\u00b0C body temperature i n the W i s t a r r a t and the g o l d e n - m a n t l e d ground s q u i r r e l 72 F i g u r e 18. Comparison between the e f f e c t s o f i n c r e a s i n g l e v e l s o f F J C O 2 o n v e n t i l a t i o n o f the W i s t a r r a t and the g o l d e n mantled ground s q u i r r e l a t 36 and 27 \u00b0C body temperature 82 i x page F i g u r e 19. Comparison between the e f f e c t s o f i n c r e a s i n g l e v e l s o f F T C C-2 on t i d a l volume o f the W i s t a r r a t and the go l d e n - m a n t l e d ground s q u i r r e l a t 36 and 2 7 \u00b0 C body temperature 84 F i g u r e 20. Comparison between the e f f e c t s o f i n c r e a s i n g l e v e l s o f Fj\u00a302 o n b r e a t h i n g f r e q u e n c y o f the W i s t a r r a t and the go l d e n - m a n t l e d ground s q u i r r e l a t 36 and 27\u00b0C body temperature 86 F i g u r e 21. The r e l a t i v e c o n t r i b u t i o n o f r e s p i r a t o r y phase d u r a t i o n s i n the normothermic (Tb=36\u00b0C) and hypothermic(Tj3=27 \u00b0C)rat a t i n c r e a s i n g l e v e l s o f i n s p i r e d CO2 88 F i g u r e 22. The r e l a t i v e c o n t r i b u t i o n o f r e s p i r a t o r y phase d u r a t i o n s i n the normothermic (Tt,=36',C) and h y p o t h e r m i c (1^=27*C) ground s q u i r r e l a t i n c r e a s i n g l e v e l s o f i n s p i r e d CO2 92 F i g u r e 23. Comparison between the e f f e c t s o f d e c r e a s i n g l e v e l s o f FjQ2 o n v e n t i l a t i o n o f the W i s t a r r a t and the g o l d e n - m a n t l e d ground s q u i r r e l a t 36 and 27 \u00b0C body temperature 94 F i g u r e 24. Comparison between the e f f e c t s o f i n c r e a s i n g l e v e l s o f FjQ2 o n b r e a t h i n g f r e q u e n c y o f the W i s t a r r a t and the g o l d e n mantled ground s q u i r r e l a t 36 and 27\u00b0C body temperature 97 F i g u r e 25. Comparison between the e f f e c t s o f i n c r e a s i n g l e v e l s o f F J Q 2 o n t i d a l volume o f the W i s t a r r a t and the g o l d e n mantled ground s q u i r r e l a t 36 and 27\u00b0C body temperature 99 F i g u r e 26. The r e l a t i v e c o n t r i b u t i o n o f r e s p i r a t o r y phase d u r a t i o n s i n the normothermic (Tb=36\u00b0C) and h y p o t h e r m i c (Tb=27\u00b0C) r a t a t d e c r e a s i n g l e v e l s o f i n s p i r e d O2 101 F i g u r e 27. The r e l a t i v e c o n t r i b u t i o n o f r e s p i r a t o r y phase d u r a t i o n s i n the normothermic (Tb=36\u00b0C and h y p o t h e r m i c (Tb=27\u00b0C) ground s q u i r r e l a t d e c r e a s i n g l e v e l s o f i n s p i r e d O2 103 X page F i g u r e 28. Mean v a l u e s f o r minute v e n t i l a t i o n o b t a i n e d d u r i n g hypothermia and h i b e r n a t i o n i n the golden - m a n t l e d ground s q u i r r e l 116 F i g u r e 29. Mean v a l u e s f o r CO2 p r o d u c t i o n o b t a i n e d d u r i n g hypothermia and h i b e r n a t i o n i n the golden - m a n t l e d ground s q u i r r e l , 118 F i g u r e 30. R e l a t i o n s h i p between t i d a l volume and r e s p i r a t o r y t imes i n the go l d e n - m a n t l e d ground s q u i r r e l d u r i n g p r o g r e s s i v e hypothermia a t 36,31 and 27\u00b0C body temperature and a t 36 and 7\u00b0C d u r i n g h i b e r n a t i o n 120 F i g u r e 31. R e l a t i o n s h i p between c e n t r a l r e s p i r a t o r y d r i v e and minute v e n t i l a t i o n i n go l d e n - m a n t l e d ground s q u i r r e l s and r a t s d u r i n g p r o g r e s s i v e h y p o t h ermia and i n the go l d e n - m a n t l e d ground s q u i r r e l d u r i n g h i b e r n a t i o n 123 x i ACKNOWLEDGEMENTS I am g r a t e f u l to my s u p e r v i s o r . Dr. W i l l i a m Kenneth Milsom, f o r h i s s u p p o r t , a d v i c e , p a t i e n c e and f r e q u e n t encouragement thr o u g h o u t t h i s s t u d y . Thanks are due to Bruce G i l l e s p i f o r the b u i l d i n g and numerous m o d i f i c a t i o n s o f the h y p o t h e r m i c chamber. I would l i k e t o thank my f r i e n d and c o l l e a g u e Heather Ann McLean f o r her s u p p o r t and e x p e d i e n t sense o f humor a t the e a r l y s t a g e s o f t h i s s t u d y . I am i n d e b t e d to my f a m i l y i n p a r t i c u l a r my husband, D a v i d N e l s o n Osborne f o r h i s unending s u p p o r t and p a t i e n c e ; my mother, P a r v i n S h e i b a n i - V e s s a l , f o r the many hours o f b a b y s i t t i n g and to my d a u g h t e r , Simone K i m b e r l y Osborne, f o r w a i t i n g . . . . 