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Effects of hypothermia on ventilation and ventilatory responses to hypercapnia and hypoxia in the golden-mantled… Osborne, Salma (Sally) 1988

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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 © 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 °C 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 °C. 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°C 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°C 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°C 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°C 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°C 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°C 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°C 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°C 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° 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 °C 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°C 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 °C 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 ° 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£02 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°C 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°C) and hypothermic(Tj3=27 °C)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 °C 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°C 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°C 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°C) and h y p o t h e r m i c (Tb=27°C) 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°C and h y p o t h e r m i c (Tb=27°C) 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°C body temperature and a t 36 and 7°C 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 °C 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°C ( 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 §_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 °C ( 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 ° 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„., 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£ 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 °C 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 °f the hydrogen i o n c o n c e n t r a t i o n and P Q02 °^ 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 °C ( 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 §_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 (£_,_ 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 r e s p i r a t o r y f r e q u e n c y ( f ) ( V E = V*T X f ) . T h i s type o f a n a l y s i s i s by no means com p l e t e , s i n c e b oth 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»poT^ 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 §_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°th. 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 ° 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 ° 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 §_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°C 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 °C) 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 °C). 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°C 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, *=»shivering 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<r). Both a i r f l o w and V<p s i g n a l s were d i s p l a y e d on a Gould c h a r t r e c o r d e r ( s e r i e s 2400/2600). C a l i b r a t i o n o f the pneumotachograph was performed b e f o r e and a f t e r each e x p e r i m e n t by i n t r o d u c i n g known volumes o f a i r a c r o s s the pneumotachograph v i a the f a c e mask. The l i n e a r i t y o f t h i s s i g n a l was checked f o r a range o f 0.2 to 3.0 ml b e f o r e and a f t e r each s e t o f e x p e r i m e n t s . High c h a r t speed (10 mm/sec) was used to a l l o w f o r b r e a t h by b r e a t h a n a l y s i s o f the i n s p i r a t o r y ( T j ) , e x p i r a t o r y ( T g ) , and t o t a l (TfOT^ r e s p i r a t o r y t i m e s . From these v a r i a b l e s , V ^ / T T , an i n d e x o f r e s p i r a t o r y d r i v e and the v e n t i l a t o r y duty c y c l e -21-(Tj/T^OT^ were de t e r m i n e d . D u r i n g h y p o t h e r m i a , Tjr was composed o f p e r i o d s w i t h e x p i r a t o r y a i r f l o w (Tgf) as w e l l as n o n - v e n t i l a t o r y or a p n e i c p e r i o d s ( T g - T g i ) . These d u r a t i o n s were a l s o d e t e r m i n e d d u r i n g the b r e a t h by b r e a t h a n a l y s i s o f r e s p i r a t o r y t i m e s . Measurement o f body temperature Body temperature was measured r e c t a l l y (Tfc) by a thermocouple probe a t t a c h e d to a d i g i t a l d i s p l a y ( S e n s o r t e k model Bat-12 / C l i f t o n , NJ) t h r o u g h o u t the e x p e r i m e n t s . T h i s probe was i n s e r t e d 8 cm w i t h i n the a n a l c a n a l . E x p e r i m e n t a l s e t u p The a n i m a l , equipped w i t h the pneumotachograph-mask u n i t and the r e c t a l probe was p l a c e d i n a s e a l e d p l e x i g l a s s box (3 l i t e r s ) s u r r o unded by a water j a c k e t ( F i g u r e 1) co n n e c t e d to a r e f r i g e r a t e d water b a t h (Lauda, model RMS 6, Brinkman: R e x d a l e , O n t a r i o ) . R e g u l a t i o n o f the water b a t h temperature (range 5-36 °C) was used to e s t a b l i s h the ambient t e m p e r a t u r e . A c o n s t a n t gas f l o w o f 1 l i t e r / m i n u t e through the a n i m a l chamber was e s t a b l i s h e d u s i n g p r e c i s i o n f l o w m e t e r s ( s e r i e s 150, model FM4333 & FM4334, L i n d e - U n i o n C a r b i d e ) . The t o t a l t u r n o v e r time f o r a i r i n the chamber was t h e r e f o r e , 3 minutes. D i r e c t measurement o f t o t a l t u r n o v e r time f o r gas i n the chamber u s i n g p r e c i s i o n gas y i e l d e d s i m i l a r v a l u e s . - 2 2 -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 R. n o r v e q i c u s . See t e x t f o r e x p l a n a t i o n . HALOTHANE VAPORIZER M Z A i N O o A N O I UJ X FLOWMETER TO GAS ANALYZERS LAUDA COOLING UNIT PRESSURE TRANSDUCER AMPLIFIER \ TO GAS ANALYZERS / CHART RECORDER INTEGRATOR - 2 4 -The gas c o m p o s i t i o n o£ both i n f l o w and o u t f l o w a i r s t r e a m s were a n a l y z e d w i t h i n +0.1% u s i n g Beckman 0M11 (O2) and LB2 (CO2) gas a n a l y z e r s , c a l i b r a t e d d a i l y w i t h room a i r and premixed 5 and 10% CO2 gas m i x t u r e s (Radiometer GMA 2 p r e c i s i o n gas s u p p l y ) . From t h i s , c a r b o n d i o x i d e p r o d u c t i o n o f the an i m a l c o u l d be c a l c u l a t e d by s u b t r a c t i n g the f r a c t i o n a l c o n c e n t r a t i o n o f CO2 i n the o u t f l o w gas from t h a t i n the i n f l o w gas and m u l t i p l y i n g by the c o n s t a n t gas fl o w t h r o u g h the chamber to g i v e ml CO2 produced per minute (VQQ2^- * t w a s assumed t h a t VQ02 changed p r o p o r t i o n a l l y w i t h VQ2 d u r i n g h y p o t hermia and thus the r e s p i r a t o r y q u o t i e n t d i d not change d u r i n g h ypothermia. E x p e r i m e n t a l P r o t o c o l : I n d u c t i o n o f hypothermia Throughout the s t u d i e s , the e x p e r i m e n t a l chamber was f l u s h e d w i t h a He-C"2 m i x t u r e (80%-20% p r e c i s i o n gas) and r e g u l a t e d l e v e l s o f v a p o r i z e d h a l o t h a n e ( F l u o t h a n e , A y e r s t L a b o r a t o r i e s ) v i a a Drager h a l o t h a n e v a p o r i z e r (Dragerwerk, Lubeck, F.R. Germany). 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 and equipped w i t h a pneumotachograph-mask u n i t , r e c t a l t h e r m i s t o r and i n the r a t s , the t a i l s were i n s u l a t e d by wrapping them i n l a y e r s o f c o t t o n wool and p l a s t i c s h e e t to p r e v e n t f r o s t b i t e . The a n i m a l s were m a i n t a i n e d a t Tb=36 °C f o r 1 /2 hour p r i o r t o measurements o f -25-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 . To induce hypothermia the water bath temperature was low e r e d to 5°C. With each 1°C drop i n body t e m p e r a t u r e , Vg and VrjQ2 were r e c o r d e d . R e g u l a t i o n o f ambient temperature a l l o w e d f o r maintenance o f r e c t a l t e m p e r a t u r e s ( r a n g i n g between 36+1 °C and 27+l°C) d e s i r e d f o r p e r i o d s up to one hour. Data A n a l y s i s A p p r o x i m a t e l y twenty seconds o f b r e a t h i n g t r a c e was r e c o r d e d a t h i g h speed (10 mm/sec) d u r i n g normothermia (36 C) and w i t h each degree C e l c i u s drop i n r e c t a l t emperature throughout p r o g r e s s i v e h y p o t h e r m i a . For each a n i m a l , s i x c o n s e c u t i v e b r e a t h s were a n a l y z e d to o b t a i n the average magnitude o f t o t a l b r e a t h d u r a t i o n (T^QT^' i n s p i r a t o r y time ( T j ) , e x p i r a t o r y t i m e s (Tg, Tg r and Tjj j -Tjri ) a n d t i d a l volume (V-p) (see F i g u r e 2 ) . The mean v a l u e s o f (ml) were then c o r r e c t e d f o r body weight and e x p r e s s e d i n ml/kg. Vf was d i v i d e d by T j t o g i v e the r e p i r a t o r y d r i v e ( V T / T T ) and T T was d i v i d e by T^OT t o g i v e the duty c y c l e ( T T / T T O T > £ o r each a n i m a l . E x p i r a t o r y t i m e s were a l s o e x p r e s s e d as a f r a c t i o n o f T T O T f o r each degree drop i n body temperature. B r e a t h i n g f r e q u e n c y ( f ) was counted as b r e a t h s per t e n second i n t e r v a l and m u l t i p l i e d by s i x t o g i v e b r e a t h s per minute. The mean V«p was m u l t i p l i e d by f to g i v e minute v e n t i l a t i o n (Vjr) i n ml/min/kg. The CC"2 p r o d u c t i o n (VrjQ2^ * n each a n i m a l was o b t a i n e d 2-3 times a t -26-each r e c t a l temperature u n t i l 2 s i m i l a r r e a d i n g s were o b t a i n e d ; t h i s v a l u e was c o r r e c t e d f o r body weight t o g i v e V c o 2 i - n ml/min/kg. V E was d i v i d e d by VQ02 to g i v e 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 (ACR). The v a l u e s o f V^, T TOT' T J , T E , T E » , T E-T E., T E / T T 0 T , T E i/T^OT' T E - T E I / T T 0 T , T J / T T Q T ' V T / T J , f, ^E' ^C02 a n < ^ ^CR f ° r i n d i v i d u a l a n i m a l s were then averaged to g i v e a mean f o r each s p e c i e s . Data p r e s e n t e d as a p e r c e n t o f 36 C v a l u e s were c a l c u l a t e d f o r each i n d i v i d u a l a n i m a l , and the n the mean v a l u e f o r each s p e c i e s