1 CHAPTER ONE GENERAL INTRODUCTION Dra m a t i c changes i n minute v e n t i l a t i o n and r e s p i r a t o r y p a t t e r n have been r e c o r d e d i n ground s q u i r r e l s d u r i n g h i b e r n a t i o n (Lyman, 1951; Landau and Dawe, 1958; Hammel e_t a l . , 1968). The c o n t i n u o u s v e n t i l a t i o n seen i n e u t h e r m i c ground s q u i r r e l s i n summer i s s i g n i f i c a n t l y reduced d u r i n g h i b e r n a t i o n i n w i n t e r and c o n v e r t e d to p e r i o d i c p a t t e r n s o f b r e a t h i n g . S u r p r i s i n g l y , as l o n g as these a n i m a l s remain e u t h e r m i c , v e n t i l a t i o n and v e n t i l a t o r y r e s p o n s e s to h y p o x i a and h y p e r c a p n i a remain c o n s t a n t over the ambient temperature range o f 5-25 \u00b0C th r o u g h o u t the year ( M c A r t h u r , 1986). T h i s s u g g e s t s t h a t the pr o f o u n d changes seen i n v e n t i l a t i o n d u r i n g h i b e r n a t i o n a r e not a f u n c t i o n o f the s e a s o n a l changes a s s o c i a t e d w i t h h i b e r n a t i o n per se but may merely r e f l e c t the d e c r e a s e s i n m e t a b o l i c r a t e and body temperature (TJ-J) t h a t o c c u r d u r i n g h i b e r n a t i o n . I f changes which o c c u r d u r i n g h i b e r n a t i o n a r e a d i r e c t consequence o f changes i n m e t a b o l i c r a t e , then minute v e n t i l a t i o n and m e t a b o l i c r a t e s h o u l d d e c r e a s e p r o p o r t i o n a l l y w i t h p r o g r e s s i v e h y p o t h e r m i a . F u r t h e r m o r e , changes i n v e n t i l a t o r y s e n s i t i v i t y t o h y p o x i c and h y p e r c a p n i c s t i m u l i s h o u l d be s i m i l a r whether m e t a b o l i c r a t e d e c r e a s e s due to h i b e r n a t i o n or hypothermia. The purpose o f t h i s s t u d y was t o t e s t t h i s h y p o t h e s i s i n the -2-g o l d e n mantled ground s q u i r r e l ( S p e r m o p h i l u s l a t e r a l i s ) and i t s u n i v e r s a l scope, i n a n o n - h i b e r n a t i n g r o d e n t o f s i m i l a r body w e i g h t , the l a b o r a t o r y r a t ( R a t t u s n o r v e g i c u s ) . Hypothermia I n homeotherms, c o r e t e m p e r a t u r e s more than one s t a n d a r d d e v i a t i o n below normal a re c o n s i d e r e d h y p o t h e r m i c . I n e u t h e r i a n mammals t h i s l e v e l would be a t or near 35\u00b0C ( B l i g h and Johnson, 1973). Except f o r mammals which e n t e r t o r p o r or h i b e r n a t i o n , such d e c r e a s e s i n body temperature do not o c c u r n o r m a l l y . The a b i l i t y o f mammalian s p e c i e s to s u r v i v e h y p o t h ermia depends on how s u c c e s s f u l l y they meet h o m e o s t a t i c demands, p r i m a r i l y those on r e s p i r a t o r y and c a r d i o v a s c u l a r f u n c t i o n , under these c o n d i t i o n s . A d i s t i n g u i s h i n g f e a t u r e o f sev e r e hypothermia i n s p e c i e s which do not undergo t o r p o r or h i b e r n a t i o n i s t h a t h e a r t and l u n g f a i l u r e o c c u r i n tandem. C h a r a c t e r i s t i c a l l y , t h e r e i s a f a l l i n minute v e n t i l a t i o n w i t h p r o g r e s s i v e body c o o l i n g , c u l m i n a t i n g i n r e s p i r a t o r y f a i l u r e f o l l o w e d by c a r d i a c a r r e s t (Adolph e_t al_. , 1961; Anderson \u00a7_t al_. , 1971; P o p o v i c and P o p o v i c , 1974; Musa c c h i a , 1984) . The h y p o t h e r m i c l i m i t f o r spontaneous b r e a t h i n g i n l a r g e u n a n e s t h e t i z e d mammals such as man and dog d u r i n g c o l d water immersion ranges between 24-29 \u00b0C ( B i g e l o w , 1955; Hegnauer, 1959). The mean l e t h a l body temperature o f a p p r o x i m a t e l y -3-25 \u00b0 C i n mammalian s p e c i e s can be s u b s t a n t i a l l y lowered by use o f a n e s t h e t i c s and r e s p i r a t o r y a s s i s t a n c e d u r i n g the co u r s e o f c o o l i n g . Most s t u d i e s e x a m i n i n g hypothermia i n mammals have been i n t e r e s t e d i n maximum l o w e r i n g o f body temperature and s u r v i v a l times a s s o c i a t e d w i t h low body te m p e r a t u r e s where a r t i f i c i a l r e s p i r a t i o n i s e i t h e r n e c e s s a r y or b e n e f i c i a l . Thus, few s t u d i e s i n v o l v i n g reduced body temperature i n mammals have measured the e f f e c t s o f hypothermia on spontaneous v e n t i l a t o r y p a t t e r n s and t h e i r c o n t r o l . I t i s g e n e r a l l y a c c e p t e d , however, t h a t h y p o t hermia has a number o f major e f f e c t s on r e s p i r a t i o n . I f s h i v e r i n g i s s u p p r e s s e d , consumption o f O2 ( V Q 2 ) a n d p r o d u c t i o n o f C02 ( V Q O 2 5 d e c r e a s e p r o g r e s s i v e l y as body temperature d e c r e a s e s and r e s p i r a t o r y r a t e , t i d a l volume, minute volume, v i t a l c a p a c i t y and a l v e o l a r v e n t i l a t i o n a r e reduced ( R o s e n f e l d , 1963; Hervey, 1973; Cooper and V e a l e , 1986). There i s an i n c r e a s e i n blood-g a s s o l u b i l i t y and a n a t o m i c a l dead space as w e l l as a l e f t s h i f t i n the oxygen e q u i l i b r i u m c u r v e ( S e v e r i n g h a u s , 1955; 1963). A d e f i n i t i v e d e s c r i p t i o n o f the changes which o c c u r i n the c o n t r o l o f b r e a t h i n g d u r i n g body c o o l i n g remain e l u s i v e however, s i n c