was o b t a i n e d . Changes i n 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 w i t h p r o g r e s s i v e c o o l i n g w i t h i n s p e c i e s were a n a l y z e d by two-way a n a l y s i s o f v a r i a n c e (ANOVA). Comparisons between s p e c i e s or two d i f f e r e n t temperature c o n d i t i o n s were made u s i n g t w o - t a i l e d s t u d e n t T - t e s t s . D i f f e r e n c e s were c o n s i d e r e d s i g n i f i c a n t a t p<0.05. -27-R e s u l t s 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 from the two s p e c i e s d u r i n g normothermia and hypothermia a re d e p i c t e d i n F i g u r e 2. F i g u r e 3 i l l u s t r a t e s the changes i n the mean v e n t i l a t i o n o f h a l o t h a n e a n e s t h e t i z e d W i s t a r r a t s and gol d e n - m a n t l e d ground s q u i r r e l s from normothermia (Tb=36 °C) through p r o g r e s s i v e ( c o l d He-C^) induced h ypothermia. F i g u r e s 4 and 5 i l l u s t r a t e the changes i n the two components o f v e n t i l a t i o n , Vf and f r e s p e c t i v e l y d u r i n g p r o g r e s s i v e h y p o t h e r m i a . A l l a n i m a l s b r e a t h e d c o n t i n u o u s l y t h r o u g h o u t the e x p e r i m e n t s . V e n t i l a t i o n and M e t a b o l i c Rate d u r i n g Normothermia At 36 °C both b r e a t h i n g f r e q u e n c y and t i d a l volume were g r e a t e r i n the r a t th a n i n the ground s q u i r r e l . Thus the average l e v e l o f minute v e n t i l a t i o n i n the r a t (823 ml/min/kg) was a p p r o x i m a t e l y 2 1/2 f o l d g r e a t e r than t h a t o f the ground s q u i r r e l (313 ml/min/kg). T h i s d i f f e r e n c e i n v e n t i l a t i o n between the two s p e c i e s was m a i n l y due t o the d i f f e r e n c e s i n mean b r e a t h i n g f r e q u e n c y (113 v e r s u s 58 b r e a t h s / m i n i n r a t and s q u i r r e l r e s p e c t i v e l y ) . T i d a l volume i n the r a t s was o n l y a p p r o x i m a t e l y 20% h i g h e r than i n the ground s q u i r r e l s (7.38 v e r s u s 6.20 ml/kg) . The t i m i n g components o f r e s p i r a t i o n i n normothermic a n i m a l s a r e t a b u l a t e d i n Table I I . The t o t a l r e s p i r a t o r y - 2 8 -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 pneumotachograph^ 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. For e x p l a n a t i o n see below: Ground squirrel 1 sec Rat w . . . . . , , I i.-h Tr=36°C ' I \::\ - ! : i • i I . I Tr=31°C Tr=27°C 1 s e c -30-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 h y p o t hermia i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . T e m p e r a t u r e ( ° C ) - 3 2 -F i g u r e 4. C o m p a r i s o n o f t i d a l v o l u m e d u r i n g n o r m o t h e r m i a a n d p r o g r e s s i v e h y p o t h e r m i a i n r a t s ( o ) a n d g r o u n d s q u i r r e l s ( • ) . E a c h p o i n t r e p r e s e n t s m e a n + / - s t a n d a r d e r r o r f o r 5 s q u i r r e l s a n d 7 r a t s . - 3 3 V j (ml/kg) 4 -3 -2 -1 --26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -34-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 h y p o t hermia i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/-s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -35-F r e q u e n c y (1 / m i n ) 0 4 — i — i — i — i 1 — i — i — i — i — I 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -36-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 d r i v e i n n o r m o t h e r m i c r a t s and ground s q u i r r e l s b r e a t h i n g He-C>2. Mean v a l u e s (X) and s t a n d a r d e r r o r o f the mean (S.E.) a r e i n seconds u n l e s s i n d i c a t e d o t h e r w i s e . Rats n=7; Ground s q u i r r e l s n=5. •TOT T I / T T O T V T / T j (ml/sec) Rat X 0. 31 SE 0.0 3 0. 34 0.03 0.66 0.06 0.47 0.01 5.98 0.52 S q u i r r e l X 0.52 SE 0.0 7 0 . 64 0.11 1. 16 0. 18 0.45 0.01 2. 98 0.29 -37-p e r i o d i n the ground s q u i r r e l was a p p r o x i m a t e l y t w i c e t h a t o f the r a t , c o r r e s p o n d i n g to the lower l e v e l s o f b r e a t h i n g f r e q u e n c y i n the l a t t e r s p e c i e s . The l o n g e r TfOT w a s d u e to both s i g n i f i c a n t l y l o n g e r i n s p i r a t o r y and e x p i r a t o r y t i m e s . Duty c y c l e i n the two s p e c i e s , however, was i d e n t i c a l ( T a ble I I ) . G i v e n the g r e a t e r v a l u e o f Tj i n the s q u i r r e l s r e l a t i v e to the r a t s , the s i m i l a r l e v e l s o f Vf i n b oth s p e c i e s r e f l e c t s a p r o p o r t i o n a t e r e d u c t i o n i n r e s p i r a t o r y d r i v e i n the s q u i r r e l s (2.98 ml/sec) r e l a t i v e to the r a t s (5.98 ml/min). F i g u r e 6 shows mean CO2 p r o d u c t i o n a t normothermia th r o u g h p r o g r e s s i v e h y p o t hermia i n b o t h r a t s and ground s q u i r r e l s . At normothermia, the average Vc02 °^ the r a t was (20 ml/min/kg) a p p r o x i m a t e l y 2 1/2 f o l d t h a t o f the ground s q u i r r e l (8 ml/min/kg), s i m i l a r t o the s p e c i e s d i f f e r e n c e i n minute v e n t i l a t i o n . The average r a t i o o f minute v e n t i l a t i o n t o CO2 p r o d u c t i o n ( a i r c o n v e c t i o n r e q u i r e m e n t ) was t h e r e f o r e not s t a t i s t i c a l l y d i f f e r e n t i n the two s p e c i e s ( F i g u r e 7 ) . V e n t i l a t i o n and M e t a b o l i c Rate d u r i n g P r o g r e s s i v e Hypothermia A g r a d u a l d e c r e a s e i n Vg i n both the l a b o r a t o r y r a t and g o l d e n - m a n t l e d ground s q u i r r e l was seen 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 . The d e c r e a s e i n Vjr, ( 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 (T D=3G °C) f o r each degree C e l c i u s drop i n body t e m p e r a t u r e , i s d e p i c t e d i n F i g u r e 8 f o r both -38-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 (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . - 3 9 -V C 0 2 (ml/min/kg) 22 20 1 8 -1 6 -1 4 -12 1 0 -8 -6 -4 -2 0 H" H F + + h H h 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -40-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 (VE/Vco2) d u r i n g normothermia and p r o g r e s s i v e h ypothermia i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -Z.1-VE/VCQ2 7 01 60-50-40- / 30-; : 20-10-0- — i i — H h + —I 1 1 1 1 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -42-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 h e r m i a 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°C body temperature i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/-s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -43-T e m p e r a t u r e ( ° C ) -44-s p e c i e s . S i m i l a r changes i n 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 are shown i n F i g u r e s 9 and 10. There was a s i g n i f i c a n t d i f f e r e n c e i n the maximum d e c r e a s e i n Vg w i t h body c o o l i n g to Tt, = 27 °C i n the two s p e c i e s . On a v e r a g e , t h e r e was a 44% drop i n VE i n the r a t as opposed to a 72% drop i n the ground s q u i r r e l . I n both s p e c i e s the drop i n v e n t i l a t i o n was due t o the change i n b r e a t h i n g f r e q u e n c y . I n the s q u i r r e l s , b r e a t h i n g f r e q u e n c y d e c r e a s e d by 69%. I n the r a t s b r e a t h i n g f r e q u e n c y droped by 42% r e p r e s e n t i n g a s i g n i f i c a n t l y lower d e c r e a s e compared to the ground s q u i r r e l . V^ d i d not d e c r e a s e a p p r e c i a b 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 i n e i t h e r s p e c i e s . U C 0 2 d e c r e a s e d i n both s p e c i e s 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 i g u r e 11 shows ^C02 e x p r e s s e d as a p e r c e n t a g e o f the normothermic v a l u e (Tb=36 °C) f o r each degree C e l c i u s drop i n body te m p e r a t u r e . J u s t as f o r minute v e n t i l a t i o n , the d e c r e a s e i n CO2 p r o d u c t i o n was s i g n i f i c a n t l y g r e a t e r i n the s q u i r r e l t h a n i n the r a t ( a t T5=27 0 C, Vrj02 d e c r e a s e d by 63% and 46% i n the s q u i r r e l s and r a t s r e s p e c t i v e l y ) . 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 , however d i d not change s i g n i f i c a n t l y w i t h hypothermia i n e i t h e r s p e c i e s ( F i g u r e 12) and was not s i g n i f i c a n t l y d i f f e r e n t between the two s p e c i e s d u r i n g p r o g r e s s i v e body c o o l i n g (2 way ANOVA). Average v a l u e s f o r r e s p i r a t o r y t i m e s , duty c y c l e -45-ure 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 h e r m i a 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°C body temperature i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/-s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -46-V T (% 3 6 C ) 140-r 1 2 0 -100 + 80 4-60 4-40 + 20 4--\ 1 1 1 1 1 1 h -I — h -26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -1 /-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 ° C body temperature i n r a t s (o) and ground s q u i r r e l s ( o ) . Each p o i n t r e p r e s e n t s mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -48-F r e q u e n c y {% 3 6 °C ) 100-r 80 + 60 + 40 + 20 + 0 ^ V H 1 1 1 1 h 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) - 4 9 -F i g u r e 11 . 