e the i n t e r p r e t a t i o n o f r e s u l t s from the few s t u d i e s t h a t do e x i s t i s c o m p l i c a t e d by the presence o f d r u g s , s h i v e r i n g and\/or trauma. _ 4 -Presence o f A n e s t h e t i c s and S h i v e r i n g d u r i n g Hypothermia Most g e n e r a l a n e s t h e t i c s are known to produce r a p i d , s h a l l o w b r e a t h i n g ; d e p r e s s the t o t a l o u t p u t o f the r e s p i r a t o r y pump and cause a dose dependant d e c r e a s e i n the v e n t i l a t o r y response to c h a n g i n g l e v e l s o f a r t e r i a l O2 and CO2 (Ngai aj\u201e., 1965; B o r i s o n , 1981; Berkenbosch e t a l , , 1982; P a v l i n and H o r n b e i n , 1986). I n a d d i t i o n to t h i s , v a r i a t i o n s i n the depth o f a n e s t h e s i a can a l t e r the l e v e l o f s h i v e r i n g t hermogenesis d u r i n g hypothermia which i n t u r n a f f e c t s v e n t i l a t i o n . S h i v e r i n g , l i k e r e s p i r a t i o n , has an u n u s u a l type o f c o n t r o l by the c e n t r a l nervous system. I t i s an i n v o l u n t a r y r esponse o f s k e l e t a l muscles which a r e u s u a l l y under v o l u n t a r y c o n t r o l . L i k e r e s p i r a t i o n , s h i v e r i n g can o c c u r i n d e c e r e b r a t e a n i m a l s and can be r e g u l a t e d w i t h i n l i m i t s by v o l u n t a r y c o n t r o l . S i n c e s h i v e r i n g i s caused by r h y t h m i c muscle c o n t r a c t i o n , i t s r e s p i r a t o r y and m e t a b o l i c e f f e c t s resemble those o f l i g h t e x e r c i s e . Both v e n t i l a t i o n and metabolism can i n c r e a s e up t o f i v e f o l d d u r i n g s h i v e r i n g (Hemingway, 1963). I n o r d e r to i s o l a t e the e f f e c t s o f l o w e r e d body temperature a l o n e on r e s p i r a t i o n , a n e s t h e t i c s must be used to s u p p r e s s s h i v e r i n g t h e r m o g e n e s i s . To a c h i e v e the o p t i m a l c o n s t a n t p l a n e o f a n e s t h e s i a r e q u i r e s c o n s t a n t adjustment t h r o u g h o u t the hypothermic p e r i o d due to changes i n m e t a b o l i c r a t e . Use o f an i n h a l a t i o n a n e s t h e t i c , such as -5-the method o\u00a3 c o n s t a n t depth h a l o t h a n e a n e s t h e s i a , s i m p l i f i e s the problem o f m a i n t a i n i n g a s t e a d y s t a t e l e v e l o f a n e s t h e s i a (Eger e_t al_. , 1965a). The minimum a l v e o l a r c o n c e n t r a t i o n (MAC) o f h a l o t h a n e n e c c e s s a r y to p r e v e n t g r o s s p u r p o s e f u l movement i n response to a s t a n d a r d , p a i n f u l s t i m u l u s and to produce a l i g h t , r e p r o d u c i b l e , s u r g i c a l p l a n e o f a n e s t h e s i a can be r e a d i l y d e t e r m i n e d . A r e c t i l i n e a r f a l l i n MAC has been demonstrated w i t h a de c r e a s e i n body temperature i n both dogs and r a t s (Eger e_t a l . , 1965b; Munson, 1970). These s t u d i e s show a 50-60% r e d u c t i o n i n h a l o t h a n e MAC f o r a 10 \u00b0C f a l l i n body temperature ( T ^ ) . T h i s r e l a t i o n s h i p c o r r e l a t e s c l o s e l y w i t h the i n c r e a s e d s o l u b i l i t y o f a n e s t h e t i c s i n l i p i d s t h a t o c c u r s a t low temperature. Hence, a g r a d u a l d e c r e a s e i n i n s p i r e d c o n c e n t r a t i o n o f h a l o t h a n e w i t h r e d u c t i o n i n T D s u f f i c i e n t t o m a i n t a i n the MAC a l l o w s f o r a s t a t e o f c o n s t a n t depth a n e s t h e s i a . Mammalian R e s p i r a t o r y C o n t r o l : E u t h e r m i a vs Hypothermia The p r i m a r y f u n c t i o n o f v e n t i l a t i o n i s to s u p p l y oxygen t o , and remove c a r b o n d i o x i d e from the b l o o d and t h e r e b y m a i n t a i n the c o n s t a n c y o f the p a r t i a l p r e s s u r e s o f these gases (P()2' ^C02^ * n t^ i e a r t e r i a l b l o o d p e r f u s i n g the t i s s u e s . T h i s t i s s u e m e t a b o l i c demand i s met by a c o n t i n u o u s -6-and r h y t h m i c pulmonary v e n t i l a t i o n t h a t i s r e g u l a t e d i n p a r t by c h e m o s e n s i t i v e s t r u c t u r e s ( chemoreceptors) and i n p a r t by o t h e r nervous r e f l e x mechanisms. I n mammals, the two major groups o f chemoreceptors i n v o l v e d i n the c h e m i c a l r e g u l a t i o n o f b r e a t h i n g are l o c a t e d i n the p e r i p h e r a l c i r c u l a t i o n a t the c a r o t i d s i n u s and a o r t i c a r c h ( c a r o t i d and a o r t i c body chemoreceptors r e s p e c t i v e l y ) , and i n the c e n t r a l c i r c u l a t i o n c l o s e t o the v e n t r a l s u r f a c e o f the m e d u l l a o b l o n g a t a , a t a y e t undetermined l o c a t i o n (Feldman, 1986). P e r i p h e r a l chemoreceptors sense a l t e r a t i o n s i n the P()2' a n <^ a^-so \u00b0f the hydrogen i o n c o n c e n t r a t i o n and P Q02 \u00b0^ the a r t e r i a l b l o o d w i t h which they a r e p e r f u s e d . C e n t r a l chemoreceptors are s t i m u l a t e d by an i n c r e a s e i n hydrogen i o n c o n c e n t r a t i o n o f the c e r e b r o s p i n a l f l u i d which i n t u r n i s a r e f l e c t i o n o f the a r t e r i a l hydrogen i o n c o n c e n t r a t i o n and Pc02 s t a t u s . I t i s g e n e r a l l y a c c e p t e d t h a t the s t e a d y s t a t e r e s p o n s e s to d e c r e a s e s i n P Q 2 ( h y p o x i a ) a r e p r i m a r i l y mediated by c a r o t i d body chemoreceptor i n p u t t o the CNS, w h i l e s t e a d y s t a t e r e s p o n s e s to i n c r e a s e s i n PC02 a r e p r i m a r i l y c o n t r o l l e d by c e n t r a l chemoreceptor i n p u t s . I n most e u t h e r m i c mammals both h y p o x i a and h y p e r c a p n i a r e s u l t i n i n c r e a s e s i n v e n t i l a t i o n l e a d i n g t o a r e t u r n o f a r t e r i a l gas t e n s i o n s t o t h e i r p h y s i o l o g i c a l l y a p p r o p r i a t e l e v e l s . R e p o r t s on the e f f e c t o f hypothermia on v e n t i l a t o r y r e s p o n s e s to h y p o x i a and h y p e r c a p n i a a re s p a r s e . Almost a l l i n v e s t i g a t o r s have found a r e d u c t i o n i n the v e n t i l a t o r y r e s p onse o f dogs t o CO2 d u r i n g h y p o t hermia a l t h o u g h the -7-e f f e c t s o f a n e s t h e s i a and s h i v e r i n g appear to a l t e r the magnitude o f the response ( S a l z a n o and H a l l , 1960; K i l m o r e and Chase, 1962; Regan and Eger, 1966; N a t s u i , 1969; Sodipo and Lee, 1971; R u i z , 1976). I n n o n - s h i v e r i n g dogs, d u r i n g c o n s t a n t depth h a l o t h a n e a n e s t h e s i a , v e n t i l a t i o n has been shown to d e c r e a s e t o a p p r o x i m a t l e y 1\/2 o f i t s normothermic v a l u e a t a T b o f 28 \u00b0C ( Q K ^ ) (Regan and Eger, 1966; Sodipo and Lee, 1971). I n a d d i t i o n , v e n t i l a t o r y r e s p o n s e s to h i g h l e v e l s o f CO2 (10-15%) d e c r e a s e d p r o g r e s s i v e l y w i t h h y p o t h ermia i n these s t u d i e s . T h i s d e c r e a s e i n the r e s p i r a t o r y s e n s i t i v i t y ( s l o p e o f t h e s e r e s p o n s e s ) to i n s p i r e d gases was r e v e r s i b l e w i t h p r o g r e s s i v e rewarming o f the dogs to normothermic v a l u e s (Regan and Eger, 1966). R e p o r t s o f the v e n t i l a t o r y r e s p o n s e s o f h y p o t h e r m i c a n e s t h e t i z e d dogs to i n h a l a t i o n o f h y p o x i c gases, however, have not been c o n s i s t e n t . I n the s t u d i e s o u t l i n e d above, n o n - s h i v e r i n g dogs under c o n s t a n t depth h a l o t h a n e a n e s t h e s i a a l s o showed a d e c r e a s e d v e n t i l a t o r y response t o low l e v e l s o f i n s p i r e d O2 (5-7% and 10-12%) w i t h h y p o t hermia which i n c r e a s e d a g a i n w i t h rewarming to normothermia (Regan and Eger, 1966; Sodipo and Lee, 1971). I n s e v e r a l o t h e r s t u d i e s , however, the v e n t i l a t o r y r e s p o n s e s o f a n e s t h e t i z e d dogs to h y p o x i a were o n l y s l i g h t l y reduced w i t h h y p o t hermia and t h i s d e c r e a s e was found not t o be s t a t i s t i c a l l y s i g n i f i c a n t ( T e r z i o g l u \u00a7_t aj^. ,1961; S a l z a n o and H a l l , 1961; N a t s u i , 1969). I n another s t u d y by K i l m o r e and Chase (1962), o f 15 b a r b i t u r a t e a n e s t h e t i z e d dogs, h y p o x i a d u r i n g hypothermia -8-reduced v e n t i l a t i o n i n s i x a n i m a l s , i n c r e a s e d i t i n s i x o t h e r s and had no e f f e c t i n t h r e e a n i m a l s . R e s p i r a t i o n i n F o s s o r i a l Mammals: Euthermia vs Hypothermia Due to t h e i r h i g h e r s u r f a c e a r e a t o volume r a t i o , s m a l l mammals l o s e heat to the environment more e a s i l y t han do l a r g e r mammals. As homeotherms, s m a l l mammals must produce more heat to m a i n t a i n t h e i r body temperature when the ambient temperature i s low. A l t h o u g h h i g h m e t a b o l i c r a t e s are c h a r a c t e r i s t i c o f these a n i m a l s , among them the f o s s o r i a l s p e c i e s have lower m e t a b o l i c r a t e s and minute v e n t i l a t i o n t h a n non-burrowing a n i m a l s o f s i m i l a r s i z e . M e t a b o l i c r a t e s i n s e v e r a l f o s s o r i a l s p e c i e s have been r e p o r t e d t o be 20-60% lower than a l l o m e t r i c p r e d i c t i o n s based on body weight (McNab, 1966; Hudson and Deavers, 1973). I n a d d i t i o n , a number o f s t u d i e s show t h a t h y p e r c a p n i c and h y p o x i c r e s p o n s e s are a l t e r e d i n b u r r o w i n g s p e c i e s compared t o n o n - f o s s o r i a l mammals ( A r i e l i and A r , 1979; F a l e s c h i n i and W h i t t e n , 1974; Holloway and Heath, 1984; S c h l e n k e r , 1985; Walker ejt al_. , 1985). F o s s o r i a l mammals i n h a b i t burrows which s e r v e as p r o t e c t i o n from p r e d a t i o n