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 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°C body temperature i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean f / -s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -50-V C 0 2 ( * 3 6 °C ) 120-r 1 0 0 -8 0 -60 + 40 + 2 0 -H h + + + + + + + 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -51 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 e r m i a 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 "C body temperature i n r a t s (o) and ground s q u i r r e l s (•). Each p o i n t r e p r e s e n t s mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -52-V E / V C 0 2 (% 36 °C ) 160-r 1 4 0 -1 2 0 -6 0 -4 0 -2 0 -0 J \ \ \ \ \ \ \ 1 \ j 26 27 28 29 30 31 32 33 34 35 36 T e m p e r a t u r e ( ° C ) -53-(Tj/T-r-oT) a n <^ r e s p i r a t o r y d r i v e (V < T / T T) i n the r a t and the ground s q u i r r e l a t body t e m p e r a t u r e s o f 36, 31 and 27 °C are d e p i c t e d i n F i g u r e 13. As shown by the peak o f these c u r v e s , t h e r e were p r o p o r t i o n a t e changes i n T j and r e s p i r a t o r y d r i v e such t h a t t h e r e was no s i g n i f i c a n t change i n the v a l u e o f V>r- w i t h hypothermia f o r e i t h e r s p e c i e s (p>0.05, 2 way ANOVA). R e d u c t i o n o f body temperature i n both s p e c i e s , l e d to an i n c r e a s e i n Tj and Tjr ( F i g u r e 13). I n c r e a s e s i n Tg ( a t Tj->=27 °C, a p p r o x i m a t e l y two f o l d i n the r a t and f i v e f o l d i n the ground s q u i r r e l ) were p r o p o r t i o n a t e l y g r e a t e r than i n c r e a s e s i n Tj and t h e r e f o r e t h e r e was a d e c r e a s e i n the duty c y c l e (Tj/T^cur) w i t h p r o g r e s s i v e h y p o t hermia ( F i g u r e 14). However, duty c y c l e was reduced a p p r e c i a b l y o n l y a t the v e r y low l e v e l s o f minute v e n t i l a t i o n o b s e r v e d 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 . A d d i t i o n a l l y , t h e r e was an emergence o f an a p n e i c p e r i o d (Tg-Tgi) a t body t e m p e r a t u r e s below 31 °C i n b o t h s p e c i e s ( F i g u r e 13). These p e r i o d s were p a r t i c u l a r l y prominent i n the h y p o t h e r m i c ground s q u i r r e l s . F i g u r e 15 i l l u s t r a t e s the c o n t r i b u t i o n o f each r e s p i r a t o r y phase d u r a t i o n to the t o t a l r e s p i r a t o r y c y c l e 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 . Accompanying hypothermia t h e r e was a r e d u c t i o n i n the f r a c t i o n o f i n s p i r e d time ( T T / T T Q T ) a n <^ a n i n c r e a s e i n the f r a c t i o n o f e x p i r a t o r y time (Tg/TfOT^ ^ u e t o a d e c r e a s e i n the f l o w g e n e r a t i n g component and an i n c r e a s e i n the a p n e i c component ( T E - T J M ) o f the e x p i r a t o r y time. A l t h o u g h the changes i n r e s p i r a t o r y t i m e s a r e q u a l i t a t i v e l y s i m i l a r i n the two - 5 4 -Figure 13. Relationship between t i d a l volume, and respiratory times at 3 d i f f e r e n t body temperatures i n the rat and ground s q u i r r e l . A l l values represent mean +/- standard error for 5 s q u i r r e l s and 7 rat s . Note: the slope representing the r e l a t i o n s h i p of V?/?! and the d i v i s i o n of respiratory times as described below: TIME (Seconds) -56-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 (Tj/TrpoT^ a n a " 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 (o) a t 36, 31 and 27 0 C body t e m p e r a t u r e . A l l v a l u e s r e p r e s e n t mean +/- s t a n d a r d e r r o r f o r 5 s q u i r r e l s and 7 r a t s . -57-1000-r 800 -CD ,E 600 + E L±J > 400 200 ~ 0 rCH 31'C 36°C 27 J * H 31°C 1 1 1 1 1 1 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 T| /T-TOT -58-Figure 15. Comparison of respiratory times as a f r a c t i o n of the t o t a l breath duration i n ground s q u i r r e l s and rats at 36 £J ,31 Q and 27°C • . A l l values represent mean +/-standard error for 5 s q u i r r e l s and 7 r a t s . FRACTION o b o o o h 01 p p bi b> —i 1— o c b < X X X X X X X X h / / / / h < X X X X X X X X X X h >////sy^ c c c c c 3 r [ FRACTION o b o o p —i— o o > at p o p bo X X X X X X X X X H • I X X X X X X X X X X h z : CO X X X X X X X X X X h z : -60-s p e c i e s , q u a n t i t a t i v e l y , t hey are much g r e a t e r i n the ground s q u i r r e l s . For example, the a p n e i c p e r i o d a t 27"C T 5 was 0.18 seconds i n the r a t and 2.0 seconds i n the ground s q u i r r e l (see F i g u r e 13). V T / T I d e c r e a s e d i n b o t h r a t s (p=0.03, 2 way A N O V A ) and ground s q u i r r e l s (p=0.007, 2 way A N O V A ) w i t h p r o g r e s s i v e hypothermia as d e p i c t e d by the s l o p e s o f the i n s p i r a t o r y p o r t i o n o f the c u r v e s i n f i g u r e 13. T h i s d e c r e a s e i n the " c e n t r a l i n s p i r a t o r y d r i v e " c o r r e l a t e s w e l l 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 the a n i m a l s a t any g i v e n body temperature ( F i g u r e 16: slope=130, r=0.96). 61-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<r7Tj) and minute v e n t i l a t i o n (Vg) i n ground s q u i r r e l s (•) and r a t s (o) 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 . A l l v a l u e s r e p r e s e n t mean +/- s t a n d a r d e r r o r i n 5 s q u i r r e l s and 7 r a t s . Slope=130, r=0.96. -62-0 1 . 2 3 4 5 6 7 8 9 10 V T / T , (ml/sec) -63-DISCUSSION D u r i n g normothermia, a n e s t h e t i z e d W i s t a r r a t 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 b r e a t h e c o n t i n u o u s l y . Both weight s p e c i f i c 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 Q Q 2 ) a r e h i g h e r i n the l a b o r a t o r y r a t compared to the ground s q u i r r e l . With s h i v e r i n g s u p p r e s s e d , i n b o t h s p e c i e s , Vjr and VrjQ2 a r e lowered p r o p o r t i o n a l l y a l o n g w i t h i n d uced d e c r e a s e s i n body tem p e r a t u r e ( T ^ ) . The p e r c e n t drop i n Vjr and V^o2 w i t h body temperature however i s g r e a t e r i n the ground s q u i r r e l compared to the l a b o r a t o r y r a t . T i d a l volume i s u n a l t e r e d by d e c r e a s e s i n body temperature and t h e r e i s a d e c r e a s e i n b r e a t h i n g f r e q u e n c y due t o the g r a d u a l d e c r e a s e i n T^OT * n both s p e c i e s w i t h d e c r e a s i n g Tvj. I n a d d i t i o n , i n s p i r a t o r y d r i v e ( V ^ / T j ) d e c r e a s e s a l o n g w i t h the d e c r e a s e i n 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 the two s p e c i e s w i t h h y p o t h e r m i a . I n both s p e c i e s t h e r e i s a g r a d u a l i n c r e a s e i n the phase d u r a t i o n s ( T j and Tg) o f 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 h ypothermia. Below 3 1 ° C T J - J , an a p n e i c p e r i o d appears which a c c o u n t s f o r a p r o g r e s s i v e l y g r e a t e r f r a c t i o n o f t o t a l r e s p i r a t o r y c y c l e (T«poT^ r e l a t i v e to the f l o w g e n e r a t i n g i n s p i r a t o r y ( T j ) and e x p i r a t o r y t imes ( T g i ) . -64-Methodology A n e s t h e s i a The p r i m a r y r e a s o n f o r a d d i t i o n o f h a l o t h a n e t o He-C-2 i n t h i s s t u d y was to e l i m i n a t e s h i v e r i n g d u r i n g the i n d u c t i o n o f h y p o t h e r m i a . I n a d d i t i o n , a n e s t h e s i a a l l o w e d d i r e c t measurement o f t i d a l volume u s i n g a f a c e mask. A l t h o u g h more a c c u r a t e , d e v i c e s such as f a c e masks, as w e l l as a n e s t h e t i c s , produce u n d e s i r a b l e a l t e r a t i o n s i n normal v e n t i l a t i o n ( G a u t i e r , 1976; F l e m i n g ejb al_. , 1983). A n e s t h e s i a and v e n t i l a t o r y d e p r e s s i o n a re i n e x t r i c a b l y l i n k e d . Choosing r e g u l a t e d l e v e l s o f h a l o t h a n e s u f f i c i e n t t o j u s t s u p p r e s s s h i v e r i n g , however, m i n i m i z e d the v e n t i l a t o r y d e p r e s s i o n due to a n e s t h e s i a t h r o u g h o u t h y pothermia and more i m p o r t a n t l y , a l l o w e d f o r s p e c i e s c o m p a r i s o n under i d e n t i c a l f u n c t i o n a l p l a n e s o f a n e s t h e s i a . I n t r i a l s t u d i e s o f t h r e e commonly used a n e s t h e t i c s i n r o d e n t s : sodium p e n t o b a r b i t o l ( S o m n o t o l ) , a t h i o b a r b i t u r a t e ( I n a c t i n ) and f l u o t h a n e ( h a l o t h a n e ) , we found h a l o t h a n e to s u p p r e s s v e n t i l a t i o n l e a s t when compared t o l i t e r a t u r e v a l u e s i n awake r a t s ( Appendix I ) . Because o f t h i s and due to the ease w i t h which the dose o f t h i s i n h a l a t i o n a n e s t h e t i c c o u l d be r e g u l a t e d 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 , h a l o t h a n e was the a n e s t h e t i c o f c h o i c e i n the p r e s e n t s t u d y . -65-Measurement o f M e t a b o l i c Rate: The use o f Vc02 a s a measure o f m e t a b o l i c r a t e i n v o l v e s the assumption t h a t the r e p i r a t o r y q u o t i e n t remained c o n s t a n t t hroughout h y p o t h e r m i a , and t h e r e f o r e changes i n VQ02 r e f l e c t e d changes i n oxygen consumption. S i n c e s h i v e r i n g was su p p r e s s e d by 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 a n e s t h e s i a , i t i s r e a s o n a b l e to assume t h a t t h e r e were no s i g n i f i c a n t changes i n the r e s p i r a t o r y q u o t i e n t (RQ). Moreover, i f the RQ d i d change t h i s would produce an e r r o r by a f a c t o r o f o n l y a p p r o x i m a t e l y +/- 0.15 i n the i n d e x f o r m e t a b o l i c r a t e ( a s s u m i n g RQ=0.85 d u r i n g normothermia and a p o s s i b l e range o f 0.7-1.0) . V e n t i l a t i o n and M e t a b o l i c Rate d u r i n g Normothermia I n g e n e r a l , v a l u e s o b t a i n e d f o r l e v e l s o f minute v e n t i l a t i o n , 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 i n the W i s t a r r a t under normothermic c o n d i t i o n s w h i l e m i l d l y a n e s t h e t i z e d a re w i t h i n the range o f v a l u e s p r e d i c t e d by S t a h l (1967) u s i n g s c a l i n g e q u a t i o n s f o r a l l mammals and s i m i l a r t o those o b s e r v e d i n u n a n e s t h e t i z e d r a t s (Pappenheimer, 1977; F a v i e r and L a c a i s s e , 1978; O l s o n and Dempsey, 1978; L a i et. al_. , 1978 A r i e l i and A r , 1979 and Holloway and Heath, 1984). The b r e a t h i n g f r e q u e n c i e s o b s e r v e d i n the p r e s e n t s t u d y were on the h i g h s i d e o f t h i s range which may r e f l e c t an a n e s t h e t i c e f f e c t . R a p id b r e a t h i n g has o f t e n been a s s o c i a t e d w i t h h a l o t h a n e a n e s t h e s i a and i t -66-has been suggested t h a t the i n c r e a s e i n b r e a t h i n g f r e q u e n c y i s due to an a c t i o n on the c e n t r a l rhythm g e n e r a t o r (Berkenbosch ejt al_. , 1982; Fukuda §_t al_. , 1982). The average m e t a b o l i c r a t e (20 ml CC^/min/kg) i n the r a t a t normothermia i n the p r e s e n t s t u d y agrees w e l l w i t h those o b t a i n e d i n u n a n e s t h e t i z e d r a t s by O l s o n and Dempsey (1978) but i s h i g h e r than t h a t o b t a i n e d i n the a n e s t h e t i z e d r a t s o f Cragg and D r y s d a l e (1983). The p e n t o b a r b i t o l a n e s t h e t i z e d r a t s i n the Cragg and D r y s d a l e s t u d y show r e l a t i v e l y lower v a l u e s o f minute v e n t i l a t i o n as w e l l , s u g g e s t i n g t h a t the d i s c r e p a n c y i n the v a l u e s f o r m e t a b o l i c r a t e and Vg s i m p l y r e f l e c t a d i f f e r e n c e i n the a n e s t h e t i c agent and/or the depth o f a n e s t h e s i a . R e s t i n g l e v e l s o f minute v e n t i l a t i o n i n the normothermic ground s q u i r r e l are a p p r o x i m a t e l y 40% o f those o b s e r v e d i n the normothermic r a t . These a r e w e l l below the v a l u e s o f Vg p r e d i c t e d by S t a h l (1967) u s i n g s c a l i n g e q u a t i o n s f o r a l l mammals. P r e d i c t e d v a l u e s f o r mammals however, do not a c c o u n t f o r s p e c i a l a d a p t a t i o n s which may have e v o l v e d w i t h i n a s p e c i e s . Indeed, low v a l u e s f o r minute v e n t i l a t i o n are commonly ob s e r v e d i n f o s s o r i a l and s e m i - f o s s o r i a l s p e c i e s ( A r i e l i & A r , 1979; Holloway & Heath, 1984; S c h l e n c h e r , 1985; Walker et. al.., 1985). Most s t u d i e s a t t r i b u t e the r e d u c t i o n i n Vg p r i m a r i l y t o d e c r e a s e s i n b r e a t h i n g f r e q u e n c y as was the case i n the p r e s e n t s t u d y . Due to r e l a t i v e l y h i g h e r r e s p i r a t o r y phase d u r a t i o n s (both h i g h e r i n s p i r a t o r y -67-and e x p i r a t o r y t imes) i n the ground s q u i r r e l compared to the r a t , b r e a t h i n g f r e q u e n c y was lower i n t h i s f o s s o r i a l s p e c i e s . I n a d d i t i o n , low v a l u e s f o r m e t a b o l i c r a t e a r e commonly o b s e r v e d i n f o s s o r i a l mammals i n c l u d i n g the pocket gopher, hedgehog, a r m a d i l l o (McNab, 1966) and i n p a r t i c u l a r i n ground s q u i r r e l s (Hudson and Deavers, 1973). I n agreement w i t h the l a t t e r r e p o r t , V*co2 w a s lower i n the ground s q u i r r e l compared to the r a t i n t h i s s t u d y . S i n c e both v a l u e s o f V"jr and Vc02 a r e p r o p o r t i o n a t e l y lower i n the s q u i r r e l as compared t o the r a t , 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 i n the two s p e c i e s i s s i m i l a r under normothermic c o n d i t i o n s d e m o n s t r a t i n g 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 r e t i g h t l y c o u p l e d i n both s p e c i e s . T h i s d a t a s u g g e s t s t h a t t h e r e a r e no d i f f e r e n c e s i n the c o n t r o l o f b r e a t h i n g i n the f o s s o r i a l ground s q u i r r e l compared to the non f o s s o r i a l r a t per se and t h a t the d i f f e r e n c e i n the l e v e l o f v e n t i l a t i o n i n the two s p e c i e s d u r i n g e u t h e r m i a r e f l e c t s the d i f f e r e n c e i n the m e t a b o l i c demand i n t h e s e a n i m a l s . S i n c e f o r most mammals, l u n g volumes s c a l e to body mass, i t i s not s u r p r i s i n g t h a t the w e i g h t - s p e c i f i c t i d a l volume i s s i m i l a r i n the ground s q u i r r e l ( r a n g i n g from 4.2-68 ml/kg) and the r a t (5.6-8.7 m l / k g ) . F u r t h e r m o r e , t h e s e r e s u l t s s uggest t h a t t h e r e may be no e f f e c t o f b u r r o w - d w e l l i n g on V T. -68-V e n t i l a t i o n and M e t a b o l i c Rate d u r i n g P r o g r e s s i v e Hypothermia The r e s u l t s o f t h i s s t u d y c l e a r l y demonstrate t h a t c o o l i n g the body causes p r o p o r t i o n a l d e c r e a s e s i n m e t a b o l i c r a t e , 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 spontaneous b r e a t h i n g , n o n - s h i v e r i n g mammals. The magnitude o f these changes i n the two s p e c i e s (see QIQ v a l u e s . Table I I I ) i s s i m i l a r t o t h a t r e p o r t e d i n dogs (Regan and Eger, 1966, Sodipo and Lee, 1971) and c a t s ( G a u t i e r and Gaudy, 1986; P o p o v i c and P o p o v i c , 1974). Because th e s e d e c r e a s e s a re p r o p o r t i o n a l , a c o n s t a n t a i r c o n v e c t i o n r e q u i r e m e n t (as i n d i c a t e d by a QIQ o f a p p r o x i m a t e l y one i n Table I I I ) , i s m a i n t a i n e d t h r o u g h o u t p r o g r e s s i v e h y p o t h e r m i a i n b o t h s p e c i e s as r e p o r t e d f o r o t h e r mammals (Lambertsen, 1980; Tenney and B a r t l e t t , 1981; Tenney and Boggs, 1986) A l t h o u g h not measured i n the p r e s e n t s t u d y , a consequence o f a c o n s t a n t ACR i s t h a t b l o o d gases and pH s h o u l d not change. Such e v i d e n c e has been documented i n the hyp o t h e r m i c dog w i t h a 10 C drop i n body temperature ( C r a n s t o n et. a l _ . , 1955; S a l z a n o and H a l l , 1960) and i n the h i b e r n a t i n g hedgehog ( C l a u s e n , 1966) and ground s q u i r r e l (Kent and P i e r c e , 1967; Musacchia and V o l k e r t , 1971) w i t h 30°C drops i n body tem p e r a t u r e . Whereas the changes i n V E and VQ02 a r e q u a l i t a t i v e l y the same i n the h y p o t h e r m i c r a t and the ground s q u i r r e l , these changes seem to d i f f e r q u a n t i t a t i v e l y i n the two -69-Table I I I : E f f e c t s o f changes i n body temperature on the average v a l u e s f o r 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 g o l d e n mantled ground s q u i r r e l s (n=5) and W i s t a r r a t s (n=7). W i s t a r 36"C 27°C V E (ml/min/kg) 820 397 VC02 (ml/min/kg) 20 9 f (b/min) 106 62 V T (ml/kg) 7.84 7.05 V E / V C 0 2 45 42 Rat Ground S q u i r r e l QlO 36°C 27°C Q 1 0 2.24 313 87 2.97 2.42 8 3 2.94 1.80 40 18 3.07 1.12 6.2 5.5 1.11 1.08 28 29 0.96 -70-s p e c i e s . For the same drop i n T^, the de c r e a s e i n Vjr and Vc02 r e l a t i v e to the normothermic v a l u e a r e a p p r o x i m a t l e y 25% more i n the ground s q u i r r e l ( F i g u r e s 8 and 11). However, t h i s q u a n t i t a t i v e d i f f e r e n c e i n p e r c e n t d e c r e a s e i n the two v a r i a b l e s i s m i s l e a d i n g s i n c e the two s p e c i e s have d i f f e r e n t i n i t a l normothermic l e v e l s o f Vg and V C Q 2 - * n the ground s q u i r r e l , the p e r c e n t change i n Vg and Vc02 * s g r e a t e r than i n the r a t s i m p l y because t h i s f o s s o r i a l s p e c i e s has s u b s t a n t i a l l y lower normothermic r e s p i r a t o r y and m e t a b o l i c r a t e s . Indeed the r a t e o f f a l l o f Vjr and VrjQ2 a s d e f i n e d by the Van't H o f f index (Qio i n Table I I I ) i s v e r y s i m i l a r i n the two s p e c i e s . The c o n s t a n c y o f the ACR 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 , i n the l a b o r a t o r y r a t and ground s q u i r r e l , s u g g e s t s t h a t minute v e n t i l a t i o n i s c o u p l e d t o the d e c r e a s i n g m e t a b o l i c demands t h a t o c c u r w i t h d e c r e a s e s i n Tj-, s i m i l a r l y i n both s p e c i e s . A d d i t i o n a l l y , b oth 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 t h e i r m e t a b o l i c demands d u r i n g hypothermia by d e c r e a s e s i n b r e a t h i n g f r e q u e n c y a l o n e ( F i g u r e 17). T i d a l volume i s not a l t e r e d by body c o o l i n g . T h e o r e t i c a l l y , p r e s e r v i n g t i d a l volume would ensure an adequate l e v e l o f a l v e o l a r v e n t i l a t i o n i n f a c e o f i n c r e a s i n g a n a t o m i c a l dead space d u r i n g hypothermia ( S e v e r i n g h a u s , 1963) and both s p e c i e s s t u d i e d i n the p r e s e n t s t u d y do indeed m a i n t a i n t h e i r normothermic l e v e l o f t i d a l volume. -71-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 ( V f ) and b r e a t h i n g f r e q u e n c y ( f ) t o minute v e n t i l a t i o n a t 36, 31 and 27°C body temperature i n the W i s t a r r a t (o) and the gol d e n - m a n t l e d ground s q u i r r e l (•). A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . -72-0 1 2 3 4 5 6 7 8 9 10 Vy (ml/kg) -73-I n t e r e s t i n g l y w i t h g r a d u a l h y p o t h e r m i a , as the l e v e l o f v e n t i l a t i o n drops a l o n g w i t h d e c r e a s i n g 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 d r i v e ( V T / T J ) i s reduced as w e l l . T h i s d e c r e a s e i n i n s p i r a t o r y d r i v e c o u l d r e s u l t from the e f f e c t o f hypothermia and/or hypometabolism on the c h e m i c a l d r i v e t o , or on the h i g h e r CNS c e n t e r s i m p i n g i n g on, 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 e f f e c t i s r e l a t e d i n the same l i n e a r f a s h i o n 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 the two s p e c i e s a t any g i v e n T 5 ( F i g u r e 16). T h i s r e l a t i o n s h i p p r o v i d e s f u r t h e r e v i d e n c e t h a t 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 p r o g r e s s i v e hypothermia i s the same i n the f o s s o r i a l ground s q u i r r e l and the n o n - f o s s o r i a l l a b o r a t o r y r a t ; the o n l y d i f f e r e n c e i n the two s p e c i e s i s the i n i t i a l e u t h e r m i c l e v e l o f m e t a b o l i c t i s s u e oxygen demand. -74-CHAPTER THREE EFFECT OF HYPOTHERMIA ON VENTILATORY RESPONSES TO CHANGING INSPIRED LEVELS OF C 0 2 AND 0 2. INTRODUCTION The " c i r c u i t diagram" o f the r e s p i r a t o r y c o n t r o l system remains l a r g e l y a " b l a c k box" w i t h p o o r l y c h a r a c t e r i z e d i n t e r n a l c o n n e c t i o n s . A number o f s t i m u l i a re known to a f f e c t v e n t i l a t i o n , such as c h a n g i n g b l o o d gases and pH, e l a s t i c and r e s i s t i v e l o a d s , e x e r c i s e , changes i n body t e m p e r a t u r e , p h a r m a c o l o g i c a l agents and p s y c h o l o g i c a l s t a t e s . I n t h i s c h a p t e r , the e f f e c t o f two n a t u r a l and most commonly used c h e m i c a l s t i m u l i , h y p e r c a p n i a and h y p o x i a on v e n t i l a t i o n and i t s components ( V f , f , T j , Tg, T<pQf, V-r/Tj, T J / T T O T ' T E f ' Tg-Tgi) w i l l be s t u d i e d d u r i n g normothermia (T)r,=36 +/- 1 C) and hypothermia (Tb = 27 +/- 1 °C) i n the l a b o r a t o r y r a t and g o l d e n mantled ground s q u i r r e l . S t u d i e s o f the s t e a d y s t a t e 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 dogs to h y p e r c a p n i a and h y p o x i a 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 (Regan and Eger, 196G and Sodipo and Lee, 1971) show t h a t the s e n s i t i v i t y ( s l o p e o f the v e n t i l a t o r y r e s p o n s e ) t o t h e s e gases drops a t a p p r o x i m a t e l y -75-the same r a t e as t h a t o f v e n t i l a t i o n (Qio=2.5). T h i s has not been s t u d i e d i n o t h e r