and c l i m a t i c extremes. Burrow c o n d i t i o n s a r e o f t e n low i n O2 c o n c e n t r a t i o n ( h y p o x i c ) and -9-h i g h i n CO2 c o n c e n t r a t i o n ( h y p e r c a r b i c ) . The p r e v a i l i n g l e v e l s o f oxygen and c a r b o n d i o x i d e depend on the number o f burrow o c c u p a n t s and t h e i r m e t a b o l i c r a t e s , on burrow geometry and on s o i l m o i s t u r e and p o r o s i t y ( W i t h e r s , 1978; A r i e l i , 1979; Maclean, 1981). R e p o r t s o f burrow gas c o m p o s i t i o n range from 20% to 8% O2 (Hayward, 1966). CO2 c o n c e n t r a t i o n i n burrows can range from 0%-13% w i t h average v a l u e s r a n g i n g 3%-4% (Hayward, 1966; McNab, 1966; S t u d i e r and P r o c t o r , 1971; Darden, 1972; B a u d i n e t t e , 1974). F o s s o r i a l mammals show c e r t a i n r e s p i r a t o r y a d a p t a t i o n s which are b e l i e v e d t o be i n response to c h r o n i c exposure to these h y p e r c a r b i c and h y p o x i c burrow c o n d i t i o n s . A c c o r d i n g l y , f o s s o r i a l s p e c i e s have a h i g h e r O2 c a r r y i n g c a p a c i t y i n the b l o o d than n o n - f o s s o r i a l s p e c i e s a r i s i n g from a h i g h e r h e m a t o c r i t and hemoglobin c o n c e n t r a t i o n (Ar e_fc. a j ^ . , 1977; B a u d i n e t t e , 1974; Chapman and B e n n e t t , 1975). I n a d d i t i o n , the 02 c a p a c i t y i s i n c r e a s e d by a l e f t - s h i f t i n the hemoglobin oxygen (Hfc,-02) e q u i l i b r i u m c u r v e ( B a r t e l e_fc. a l . , 1969; B a u d i n e t t e , 1974) a l l o w i n g f o r f u l l s a t u r a t i o n o f b l o o d a t the lower O2 p a r t i a l p r e s s u r e s e x p e r i e n c e d i n the burrow. The r e s u l t a n t O2 h a l f - s a t u r a t i o n p r e s s u r e s (P50) r a n g i n g from 23 t o 33 t o r r (Harkness e t al_. , 1974; H a l l , 1965; B a u d i n e t t e , 1974; Lec h n e r , 1976) a r e lower than those of a n o n - f o s s o r i a l r o d e n t o f comparable s i z e , the r a t which has a P50 o f 38-39 t o r r ( H a l l , 1965; L e c h n e r , 1976). These a d a p t a t i o n s suggest t h a t f o s s o r i a l mammals s h o u l d be more t o l e r a n t o f moderate l e v e l s o f h y p o x i a than n o n - f o s s o r i a l -10-homeotherms. S t u d i e s t h a t have examined the h y p o x i c s e n s i t i v i t y o f f o s s o r i a l mammals, however, show t h a t some b u r r o w - d w e l l i n g s p e c i e s such as the D j u n g a r i a n hamster ( S c h l e n k e r , 1985) are as s e n s i t i v e ; o t h e r s such as the g o l d e n hamster and the g o l d e n mantled ground s q u i r r e l (Holloway and Heath, 1984; Walker e t a l , 1985; McA r t h u r , 1986) are more s e n s i t i v e and s t i l l o t h e r s such as the f o s s o r i a l mole r a t ( A r i e l i and A r , 1979) are l e s s s e n s i t i v e t o h y p o x i a than the n o n - f o s s o r i a l r a t . I n c o n t r a s t a h i g h t o l e r a n c e t o h y p e r c a p n i a i s t y p i c a l o f almo s t a l l f o s s o r i a l mammals (Darden, 1972; L e i t n e r and Malan, 1973; A r i e l i and A r , 1979; Holloway and Heath, 1984; Walker e t a l , 1985). The t h r e s h o l d f o r v e n t i l a t o r y r e s p o n s e s to CO2 appears e l e v a t e d , w h i l e CO2 s e n s i t i v i t y i s reduced. Hence, whereas a r e d u c t i o n i n v e n t i l a t o r y r e s ponse to h y p e r c a p n i a i s a common c h a r a c t e r i s t i c o f b u r r o w - d w e l l i n g mammals, reduced v e n t i l a t o r y s e n s i t i v i t y t o h y p o x i a i s not a u n i v e r s a l phenomenon d u r i n g e u t h e r m i a . I n view o f these a d a p t a t i o n s , i t has been suggested ( L e i t n e r and Malan, 1973; Milsom and McA r t h u r , 1987) t h a t v e n t i l a t i o n i n e u t h e r m i c f o s s o r i a l h i b e r n a t o r s may be p r i m a r i l y r e g u l a t e d by the p e r i p h e r a l chemoreceptors m o n i t o r i n g a r t e r i a l O2 t e n s i o n s . G i v e n the h i g h O2 c a r r y i n g c a p a c i t y i n the b l o o d o f these s p e c i e s , i t i s not s u r p r i s i n g then t h a t t h e i r v e n t i l a t o r y r a t e s a re s i g n i f i c a n t l y lower than a n i m a l s o f s i m i l a r body s i z e . No i n f o r m a t i o n e x i s t s on spontaneous b r e a t h i n g p a t t e r n and v e n t i l a t o r y response d u r i n g p r o g r e s s i v e - 1 1 -hypothermia i n f o s s o r i a l mammals. Obj e c t i v e s The o b j e c t i v e s o f the p r e s e n t study were t o a s c e r t a i n a method f o r i n d u c t i o n o f hypothermia w i t h a c o n s t a n t and r e p r o d u c i b l e p l a n e o f a n e s t h e s i a s u f f i c i e n t t o j u s t i n h i b i t s h i v e r i n g thermogenesis and a l l o w measurement o f r e s p i r a t o r y and m e t a b o l i c v a r i a b l e s d u r i n g i n d u c t i o n o f h y pothermia i n the f o s s o r i a l g o l d e n - m a n t l e d ground s q u