mammals. Examining the changes i n the t h r e s h o l d and the s e n s i t i v i t y o f 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 d u r i n g h y p o t hermia i n the f o s s o r i a l ground s q u i r r e l and n o n - f o s s o r i a l r a t would de t e r m i n e whether such changes, p r o p o r t i o n a l to those o f 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 w i t h body c o o l i n g , a re w i d e s p r e a d . I f so, t h i s would s u p p o r t the c o n c l u s i o n o f the p r e c e e d i n g c h a p t e r s u g g e s i n g t h a t v e n t i l a t o r y c o n t r o l i s t i g h t l y c o u p l e d to changes i n m e t a b o l i c r a t e and i s independant o f the s e a s o n a l phenomenon o f h i b e r n a t i o n i n the ground s q u i r r e l . -76-MATERIALS AND METHODS Animals S i x a d u l t female W i s t a r r a t s (R. n o r v e q i c u s ) and s i x go l d e n - m a n t l e d ground s q u i r r e l s (S_. l a t e r a l i s ) were used i n t h i s s t u d y . The s o u r c e , weight r a n g e , d i e t and h o u s i n g c o n d i t i o n s o f the a n i m a l s are i d e n t i c a l to those d e s c r i b e d i n Chapter Two. Measurement o f Gas Responses A c o n s t a n t f l o w o f 1 l i t e r / m i n u t e o f He-02 (80%-20%) through the a n i m a l chamber was e s t a b l i s h e d u s i n g p r e c i s i o n f l o w m e t e r s ( s e r i e s 150, model FM1433 and FM4334, L i n d e - U n i o n C a r b i d e ) t h r o u g h o u t t h e s e s t u d i e s . I n a d d i t i o n , t e s t gases were c r e a t e d by m i x i n g 100% C O 2 , O 2 or N 2 u s i n g f l o w m e t e r s . The gas c o m p o s i t i o n o f both i n f l o w and o u t f l o w a i r s t r e a m s was a n a l y z e d w i t h i n +/- 0.1% u s i n g Beckman 0M11 O 2 and LB2 C O 2 gas a n a l y z e r s which were c a l i b r a t e d p r i o r t o e x p e r i m e n t s w i t h room a i r and premixed 5 and 10% C O 2 (Radiometer GMA 2 p r e c i s i o n gas s u p p l y ) . E x p e r i m e t a l p r o t o c o l Both the l e v e l s o f a n e s t h e s i a chosen t h r o u g h o u t the expe r i m e n t and the method o f i n d u c t i o n o f hypothermia were - 7 7 -i d e n t i c a l to e x p e r i m e n t s d e s c r i b e d i n Chapter Two. The ani m a l was p l a c e d i n t o the chamber and m a i n t a i n e d a t 36+l°C f o r 1/2 hour b e f o r e exposure to the t e s t gases. At the end o f t h i s p e r i o d a c o n t r o l b r e a t h i n g t r a c e was r e c o r d e d . The animal was then exposed to t e s t gas m i x t u r e s i n random o r d e r and a l t e r n a t i n g sequence w i t h He-C>2 c o n t r o l s . The f o l l o w i n g t e s t gases were used: H y p o x i a : ?I02= 2 0 % ' 1 7 % ' 1 5 % o r 1 3 % w i t h 80% h e l i u m and b a l a n c e n i t r o g e n . H y p e r c a p n i a : F I C 0 2 = 0 % ' 1 % ' 3 % o r 5 % w i t h FJQ2= 21% and b a l a n c e h e l i u m . The d u r a t i o n o f exposure t o each o f the t e s t gas m i x t u r e s was 15 minutes and b r e a t h i n g t r a c e s f o r a n a l y s i s were r e c o r d e d d u r i n g the l a s t minute o f exposure. The d u r a t i o n o f exposure to He-C-2 c o n t r o l s v a r i e d from 15-30 minutes u n t i l the r e t u r n o f v e n t i l a t i o n t o the o r i g i n a l c o n t r o l v a l u e was ensu r e d . The an i m a l was then c o o l e d t o Tb=27°C ±1°C and a d m i n i s t e r e d the same gas m i x t u r e s i n the same o r d e r f o r the same d u r a t i o n s as d u r i n g normothermia. The t o t a l d u r a t i o n o f an e x p e r i m e n t a l s e r i e s was 6-7 hou r s : a p p r o x i m a t e l y 2 hours f o r i n d u c t i o n o f hyp o t h e r m i a and 2 hours f o r gas s t u d i e s a t each body tem p e r a t u r e . - 7 8 -Data A n a l y s i s Twenty seconds o f b r e a t h i n g t r a c e was r e c o r d e d f o r a n a l y s i s a t h i g h speed (10 mm/sec) a t the end o f each c o n t r o l or t e s t gas exposure a t each tem p e r a t u r e . From these t r a c e s , v a l u e s o f V-p ( m l / k g ) ; f (1/min) ; Vg (ml/min/kg) and a c t u a l v a l u e s o f phase d u r a t i o n s ( T j , Tg, T g r , Tjr-Tgi) as w e l l as t h e i r f r a c t i o n a l c o n t r i b u t i o n to T<roT were o b t a i n e d as e x p l a i n e d i n Chapter Two f o r each a n i m a l . These v a l u e s were t h e n averaged t o g i v e the mean f o r each s p e c i e s . Data p r e s e n t e d as p e r c e n t change from c o n t r o l were c a l c u l a t e d as the d i f f e r e n c e between the t e s t gas v a l u e and the c o n t r o l v a l u e , as a p e r c e n t o f the l a t t e r , f o r each i n d i v i d u a l . These t r a n s f o r m e d d a t a were then averaged to g i v e the mean p e r c e n t change v a l u e f o r each s p e c i e s or s t a t e . Changes i n v e n t i l a t i o n (ml/min/kg) from c o n t r o l w i t h t e s t gases were a n a l y z e d by two way a n a l y s i s o f v a r i a n c e (ANOVA). S l o p e s o f the s e gas r e s p o n s e s were e s t i m a t e d by the method o f l e a s t s q u a r e s . L i n e a r r e g r e s s i o n and r e g r e s s i o n s o f l o g a r i t h m i c and h y p e r b o l i c t r a n s f o r m a t i o n s were f i t t e d but i t was found t h a t l i n e a r r e g r e s s i o n gave the b e s t c o r r e l a t i o n c o e f f i c i e n t f o r Vg. The s l o p e s o f the normothermic r e s p o n s e s were then compared t o the s l o p e s o f the re s p o n s e s o b t a i n e d d u r i n g hypothermia u s i n g S t u d e n t s ' s T - t e s t f o r the r e g r e s s i o n c o e f f i c i e n t s ( Z a r , 1984) a t each tem p e r a t u r e . The v e n t i l a t o r y t h r e s h o l d s were d e t e r m i n e d by comparing ( S t u d e n t - 7 9 -T - t e s t , p a i r e d ) the response o f i n d i v i d u a l s a t d i f f e r e n t l e v e l s o f i n s p i r e d h y p o x i a or h y p e r c a p n i a t o t h e i r c o n t r o l s . D i f f e r e n c e s were c o n s i d e r e d s i g n i f i c a n t a t p<0.05 u n l e s s i n d i c a t e d o t h e r w i s e . -80 RESULTS Responses to H y p e r c a p n i a The e f f e c t o f i n c r e a s i n g F i C02 o n v e n t i l a t i o n i n the r a t and ground s q u i r r e l i s shown i n F i g u r e 18. A l l s i x r a t s showed a g r a d u a l i n c r e a s e i n v e n t i l a t i o n i n response to h y p e r c a p n i a d u r i n g normothermia. On a v e r a g e , a n e s t h e t i z e d , normothermic r a t s respond to moderate h y p e r c a p n i a ( F I C 0 2 = 5 % ) w i t h a 50% i n c r e a s e (range=34-99%) i n V £ over normocapnic c o n t r o l l e v e l s . T h i s i n c r e a s e i n V J J was due to a s i g n i f i c a n t i n c r e a s e i n (p<0.001. F i g u r e 19). S i m i l a r l y d u r i n g h y p o t h e r m i a , a l l r a t s i n c r e a s e d t h e i r v e n t i l a t i o n i n r e s p o n s e t o h y p e r c a p n i a by a s i g n i f i c a n t i n c r e a s e i n V*rj» (p<0.0001). Mean v e n t i l a t i o n under moderate h y p e r c a p n i a ( F J C Q 2 = 5 % ) i n c r e a s e d on average 83% (range 16-88%) over the normocapnic l e v e l a t t h i s body t e m p e r a t u r e . The h y p e r c a p n i c 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 a t both body t e m p e r a t u r e s was between 1-3% ^IC02' There were no s i g n i f i c a n t changes i n b r e a t h i n g f r e q u e n c y w i t h h y p e r c a p n i a a t e i t h e r body t e m p e r a t u r e s ( F i g u r e 20) and the r e l a t i v e c o n t r i b u t i o n o f i n s p i r a t o r y and e x p i r a t o r y phase d u r a t i o n s to the t o t a l b r e a t h c y c l e time were not a l t e r e d ( F i g u r e 21). 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 a t 36 and 27°C, the c o r r e l a t i o n c o e f f i c i e n t , r , o f t h e s e s l o p e s and the Q^g e f f e c t on the s l o p e s a r e t a b u l a t e d i n Table I V . As i n d i c a t e d by the Q\Q o f a p p r o x i m a t l e y one, the s l o p e s o f the -81-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 Fjco2 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 (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27°C body temp e r a t u r e . A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% F J C Q 2 * -82-V E (ml/min/kg) 200 000 800 600- -400- -200- -X \ CZ]FIC02= o* ESFIC02= 1* IZZZFIC02= 3* mnco2= 5* Rat 36'C Rat 27'C GMGS 36'C GMGS 27*C 83-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 Ficc-2 o n t i d a l volume o f the W i s t a r r a t (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27"C body te m p e r a t u r e . A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% Fjco2--84-V T (ml/kg) 12 104-8 4 -0 A • FIC02= 0% ESFIC02= 1* IZZ2FIC02= 3% mnco2= 5* Rat 36 'C Rat 27 # C GMGS 36*C GMGS 27* C -85-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 FiC02 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 (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27°C body temperature. A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% F I C 0 2 -- 8 6 -uency (b/min) I CZ1FIC02= 0% ESFIC02= 1* ZZ7JFIC02= 3% ranco2= 5% Rat 36*C Rat 27'C GMGS 36*C GMGS 27'C -87-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 (Tt,=36°C) and hy p o t h e r m i c (T]3=27 0 C) r a t 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 CC-2-F I C O 2 = 0 % D ; F I C O 2 = 1 % 0 ; F I C O 2 = 3%[2 ; F I C 0 2 = 5 % S • -88-0.8 0.7 -0.6 • 0.5 o Q 0.4 0.3 0.2 0.1 -0.0 -S X \ X S X S X S X \ X \ X \ X T r = 3 6 • C \ x s x \ x \ X \ X \ X S X S X V X S X V X \ X S X \ X S X \ X S X S X S X \ X S X S X V T T O T T E / T - T O T T C / T T D T 0.8 0.7 0.6 O ° ' 5 5 0.4 0.3 0.2 0.1 0.0 T r - 2 7 * C |X. i X V X X X X V X V X V X S X S X S X V X V X V X V X S X S X i l k V T T C T T E / T T O T T C / T T O T T E - T C / T - T O T - 8 9 -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 of the r e l a t i o n 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 03 and CO2 f r a c t i o n i n r a t s and golden-mantled ground s q u i r r e l s a t 36 and 27°C body temperature. The c o r r e l a t i o n c o e f f i c i e n t s , r ; Y i n t e r c e p t s , b; and the sl o p e s of the gas response curves and the Q\Q v a l u e s f o r the e f f e c t of change i n body temperature are t a b u l a t e d f o r n=6 i n each group. T b(°C) Test Gas Slope Q-10 Rat Rat 36 27 S q u i r r e l 36 S q u i r r e l 27 Rat Rat 36 27 S q u i r r e l 36 S q u i r r e l 27 Hypercapnia 65.6 Hypercapnia 70.6 Hypercapnia 21,7 Hypercapnia 12,8 Hypoxia Hypoxia Hypoxia Hypoxia -26. 4 -30.0 -4.9 -2.5 0.99 0 . 96 0.94 0.97 0.91 0.91 0. 