i r r e l (S^_ l a t e r a l i s ) . T h i s would p e r m i t t e s t i n g o f the h y p o t h e s i s t h a t minute v e n t i l a t i o n and m e t a b o l i c r a t e d e c r e a s e p r o p o r t i o n a l l y w i t h p r o g r e s s i v e h y p o t hermia which would i n t u r n suggest t h a t the changes i n minute v e n t i l a t i o n which o c c u r d u r i n g h i b e r n a t i o n i n the ground s q u i r r e l a re merely the consequence o f d e c r e a s e s i n body temperature and m e t a b o l i c r a t e r a t h e r than the r e s u l t o f s e a s o n a l changes a s s o c i a t e d w i t h h i b e r n a t i o n (Chapter Two). I n a d d i t i o n , e x a m i n i n g the changes i n the t h r e s h o l d and s e n s i t i v i t y o f the v e n t i l a t o r y r e s p o n s e s to h y p o x i a and h y p e r c a p n i a , would determine whether changes i n v e n t i l a t o r y s e n s i t i v i t y d u r i n g h y p o t hermia are s i m i l a r to changes i n m e t a b o l i c r a t e (Chapter T h r e e ) . I f so, t h i s would suggest t h a t the c o n t r o l o f v e n t i l a t i o n i s r e g u l a t e d by m e t a b o l i c demand a t low body t e m p e r a t u r e s . An a d d i t i o n a l o b j e c t i v e was to t e s t the scope o f t h i s h y p o t h e s i s i n a n o n - f o s s o r i a l n o n - h i b e r n a t i n g r o d e n t o f s i m i l a r body w e i g h t , the l a b o r a t o r y r a t (\u00a3_,_ n o r v e g i c u s ) . -12-CHAPTER TWO VENTILATION DURING PROGRESSIVE HYPOTHERMIA INTRODUCTION C o n t r o l o f v e n t i l a t i o n i n the CNS i s b e l i e v e d t o oc c u r a t the l e v e l o f b r a i n s t e m n e u r o n a l p o o l s which a c t as the \" c e n t r a l r e s p i r a t o r y c o n t r o l l e r \" . T h i s c e n t r a l c o n t r o l l e r r e c e i v e s i n p u t s from v a r i o u s s o u r c e s : c h e m o r e c e p t o r s , mechanoreceptors and h i g h e r CNS c e n t e r s and g e n e r a t e s a r h y t h m i c o u t p u t to r e s p i r a t o r y motoneurons. A f t e r m o d i f i c a t i o n a t the motoneuron l e v e l , t h i s o u t p u t i s t r a n s l a t e d i n t o a p a r t i c u l a r temporal sequence o f r e c i p r o c a t i n g i n s p i r a t o r y and e x p i r a t o r y f l o w . U n t i l r e c e n t l y , the r e s u l t i n g t i d a l p a t t e r n had been examined i n terms o f t i d a l volume (V
p and f a r e time dependent v a r i a b l e s . I n f a c t , f i s i n v e r s e l y p r o p o r t i o n a l t o t o t a l r e s p i r a t o r y time (T\u00bbpoT^ a n a \" c a n D e changed as a r e s u l t o f a change i n i n s p i r a t o r y ( T j ) or e x p i r a t o r y time (Tg). Vq> on the o t h e r hand depends on b o t h T T and i n s p i r a t o r y f l o w . R e c e n t l y the c o n t r i b u t i o n o f mean i n s p i r a t o r y f l o w and r e s p i r a t o r y phase d u r a t i o n s t o Vj; have been s e p a r a t e d by -13-the f o l l o w i n g e q u a t i o n ( M i l i c - E m i l i e t a l . . , 1975, 1976): V E = V T \/ T : X T J \/ T T Q T where V T \/ T J i s mean i n s p i r a t o r y f l o w r a t e and T T \/ T T O T i s a d i m e n s i o n l e s s number which can be d e f i n e d as i n s p i r a t o r y \"duty c y c l e \" a term used i n e l e c t r i c a l e n g i n e e r i n g to denote the f r a c t i o n o f a p e r i o d i c c y c l e t h a t i s e n e r g e t i c a l l y a c t i v e ( M i l i c - E m i l i \u00a7_t al_. , 1981). A c c o r d i n g t o t h i s e x p r e s s i o n , a change i n v e n t i l a t i o n can r e s u l t from a change i n V < T \/ T T or T T \/ T T C - T o r D\u00b0th. A change i n T T \/ T T - Q T o c c u r s when T j changes d i s p r o p o r t i o n a t e l y w i t h r e s p e c t to Tg and r e f l e c t s changes i n the t i m i n g (rhythm g e n e r a t i o n ) o f the c e n t r a l r e s p i r a t o r y c o n t r o l l e r . Vip\/Tj i s a me c h a n i c a l t r a n s f o r m o f the r a t e o f r i s e o f a c t i v i t y o f i n s p i r a t o r y a l p h a motoneurons and has been used as an i n d e x o f i n s p i r a t o r y d r i v e by a number o f i n v e s t i g a t o r s ( B r a d l e y , 1977; G a u t i e r , 1975; Remmers, 1976), The r a t e o f i n c r e a s e o f r e s p i r a t o r y d r i v e remains c o n s t a n t d u r i n g many r e s p o n s e s such as v a g a l and i n t e r c o s t a l r e f l e x e s , which a l t e r V>r- and f t o g e t h e r . T h e r e f o r e a change i n V f \/ T j s h o u l d o n l y r e f l e c t changes i n r e s p i r a t o r y d r i v e due to the chemoreceptor or h i g h e r CNS i n p u t s t o the c e n t r a l c o n t r o l l e r . I n terms o f o u t p u t however, the l i n k a g e between the r e s p i r a t o r y c o n t r o l l e r and the movement o f a i r i n c l u d e s the b e h a v i o u r o f r e s p i r a t o r y m u s c l e s , and the m e c h a n i c a l p r o p e r t i e s o f the - 1 4 c h e s t w a l l , diaphragm and l u n g s . T h e r e f o r e changes i n any o f the above, i n a d d i t