97 0.85 678 454 352 113 1210 1110 459 140 0 . 93 1. 70 0 . 88 1. 96 - y u -v e n t i l a t o r y r e s p o n s e s i n the r a t remained the same i r r e s p e c t i v e o f body tem p e r a t u r e . V e n t i l a t o r y r e s p o n s e s o f s q u i r r e l s to 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 C O 2 are d e p i c t e d i n F i g u r e 18 as w e l l . D u r i n g normothermia, i n s p i r a t i o n o f 5% C O 2 l e d t o a s l i g h t but s i g n i f i c a n t i n c r e a s e (26%) i n Vj; over the normocapnic l e v e l . T h i s response was c o n s i d e r a b l e b l u n t e d i n comparison w i t h the r a t (26 v e r s u s 50% i n c r e a s e i n average Vg r e s p e c t i v e l y ) . With h y p o t h e r m i a , the g r a d u a l i n c r e a s e i n V{£ w i t h i n c r e a s i n g l e v e l s o f h y p e r c a p n i a was m a i n t a i n e d but was not s t a t i s t i c a l l y s i g n i f i c a n t . The s e n s i t i v i t y t o h y p e r c a p n i a as i n d i c a t e d by the s l o p e o f t h i s v e n t i l a t o r y r e s p onse was reduced by a p p r o x i m a t e l y 1/2 w i t h the 10°C drop i n Tb ( T a b l e I V ) . At b o t h body t e m p e r a t u r e s , the v e n t i l a t o r y r e s p o n s e s were due t o i n c r e a s e s i n Vf ( F i g u r e 19). As t h e r e were no s i g n i f i c a n t changes i n the i n s p i r a t o r y and e x p i r a t o r y phase d u r a t i o n s ( F i g u r e 2 2 ) , b r e a t h i n g f r e q u e n c y d i d not change s i g n i f i c a n t l y w i t h 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 C O 2 ( F i g u r e 20) and the h y p e r c a p n i c t h r e s h o l d s f o r the e u t h e r m i c v e n t i l a t o r y r e s p o n s e s a t b o t h body t e m p e r a t u r e s f e l l between 1-3% C O 2 . Responses to Hypoxia The v e n t i l a t o r y r e s p o n s e s o f r a t s and ground s q u i r r e l s (n=6 i n each group) to 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 O 2 are - 9 1 -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 (Tfc,=36 °C) and hyp o t h e r m i c (T b=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. F T C 0 2 = 0 % F T C 0 2 = 1 % ^ ; F l C 0 2 = 3 % S;F I C 0 2 = 5 % 0 . »*xxxx><xxxx><xx^  HXXXXXXXXXXXX H ^ Z / / / / / / / / / / O H — CO ? *KO<XXX><XXXXXXXXXX^^ HXXXXXXXXX XTTX *\// // // // / r^nr H^XXXXXXX^KXX HXXXXXXX) H / / / / ^ 7 " ^ o q r ^ « ) i 0 ' * n c > j « - ; 0 d o o o ' d o o o o N0I10VUJ •—KXXXX><XX>«XXXXXXXXX; H y y y y y y y y y y s s s s s s s / s P^T C^KXXKXXX MX X X ^ >-KX><XX>WXX>&OW HXXXXXXXXXXXXXX H X X X X X. H / / / / z : < D r « : « o i O ' * - » o c M » - o d d d d d d d d d NOLLOVHd -93-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 F J Q 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 (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27° C body temp e r a t u r e . A l l v a l u e s r e p r e s e n t the mean +/-s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% FjQ2-- 9 4 -V E (ml/min/kg) 1200-r I000--800 600 --400- -200- -0 • i i IZDFI02= 20% ESFI02= 17% IZZ2FI02= 15% LXlFI02= 13% ft t l '4 Rot 36*C Rat 27'C GMGS 36*C GMGS 27'C -95-shown i n F i g u r e 23. D u r i n g normothermia, 15% i n s p i r e d O2 or l e s s l e d to a g r a d u a l i n c r e a s e i n v e n t i l a t i o n i n the r a t t (p<0.05). On average, a 26% i n c r e a s e i n Vg above the normoxic c o n t r o l l e v e l s was o b s e r v e d w i t h 13% i n s p i r e d O2 i n t h i s s p e c i e s . At t h i s l e v e l o f h y p o x i a o n l y one o f the r a t s t e s t e d f a i l e d t o i n c r e a s e v e n t i l a t i o n above the normoxic v a l u e . The v e n t i l a t o r y response o b s e r v e d was due to an i n c r e a s e i n b r e a t h i n g f r e q u e n c y a l o n e (p<0.001; F i g u r e 24) as V<j> as not a l t e r e d s i g n i f i c a n t l y d u r i n g normothermic h y p o x i a ( F i g u r e 25). V e n t i l a t i o n i n c r e a s e d i n r e sponse to h y p o x i a i n the r a t d u r i n g h y p o t hermia as w e l l (35%) but no o b v i o u s t h r e s h o l d l e v e l s were o b s e r v e d d u r i n g h y p o t h e r m i a . I n c o n t r a s t to the h y p o x i c response d u r i n g normothermia, the i n c r e a s e i n v e n t i l a t i o n d u r i n g h y p o t hermia was due t o g r a d u a l i n c r e a s e s i n b oth b r e a t h i n g f r e q u e n c y and Vy ( F i g u r e s 24 &25). There were no changes i n the i n s p i r a t o r y o r e x p i r a t o r y t i m e s but w i t h 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, a c t i v e e x p i r a t o r y time ( T E I / T ^ O T ^ i n c r e a s e d as the a p n e i c p e r i o d ( T g - T g ' / T T Q T ) s h o r t e n e d ( F i g u r e 26). B r e a t h i n g f r e q u e n c y d i d not change s i g n i f i c a n t l y under these c o n d i t i o n s ( F i g u r e 24). The s l o p e o f the normothermic v e n t i l a t o r y r e s p onse was not s i g n i f i c a n t l y d i f f e r e n t from t h a t d u r i n g h y p o t h e r m i a ( T a b l e I V ) . F i g u r e 23 a l s o shows the h y p o x i c r e sponse o f ground s q u i r r e l s to 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. As the s l o p e s o f t h e s e r e s p o n s e s suggest ( T a b l e I V ) , v e n t i l a t i o n was -96-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 F J C O 2 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 (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27°C body tem p e r a t u r e . A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% FiC02--97-Frequency (b/min) 140-r 120-100-80-60-40 20-f 0 T 1VZ 0 • FI02= 20% ESFI02= 17% IZZZFI02= 15% mFI02= 13% Rat 36*C Rat 27*C GMGS 36*C GMGS 27*C -98-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 on t i d a l volume o f the W i s t a r r a t (n=6) and the g o l d e n mantled ground s q u i r r e l (n=6) a t 36 and 27°C body tem p e r a t u r e . A l l v a l u e s r e p r e s e n t the mean +/- s t a n d a r d e r r o r . * r e p r e s e n t s v a l u e s s t a t i s t i c a l l y d i f f e r e n t ( T - T e s t , p<0.05 p a i r e d ) from 0% F J Q 2 --99-V J (ml/kg) 12 10 • Fi02= 20% E 3 F I 0 2 = 17% EZ2FI02= 15% CX3FI02= 13% 8 -6 -4--2 - -0 77s 'A A. Rat 36'C Rat 27'C GMGS 36'C GMGS 27'C -100-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 (T^=3SCC) and h y p o t h e r m i c (Tb=27 °C) 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 O 2 . F I O 2 = 20%D ; F I 0 2 = 17% 0 ; F I 0 2 = 15% 3 ; F I 0 2 = 1 3 % S . -101-0.8 0.7 r 0.6 0.5 r OA 0.3 0.2 r 0.1 0.0 Tr - 3 6 ^ C v X N X N X S X ^ X IN* s x s x s x s x V x s x s x s x \ X m IN* s x s x s x N X s x V x s x s x V x N X T l /TTOT T E / T-ro T T p/TTOT 0.8 0.7 I-0.6 0.5 • 0.4 -0.3 • 0.2 0.1 0.0 \ x S X S X S X S X S X S X Tr = 2 7 *C \ X \ X \ X V X \ X \ X s x S X V X S X V X S X 18 I I S V T T O T T E / T T O T T p / T T O T T E - T P / T T O T -102-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°C) and hy p o t h e r m i c (Tb=27 °C) 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 0 2. F I O2=20% • ; F T O 2 = 1 7 % 3^ ; F T O 2 = 1 5 % ^ l F I 0 2 = 1 3 % 0 . -103-0.8 0.7 0.6 r— Q 0.4 •J- 0.3 0.2 0.1 0.0 Tr = 3 6 B C i JL _L \ X _ S X a s X s s X 5 X 6 s X R s X ft w ft s s s s s s s \ \ \ \ s s s T I A T O T T E / T T O T T P / T T O T 0.8 0.7 0.6 o °-5 p Q 0.4 0.3 0.2 0.1 0.0 ^ x ^ x ^ x ^ X Tr = 27'C f jx s x s x s x v x s x V x s x V x s x s x s, X \ x 11 \ X \ X \ X X ^ X ^ X v X v X v X N X ^ X M L T l / T T 0 T T E / T T O T T E - / T T O T TE—TE*/ T JQJ -104-not i n c r e a s e d s i g n i f i c a n t l y between 20 and 13% F J Q2- There were l i t t l e change i n r e s p i r a t o r y phase d u r a t i o n s ( F i g u r e 2 7 ) , b r e a t h i n g f r e q u e n c y ( F i g u r e 24) or Vrp ( F i g u r e 25) w i t h d e c r e a s i n g l e v e l s o f F J Q2- T h e h y p o x i c r esponses o f h y p o t h e r m i c ground s q u i r r e l s were h i g h l y v a r i a b l e . I n 4/6 s q u i r r e l s , Vg d i d not change a p p r e c i a b l y under any of the h y p o x i c t e s t gases. By c o n t r a s t , two s q u i r r e l s were v e r y s e n s i t i v e to h y p o x i a (40% and 200% i n c r e a s e i n Vg above normoxia a t F j o 2 = 0 ' i 3 ) . Comparison o f 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 t o h y p o x i a a t 36 and 27°C T D i n the ground s q u i r r e l s ( T a b l e I V ) i n d i c a t e s a 50% d e c r e a s e i n s e n s i t i v i t y t o t h i s gas w i t h h y p o t hermia but due to the s m a l l r e s p o n s e s and h i g h v a r i a b i l i t y t h i s d i f f e r e c e was not s t a t i s t i c a l l y s i g n i f i c a n t . -105-D i s c u s s i o n Both normothermic ground s q u i r r e l s and r a t s show a g r a d u a l i n c r e a s e i n minute v e n t i l a t i o n i n response to i n c r e a s e d l e v e l s o f i n s p i r e d C O 2 and reduced l e v e l s o f i n s p i r e d O 2 . The r e s p o n s e s t o b o t h t e s t gases however are c o n s i d e r a b l y b l u n t e d i n the ground s q u i r r e l compared to the l a b o r a t o r y r a t . T h i s i s even more so d u r i n g hypothermia. With d e c r e a s e s i n body t e m p e r a t u r e , the s l o p e o f these r e s p o n s e s i s reduced to a p p r o x i m a t e l y h a l f ( Q I Q = 1 . 