i o n to the b e h a v i o u r o f the c e n t r a l c o n t r o l l e r would l e a d to a change i n V ^ \/ T j . G a u t i e r and Gaudy ( 1 9 8 6 ) have r e c e n t l y a p p l i e d t h i s type o f v e n t i l a t o r y p a t t e r n a n a l y s i s to a n e s t h e t i z e d c a t s r e c o v e r i n g from hypothermia and t h e i r r e s u l t s suggest s e q u e n t i a l r a t h e r t h a n p a r a l l e l changes i n V^\/Tj and Tl\/TfOT w i t h i n c r e a s e s i n body tem p e r a t u r e . D u r i n g rewarming from h y p o t h e r m i a , i n t h e i r e x p e r i m e n t s on c a t s , a p r o g r e s s i v e i n c r e a s e i n v e n t i l a t i o n was o b s e r v e d r e s u l t i n g from an i n c r e a s e i n f between 2 8 and 3 8 \" C ( Q I Q = 2 . 5 ) . Between 2 8 and 3 2 \u00b0 C , the i n c r e a s e i n v*jr was due to a marked i n c r e a s e i n t i m i n g ( T J \/ T T Q T ) a l o n e whereas above 3 4 \u00b0 C , d r i v e ( V-p\/Tj) i n c r e a s e d w h i l e T J \/ T ^ Q T not e x h i b i t any f u r t h e r changes. These r e s u l t s i m p l y t h a t d i f f e r e n t components o f minute v e n t i l a t i o n a r e r e g u l a t e d d u r i n g h y p o t h ermia a t d i f f e r e n t body t e m p e r a t u r e s . S u r p r i s i n g l y few r e p o r t s document the spontaneous b r e a t h i n g p a t t e r n o f s m a l l f o s s o r i a l or non f o s s o r i a l mammals d u r i n g h y p o t h e r m i a . As e x p e r i m e n t a l models th e s e a n i m a l s , have o f t e n been the f o c u s o f s t u d i e s e s t a b l i s h i n g c o o l i n g mehods t h a t maximize s u r v i v a l time a t body t e m p e r a t u r e s below the l e v e l o f c e s s a t i o n o f b r e a t h i n g . These t e c h n i q u e s o f t e n a l l o w f o r s h i v e r i n g and i n v o l v e a r t i f i c i a l v e n t i l a t i o n or b r e a t h i n g o f h y p o x i c h y p e r c a p n i c a i r m i x t u r e s d u r i n g the -15-c o u r s e o f i n d u c t i o n (Andjus and Smith, 1955; P o p o v i c , 1960a; P o p o v i c , 1960b; Adolph \u00a7_t a l _ - , 1961; Rogers and H i l l m a n , 1970). More r e c e n t s t u d i e s e xamining the s u r v i v a l t imes o f r o d e n t s a t low body t e m p e r a t u r e s have t a k e n advantage o f the combined e f f e c t s o f h i g h t h e r m a l c o n d u c t i v i t y o f h e l i u m (6 f o l d compared to n i t r o g e n ) and a temperature g r a d i e n t between s k i n and ambient a i r to r a p i d l y induce hypothermia i n these a n i m a l s ( F i s c h e r and M u s a c c h i a , 1968; Anderson e t al_. , 1971; Musacchia and J a c o b s , 1973; S t e f f e n and M u s a c c h i a , 1985). The a d d i t i o n o f v a p o r i z e d r e g u l a t e d h a l o t h a n e t o the c h i l l e d He-C-2 m i x t u r e not o n l y a l l o w s f o r c o n s t a n t depth o f a n e s t h e s i a t o i n h i b i t s h i v e r i n g , but has a l s o been shown to enhance the s u r v i v a l o f h y p o t h e r m i c a n i m a l s by f o u r f o l d ( V o l k e r t and M u s a c c h i a , 1976). D e s p i t e the advantages o f the h e l o x method and i t s s u i t a b i l i t y f o r a n a l y s i s o f the changes i n v e n t i l a t o r y p a t t e r n d u r i n g h y p o t hermia i n s m a l l mammals, t h i s i s s u e has not been s p e c i f i c a l l y a d d r e s s e d i n p r e v i o u s s t u d i e s . I n t h i s c h a p t e r , the changes i n Vg and m e t a b o l i c r a t e d u r i n g p r o g r e s s i v e h y p o t hermia w i l l be examined to d e t e r m i n e whether i n non s h i v e r i n g f o s s o r i a l and n o n - f o s s o r i a l mammals, V * E and MR do indeed d e c r e a s e p r o p o r t i o n a l l y d u r i n g h y p o t h e r m i a as h y p o t h e s i z e d . T h i s would suggest t h a t the d e c r e a s e i n v e n t i l a t i o n which o c c u r s d u r i n g h i b e r n a t i o n i s due t o a r e d u c t i o n i n m e t a b o l i c demand w i t h reduced body temperature r a t h e r than a s e a s o n a l phenomenon. I n a d d i t i o n components o f v e n t i l a t i o n (as d e s c r i b e d above) w i l l be - 1 6 -examined to d e t e r m i n e whether hypothermia has d i f f e r e n t i a l e f f e c t s on the r e g u l a t i o n o f the components v e n t i l a t i o n a t lowered body t e m p e r a t u r e s . MATERIALS AND METHODS Animals A l l s t u d i e s were c a r r i e d out on a d u l t , female W i s t a r r a t s ( R a t t u s n o r v e g i c u s ) and g o l d e n - m a n t l e d ground s q u i r r e l s ( S p e r m o p h i l u s l a t e r a l i s ) ( o b t a i n e d from commercial s o u r c e s ) between 170-270 gram body weight. These a n i m a l s were housed i n p a i r s i n p l e x i g l a s s cages (45cm x 25 cm x 20cm) w i t h w i r e mesh tops and S a n i c e l l and wood s h a v i n g s f o r bedding. P u r i n a l a b chow and water were p r o v i d e d j y | l i b i t u m . E x p e r i m e n t s on s q u i r r e l s were conducted o n l y d u r i n g the n o n - h i b