7 and 1.96) o f the normothermic v a l u e s i n the ground s q u i r r e l w h i l e i n the r a t the s l o p e o f the v e n t i l a t o r y r e s p o n s e s i s not a l t e r e d (Qio=0.93 and 0.88) w i t h d e c r e a s e s i n body te m p e r a t u r e . Methodology "Steady S t a t e " The t e c h n i q u e used i n the p r e s e n t s t u d y r e l i e s on measuring minute v e n t i l a t i o n i n response to a s t e a d y m a i n t a i n e d s t i m u l u s . To q u a n t i f y the r e s u l t s a c c u r a t e l y , a t l e a s t t h r e e l e v e l s o f F ' l C 0 2 / ' F I 0 2 must be chosen to deter m i n e the s l o p e o f the r e l a t i o n s h i p between Vg and F I C 0 2 / ' F I 0 2 ' R e c o r d i n g v e n t i l a t o r y r e s p o n s e s t o changes i n i n s p i r e d gas c o m p o s i t i o n however, can o n l y g i v e an - 1 0 6 -e s t i m a t e o f the 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 e r c a p n i a or h y p o x i a . True v e n t i l a t o r y s e n s i t i v i t i e s a r e q u a n t i f i e d by measurements o f e n d - t i d a l o r p r e f e r a b l y , a r t e r i a l b l o o d gas t e n s i o n s . I n the p r e s e n t s t u d y , i t i s assumed t h a t a r t e r i a l b l o o d gases r e f l e c t e d the changes i n i n s p i r e d F T C O 2 a n a " F J O 2 - * t * s noteworthy t h a t v e n t i l a t o r y r e s p o n s e s w i t h i n any s p e c i e s a r e r e m a r k a b l y v a r i a b l e and not n e c c e s s a r i l y dependant on d i f f e r e n c e s i n a n e s t h e s i a / c o n s c i o u s n e s s or the t e c h n i q u e used. Among h e a l t h y men f o r example, the range o f v e n t i l a t o r y r e s p o n s e s i s a p p r o x i m a t l e y 16 f o l d (Rebuck and S l u t s k y , 1981). A n e s t h e s i a A c c o r d i n g t o r e c e n t a n a l y s e s on l a r g e mammalian s p e c i e s h a l o t h a n e i n d u c e s a dose dependant d e p r e s s i o n i n the v e n t i l a t o r y r e s ponse t o CO2 and 0 2 (For r e v i e w s see: H i c k e y and S e v e r i n g h a u s , 1981 and P a v l i n and H o r n b e i n , 1986). The e f f e c t o f h a l o t h a n e on the s e v e n t i l a t o r y r e s p o n s e s has been shown t o be s p e c i e s s p e c i f i c . Whereas MAC o f h a l o t h a n e i n dogs and c a t s reduces the v e n t i l a t o r y response t o h y p e r c a p n i c and h y p o x i c s t i m u l i ( D a v i e s , §_t aJL.. , 1982; Hirshman e t a l , , 1977 and Weiskopf e t a l . , 1974), i t has no e f f e c t on the h y p e r c a p n i c response i n humans but a b o l i s h e s t h e i r h y p o x i c response ( K n i l l and G e l b , 1978). There a r e two r e p o r t s on the v e n t i l a t o r y -107-r e s p o n s e s t o i n s p i r e d l e v e l s o f CO2 and 0 2 i n the awake e u t h e r m i c g o l d e n - m a n t l e d ground s q u i r r e l s from our l a b o r a t o r y ( M c A r t h u r , 1986 and Webb, 1987). I n c r e a s e s i n v e n t i l a t i o n t o 5% CO2 are i d e n t i c a l t o those o b t a i n e d i n the p r e s e n t study s u g g e s t i n g t h a t h a l o t h a n e has no e f f e c t on the h y p e r c a p n i c response o f the ground s q u i r r e l s . S i m i l a r l y , t h e r e i s l i t t l e d i f f e r e n c e i n the h y p o x i c response o f the awake ground s q u i r r e l to 13% O2 s t u d i e d by McArthur (1986) compared to the a n e s t h e t i z e d a n i m a l s i n the p r e s e n t s t u d y . I n c o n t r a d i c t i o n t o the former r e p o r t , Webb (1987) f i n d s a s i g n i f i c a n t i n c r e a s e i n minute v e n t i l a t i o n a t 13% O2 i n the awake g o l d e n - m a n t l e d ground s q u i r r e l . G i v e n the v a r i a b i l i t y o f v e n t i l a t o r y r e s p o n s e s t o h y p o x i a and the l a c k o f b l o o d gas d a t a i n the above s t u d i e s , i t i s d i f f i c u l t t o c o n c l u d e whether h a l o t h a n e has any s i g n i f i c a n t i n h i b i t o r y e f f e c t on the h y p o x i c r e s p o n s e s o f the ground s q u i r r e l s . Responses t o H y p e r c a p n i a Normothermia R e p o r t s on the p e r c e n t i n c r e a s e i n minute v e n t i l a t i o n o f the l a b o r a t o r y r a t to 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 C 0 2 v a r y c o n s i d e r a b l y , r a n g i n g from 50 to 200 % i n c r e a s e s a t an -108-FlC02 °f 5% ( L a i §_t al_- r A r i e l i and A r , 1979; Cragg and D r y s d a l e , 1983; Holloway and Heath, 1984). Some o f the d i f f e r e n c e s i n r e p o r t e d v a l u e s a re due to d i f f e r e n t t y p e s and p l a n e s o f a n e t h e s i a masking the r e s p o n s e s o f the a n i m a l s , and o t h e r s may be due to d i f f e r e n c e s i n the s t r a i n o f the l a b o r a t o r y r a t i n v e s t i g a t e d . The magnitude o f i n c r e a s e i n minute v e n t i l a t i o n o f W i s t a r r a t s used i n t h i s s t u d y , i s s i m i l a r t o t h a t o b t a i n e d i n the u n a n e s t h e t i z e d W i s t a r r a t s o f A r i e l i and Ar (1979). The magnitude o f the v e n t i l a t o r y r e s ponse o f the go l d e n - m a n t l e d ground s q u i r r e l t o the same range o f i n s p i r e d CO2 i s s i g n i f i c a n t l y b l u n t e d as compared t o the l a b o r a t o r y r a t . B l u n t e d h y p e r c a p n i c s e n s i t i v i t y has been w e l l documented i n a v a r i e t y o f f o s s o r i a l s p e c i e s such as the poc k e t gopher, marmot, mole r a t , g o l d e n hamster and the gol d e n - m a n t l e d and Columbian ground s q u i r r e l (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; McA r t h u r , 1986; and Webb, 1986). T h i s appears t o be an a d a p t a t i o n t o the burrow environment and c o i n c i d e s w i t h the h i g h r e s t i n g Pacc-2 a n < ^ ^ o w v e n t i l a t i o n o f t e n found i n f o s s o r i a l mammals and b i r d s ( A r i e l i and A r , 1979; Boggs ejb a l . , 1984; See C h a p t e r s One and Two as w e l l ) . The o v e r a l l r e d u c t i o n i n the v e n t i l a t o r y response to a l l l e v e l s o f i n s p i r e d CO2 i n f o s s o r i a l s p e c i e s may r e s u l t from d e c r e a s e s i n chemoreceptor s e n s i t i v i t y or i n c r e a s e s i n the b l o o d b u f f e r i n g c a p a c i t y o f the s e a n i m a l s through i n c r e a s e d b l o o d -109 b i c a r b o n a t e or through r e n a l a d j u s t m e n t s (Boggs §_t a l . , 1984 ) . Whatever the d i f f e r e n c e ( s ) , i t appears t o be g e n e t i c a l l y d e t e r m i n e d as r e p o r t s on c h r o n i c p e r i n a t a l CO2 exposure i n r a t s and mice show no d i f f e r e n c e s compared to the a d u l t h y p e r c a p n i c Vg response ( B i r c h a r d e_t al_. , 1984). 4 I n the p r e s e n t s t u d y , the i n c r e a s e i n Vg i n both r a t s and ground s q u i r r e l s i n response to CO2 was due to s i g n i f i c a n t i n c r e a s e s i n V-r- a l o n e ( F i g u r e s 19, 20 &22). Response p a t t e r n s t o h y p e r c a p n i a r e p o r t e d p r e v i o u s l y i n r a t s and ground s q u i r r e l s b o th awake and a n e s t h e t i z e d show s m a l l or e q u i v a l e n t i n c r e a s e s i n b r e a t h i n g f r e q u e n c y as w e l l (Pappenheimer, 1977; L a i e_t a_l_. , 1978; Cragg and D r y s d a l e ,1983; M c A r t h u r , 1986 and Webb, 1986). The l a c k o f any fr e q u e n c y response i n the two s p e c i e s d u r i n g h y p e r c a p n i a i n t h i s s t u d y c o u l d w e l l be due to the c e n t r a l e f f e c t o f h a l o t h a n e on the m e d u l l a r y r e s p i r a t o r y c e n t e r (Ngai §_t a l , 1965). Responses t o Hypoxia The o v e r a l l r e s p o n s e s o f r a t s and ground s q u i r r e l s t o 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 0 2 i n t h i s s t u d y a re l e s s t han those p r e v i o u s l y r e p o r t e d (Pappenheimer, 1977; F a v i e r and L a c a i s s e , 1978; A r i e l i and A r , 1979; Cragg and D r y s d a l e , 1983; Holloway and Heath, 1984 ; and Webb,, 1986). A l t h o u g h - 1 1 0 -the e f f e c t s o f h a l o t h a n e a n e s t h e s i a i n these two s p e c i e s on the v e n t i l a t o r y response t o h y p o x i a i s unknown, the p o t e n t i n h i b i t o r y e f f e c t o f t h i s drug on v e n t i l a t o r y reponses o f man and dog suggest t h a t t h e r e c o u l d w e l l be some masking o f the t r u e response i n the p r e s e n t s t u d y . A d d i t i o n a l l y , as Cragg and D r y s d a l e ( 1 9 8 3 ) p o i n t o u t , the f u l l e x p r e s s i o n o f the h y p o x i c response o f the a n e s t h e t i z e d r a t depends on the maintenance o f i s o c a p n i a . D u r i n g exposure t o 15 or 10% O2, they found a d e c r e a s e i n Pac02 t h a t c o u l d be o f f s e t w i t h a d d i t i o n o f 6% CO2 to the i n s p i r a t e . End t i d a l o r a r t e r i a l gas t e n s i o n s were not measured i n the p r e s e n t s t u d y , and no a t t e m p t s were made to m a i n t a i n the a n i m a l s i s o c a p n i c . Hence, h y p o c a p n i a c o u l d have p o s s i b l y c o n t r i b u t e d t o the d e p r e s s e d h y p o x i c response o f the r a t and ground s q u i r r e l i n t h i s s t u d y . A l t e r n a t i v e l y , an F J O 2 = 0 - 1 3 may not have been s t r o n g enough t o e l i c i t a l a r g e r e s p i r a t o r y r e s p o n s e i n e i t h e r s p e c i e s . For example, r e p o r t s i n a n e s t h e t i z e d r a t s (Cragg and D r y s d a l e , 1 9 8 3 ) and u n a n e s t h e t i z e d r a t s and ground s q u i r r e l s (Pappenheimer, 1 9 7 7 ; Holloway and Heath,1 9 8 4 ; and McA r t h u r , 1986 ) suggest t h a t the v e n t i l a t o r y t h r e s h o l d t o h y p o x i a i s c l o s e r t o 10% O2. N o n e t h e l e s s , a co m p a r i s o n o f the s l o p e s o f the v e n t i l a t o r y r e s ponse c u r v e s to v a r y i n g l e v e l s o f i n s p i r e d CO2 ( T a b l e I V ) , i l l u s t r a t e s t h a t i n the go l d e n - m a n t l e d ground s q u i r r e l , v e n t i l a t o r y s e n s i t i v i t y i s d e c r e a s e d w i t h h y p o t h ermia to a s i m i l a r e x t e n t as Vfjrj2 (QlO ~ 2 ) . - 1 1 1 -A l t h o u g h a comparison o f the s l o p e s o f the v e n t i l a t o r y response c u r v e s to h y p o x i a ( T a b l e IV) s h o u l d be t r e a t e d w i t h c a u t i o n s i n c e these s l o p e s a re not s i g n i f i c a n t l y d i f f e r e n t from z e r o , they do i n d i c a t e t h a t i n the ground s q u i r r e l , 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 reduced to the same e x t e n t as V r j Q 2 (QlO~ 2 ) . These comparisons s u g g e s t t h a t v e n t i l a t o r y c o n t r o l i s not a l t e r e d d u r i n g h y p o t h e r m i a but s i m p l y reduced i n g a i n t o the same e x t e n t as the m e t a b o l i c r e q u i r e m e n t s o f the c o l d a n i m a l . S u r p r i s i n g l y 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 t o i n s p i r e d l e v e l s o f C O 2 and O 2 i n the l a b o r a t o r y r a t d u r i n g hypothermia were not s i g n i f i c a n t l y d i f f e r e n t from the normothermic v a l u e s . T h i s i n d i c a t e s t h a t d e s p i t e the r e d u c t i o n i n metabolism i n the h y p o t h e r m i c a n i m a l the r e s p i r a t o r y system g a i n remains the same, r e s u l t i n g i n g r e a t e r r e l a t i v e s e n s i t i v i t y to i n s p i r e d gases a t low body t e m p e r a t u r e s . These r e s u l t s a r e a l s o i n c o n s i s t a n t w i t h those found i n l a r g e r mammals d u r i n g p r o g r e s s i v e h y p o t h e r m i a . G i v e n the r e m a r k a b l e v a r i a b i l i t y o f v e n t i l a t o r y s e n s i t i v i t y w i t h i n any s p e c i e s , the s l o p e s o f v e n t i l a t o r y r e s p o n s e s i n the r a t i n the p r e s e n t s t u d y c o u l d s i m p l y r e f l e c t the b e h a v i o r o f t h i s s m a l l sample s i z e . A l t e r n a t i v e l y , i f the s l o p e s o b t a i n e d i n t h i s s t u d y a r e t r u e i n d i c e s o f r e s p i r a t o r y s e n s i t i v i t y i n the r a t , the mechanisms u n d e r l y i n g the d i f f e r e n c e s i n c o n t r o l o f v e n t i l a t i o n i n the W i s t a r r a t d u r i n g h y o t h e r m i a need f u r t h e r assessment. 