e r n a t i n g season ( A p r i l - S e p t e m b e r ) w h i l e the r a t s were s t u d i e d t h r o u g h o u t the y e a r . The e n v i r o n m e n t a l chamber h o u s i n g the a n i m a l c o l o n y was k e p t c o n s t a n t a t 20 +\/-1\u00b0C on a 12 hour l i g h t : 1 2 hour dark p h o t o p e r i o d . D e t e r m i n i n g the a p p r o p r i a t e p l a n e o f a n e s t h e s i a P i l o t s t u d i e s were conducted to d e t e r m i n e the minimum l e v e l o f h a l o t h a n e a n e s t h e s i a s u f f i c i e n t t o a l l o w f o r maintenance o f a pneumotachogragh mask u n i t on the a n i m a l s snout d u r i n g normothermia (T D=36 \u00b0C) and s u f f i c i e n t t o s u p p r e s s s h i v e r i n g w i t h p r o g r e s s i v e hypothermia ( t o Tj-,= 27 \u00b0C). A l l a n i m a l s were i n i t i a l l y a n e s t h e t i z e d w i t h 5% h a l o t h a n e , equipped w i t h a pneumotachograph-face ma s k - u n i t , a r e c t a l temperature probe and p l a c e d i n the e x p e r i m e n t a l -18-chamber. D u r i n g normothermia, a n e s t h e s i a was m a i n t a i n e d w i t h 2-2.5% h a l o t h a n e f o r a 30 minute i n t e r v a l . V e n t i l a t i o n and m e t a b o l i c r a t e were r e c o r d e d a t the end o f t h i s i n t e r v a l , a f t e r which the a n i m a l s were exposed to c o n s e c u t i v e s t e p d e c r e a s e s i n the l e v e l o f a n e s t h e s i a , each l a s t i n g 15 minutes. At the end o f each 15 minute i n t e r v a l , Vg and VC02 were r e c o r d e d a g a i n . As a n e s t h e s i a was reduced, a l e v e l was r e a c h e d where n o t i c e a b l e i n c r e a s e s i n both Vjr and VC02 o c c u r r e d . These i n c r e a s e s were a s s o c i a t e d w i t h the onset o f s h i v e r i n g and\/or a t t e m p t s by the a n i m a l to remove the f a c e mask. The l e v e l o f a n e s t h e s i a was then i n c r e a s e d u n t i l s h i v e r i n g and\/or movement d i s a p p e a r e d . The l e v e l o f a n e s t h e s i a j u s t r e q u i r e d to su p p r e s s s h i v e r i n g was d e f i n e d as the s h i v e r i n g t h r e s h o l d ( T a b l e 1 ) . The an i m a l was th e n c o o l e d to 27 +\/- 1\u00b0C r e c t a l t e mperature and the s h i v e r i n g t h r e s h o l d a t t h i s t e mperature was o b t a i n e d i n the same manner as d e s c r i b e d above, w i t h the e x c e p t i o n t h a t the i n i t i a l a n e s t h e t i c l e v e l s were reduced w i t h the r e d u c t i o n i n body te m p e r a t u r e . To ensure t h a t none o f the e x p e r i m e n t a l a n i m a l s would s h i v e r , the l e v e l o f r e g u l a t e d h a l o t h a n e was m a i n t a i n e d 10% above the average s h i v e r i n g t h r e s h o l d o b t a i n e d i n t h i s p i l o t s t u d y ( T a b l e I ) . Hence the r a t s were a n e s t h e t i z e d w i t h 1% h a l o t h a n e a t Tj-,=3G * C and t h i s l e v e l was g r a d u a l l y reduced to 0.6% h a l o t h a n e by Tb=27 0 C . The ground s q u i r r e l s however, i n s p i r e d 2% h a l o t h a n e d u r i n g normothermia and w i t h p r o g r e s s i v e body c o o l i n g the a n e s t h e t i c l e v e l was g r a d u a l l y TABLE I: E f f e c t of lowering lev e l s of halothane anesthesia on v e n t i l a t i o n and C02 production at 36 and 27\"C body temperature in the rat and ground s q u i r r e l . A l l values are i n ml\/min\/kg. Tr=36 C Rat Squirrel Halothane Subject A Subject B Subject C Subject A Subject B Subject C (%) VE VC02 VE VC02 VE VC02 VE VC02 VE VC02 VE VC02 2.50 2. 25 2. 00 617 12 532 1. 75 569 1. 50 600 13 550 1. 25 598 1 *624 14 *620 0. 85 #711 19 #698 0.70 0.60 234 *252 13 443 16 12 408 16 13 14 422 16 14 451 16 18 458 16 *552 18 #885 25 7 7 285 7 338 7 *293 8 *335 6 #370 11 #376 11 Tr=27 C 1.50 138 1.25 1.00 236 9 *149 2 0.85 403 8 205 9 #255 5 0.70 0.60 0.50 0.40 0. 30 6 3 5 418 8 182 7 175 9 *429 10 228 7 186 9 #569 19 *263 7 *207 9 #314 15 #241 295 10 16 96 3 114 4 *123 3 *179 4 #129 6 #499 7 ^\"shivering observed, *=\u00bbshivering threshold -20-reduced to 1% h a l o t h a n e . Measurement o f v e n t i l a t i o n V e n t i l a t o r y f l o w was measured w i t h a s m a l l p l e x i g l a s s pneumotachograph a t t a c h e d to the open end o f a form f i t t i n g f a c e mask. The t o t a l deadspace f o r the u n i t was 0.30 ml. The pneumotachograph-mask u n i t was s e c u r e l y f a s t e n e d to the snout o f the a n e s t h e t i z e d a n i m a l f o l l o w i n g the i n d u c t i o n o f a n e s t h e s i a . The pneumotachograph was c o n n e c t e d to a V a l i d y n e d i f f e r e n t i a l p r e s s u r e t r a n s d u c e r (model DP 103-18, N o r t h r i d g e , C a l i f o r n i a ) , and the a i r f l o w s i g n a l was a m p l i f i e d (Gould U n i v e r s a l A m p l i f i e r , Model 13-4615-58), and c o u p l e d to an e l e c t r o n i c i n t e g r a t o r (Gould i n t e g r a t o r a m p l i f i e r , model 13-4615-17) to y i e l d measurements o f t i d a l volume (V