112-CHAPTER FOUR GENERAL DISCUSSION A p r e v i o u s r e p o r t from our l a b o r a t o r y demonstrated t h a t as l o n g as the go l d e n - m a n t l e d ground s q u i r r e l remained 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 e r c a p n i a and h y p o x i a r e m a i n c o n s t a n t over the ambient temperature range o f 5-25°C throughout the year ( M c A r t h u r , 1986). D r a m a t i c r e d u c t i o n s i n minute v e n t i l a t i o n i n c o n j u n c t i o n w i t h m e t a b o l i c r a t e however,are c h a r a c t e r i s t i c o f e n t r a n c e i n t o h i b e r n a t i o n i n grounds s q u i r r e l s . The p r e s e n t s t u d y was 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 t h e s e r e d u c t i o n s i n v e n t i l a t i o n a r e independant 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 but r e f l e c t a t i g h t c o u p l i n g between 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 ground s q u i r r e l s when body temperature i s reduced. I n a d d i t i o n , the scope o f t h i s h y p o t h e s i s was t e s t e d by e x a m i n i n g the e f f e c t s o f h y p o t h e r m i a on minute v e n t i l a t i o n and i t s c o n t r o l i n the n o n - h i b e r n a t i n g l a b o r a t o r y r a t . The p r e s e n t s t u d y shows t h a t i n the a n e s t h e t i z e d , non s h i v e r i n g , g o l d e n - m a n t l e d ground s q u i r r e l , p r o g r e s s i v e h ypothermia r e s u l t s i n p r o p o r t i o n a t e d e c r e a s e s i n metabolism and minute v e n t i l a t i o n as r e f l e c t e d by CO2 consumption and b r e a t h i n g f r e q u e n c y r e s p e c t i v e l y . D u r i n g p r o g r e s s i v e h y p o t h ermia t o 27°C body tem p e r a t u r e b r e a t h i n g remains - 1 1 3 -c o n t i n u o u s . The r e d u c t i o n i n v e n t i l a t i o n i n t h i s range o f hypothermia i s w e l l c o n t r o l l e d . T i d a l volume i s p r e s e r v e d i n fa v o u r o f adequate a l v e o l a r v e n t i l a t i o n . R e d u c t i o n s i n minute v e n t i l a t i o n due to d e c r e a s e s i n b r e a t h i n g frequency match w e l l the de c r e a s e i n m e t a b o l i c demand 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 . 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 remains the same th r o u g h o u t body c o o l i n g , r e f l e c t i n g the t i g h t c o u p l i n g between r e s p i r a t i o n and metabolism i n the two s p e c i e s . I f we compare the e f f e c t o f body temperature on minute v e n t i l a t i o n , 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 d u r i n g hypothermia ( i n the p r e s e n t s t u d y ) t o those o b t a i n e d d u r i n g deep h i b e r n a t i o n ( M c A r t h u r , 1986; Webb, 1987), we f i n d s t r i k i n g l y s i m i l a r QIQ e f f e c t s ( T a b l e V ) . The average v a l u e s f o r 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 o b t a i n e d i n d e e p l y h i b e r n a t i n g g o l d e n - m a n t l e d ground s q u i r r e l s a t 7*C body tem p e r a t u r e ( M c A r t h u r , 1986 and Webb 1987), f a l l c l o s e to the e x p e c t e d v a l u e s based on the r e g r e s s i o n e q u a t i o n s d e f i n i n g the v a l u e s o b t a i n e d 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 ( F i g u r e s 28 and 29 r e s p e c t i v e l y ) . S i m i l a r l y , changes i n the 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 i b e r n a t i o n a r e an e x t e n t i o n o f changes o b s e r v e d w i t h p r o g r e s s i v e h y p o thermia ( F i g u r e 30). The 7°C h i b e r n a t i n g ground s q u i r r e l has p r o p o r t i o n a l l y g r e a t e r r e s p i r a t o r y phase d u r a t i o n s , and hence lower b r e a t h i n g f r e q u e n c y , but i t s t i d a l volume, l i k e the hyp o t h e r m i c a n i m a l i s not a l t e r e d w i t h r e d u c e d body t e m p e r a t u r e . I n s p i r a t o r y f l o w r a t e i s reduced -114-Table V: E f f e c t s o f changes i n body temperature on the average v a l u e s f o r 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 hypothermic (n=5) 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 (n=7). H i b e r n a t i n g Hypothermic 36°C 7°C Q 1 0 36°C 27°C Q 1 0 V E (ml/min/kg) *319 *13 3.02 313 87 2.97 V C 0 2 (ml/min/kg) **8 **0.3 3.10 8 3 2.94 f (b/min) 59 2.6 2.88 40 18 3.07 *data from McArthur (1986) ** d a t a from Webb (1987) o -115-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 h y p o t h e r m i a i n g o l d e n - m a n t l e d ground s q u i r r e l s (o, n=5). o.l3Tb The e x p o n e n t i a l r e g r e s s i o n : Vjj;=2.56e d e f i n e s t h i s r e l a t i o n s h i p w i t h r=0.97. * r e p r e s e n t s mean v a l u e s o b t a i n e d d u r i n g e u t h e r m i a and h i b e r n a t i o n (n=7, McA r t h u r , 1987) i n the same s p e c i e s . -117-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 h y p o t h ermia i n g o l d e n - m a n t l e d ground s q u i r r e l s (o, n=5). The e x p o n e n t i a l r e g r e s s i o n : VrjO2 =0-091 e d e f i n e s t h i s r e l a t i o n s h i p w i t h r= 0.98. * r e p r e s e n t s the mean v a l u e s o b t a i n e d d u r i n g e u t h e r m i a and h i b e r n a t i o n (Webb, 1987) i n t h i s s p e c i e s . VC02 (ml/min/kg) 6 12 18 24 30 36 T e m p e r a t u r e (°C) - 1 1 9 -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 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 g o l d e n mantled 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 0 C body temperature (n=5) and a t 36 and 7 C (n=7) d u r i n g h i b e r n a t i o n . A l l v a l u e s r e p r e s e n t mean +/-s t a n d a r d e r r o r f o r 5 s q u i r r e l s . -120-100- r 9 0 -8 0 -7 0 -CJ 0 CD 6 0 -ro 5 0 -I— 4 0 -1 > 3 0 -2 0 -1 0 -0 -,», T t . \ i , I \ \ \ f/ I 1 1 \ ! i \ 1 t \ \ \ V L J-36°C — - 3 r c 27°C — 7'C • I i I • I i 1 • I i 1 i I i I i I i I i 1 i I ' I 0 1 2 3 4 5 6 7 8 9 10 1 1 1 2 TIME (Seconds) -121-f u r t h e r than t h a t o f the 27°C h y p o t h e r m i c a n i m a l s but the r e l a t i o n s h i p between t h i s index o f 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 s not s i g n i f i c a n t l y d i f f e r e n t from t h a t found i n hypothermia ( F i g u r e 31). The s e n s i t i v i t y o f the r e s p i r a t o r y system as d e f i n e d by the s l o p e 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 e r c a p n i c and h y p o x i c s t i m u l i i s changed w i t h s i m i l a r t e mperature c o e f f i c i e n t (Qirj-3) a s 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 w i t h p r o g r e s s i v e hypothermia. These o b s e r v a t i o n s show t h a t a l t h o u g h h i b e r n a t i o n and hyp o t h e r m i a are f u n d a m e n t a l l y d i f f e r e n t phenomena, r e g u l a t i o n o f v e n t i l a t i o n a t reduced body t e m p e r a t u r e s i s independant o f the p h y s i o l o g i c a l changes t h a t a r e unique t o h i b e r n a t i o n and i s t i g h t 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 , S. L a t e r a l i s . The p r e s e n t s t u d y a l s o c o n f i r m s p r e v i o u s r e p o r t s t h a t weight s p e c i f i c m e t a b o l i c r a t e and minute v e n t i l a t i o n a r e s i g n i f i c a n t l y lower i n the e u t h e r i a n burrow d w e l l i n g 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 ) compared to the n o n - f o s s o r i a l l a b o r a t o r y r a t (R. n o r v e q i c u s ) . A p a r t from t h i s d i f f e r e n c e , the changes i n minute v e n t i l a t i o n , b r e a t h i n g f r e q u e n c y , m e t a b o l i c r a t e , r e s p i r a t o r y p a t t e r n and the i n s p i r a t o r y f l o w r a t e a r e i d e n t i c a l i n the two s p e c i e s . I n a d d i t i o n , a s i m i l a r c o r r e l a t i o n d e s c r i b e s the l i n e a r r e l a t i o n s h i p between i n s p i r a t o r y d r i v e and v e n t i l a t o r y r e q u i r e m e n t o f e i t h e r s p e c i e s a t any g i v e n body tem p e r a t u r e . -122-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 (Vip/Tj) and minute v e n t i l a t i o n (Vg) i n g o l d e n mantled ground s q u i r r e l s (o, n=5) and r a t s (•, n=7) d u r i n g p r o g r e s s i v e h y p o t h e r m i a and i n the g o l d e n mantled 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 ( A , n=7). A l l v a l u e s r e p r e s e n t means +/- s t a n d a r d e r r o r . -123-0 1 2 3 4 5 6 7 8 9 1 0 V T / T , (ml/sec) -124-Hence, i t i s c o n c l u d e d t h a t i n the n o n - h i b e r n a t i n g r a t , minute v e n t i l a t i o n i s a l s o t i g h t l y c o u p l e d to m e t a b o l i c demand d u r i n g p r o g r e s s i v e h y p o t h e r m i a . However, due to i n c o n c l u s i v e r e s u l t s , i t remains u n r e s o l v e d whether the v e n t i l a t o r y system c o n t r o l as r e f l e c t e d by changes i n the s l o p e o f the v e n t i l a t o r y response to i n s p i r e d CO2 and 0 2 i s a l t e r e d s i m i l a r l y i n the r a t or not. -125-LITERATURE CITED A d o l f , E.F., A. N a b e r s c h n i g , D.P. Orchard. 1961. V e n t i l a t i o n o f l u n g s i n d e e p l y h y p o t h e r m i c r a t s . J . A p p l . P h y s i o l . 16: 819-826. Anderson, G.L., W.A. V o l k e r t and X.J. Musacchia. 1971. Oxygen consumption, e l e c t r o c a r d i o g r a m and spontaneous r e s p i r a t i o n i n hyp o t h e r m i c hamster. Am. J . P h y s i o l . 221: 1774-1778. Andjus, R.K. and A.U. Smith. 1955.Reanimation o f a d u l t r a t s from body t e m p e r a t u r e s between 0 and 2°C. J . P h y s i o l . 123: 446-472. A r , A., R. A r i e l i , and A. S h k o l n i c . 1977. B l o o d gas p r o p e r t i e s and f u n c t i o n i n the f o s s o r i a l mole r a t under normal and h y p o x i c - h y p e r c a p n i c a t m o s p h e r i c c o n d i t i o n s . R e s p i r . P h y s i o l . 30: 201-218. A r i e l i , R. 1979. The a t m o s p h e r i c environment o f the f o s s o r i a l mole r a t ( S p a l a x e h r e n b e r g i ) : e f f e c t s o f season, s o i l t e x t u r e , r a i n , t e m p e r a t u r e , and a c t i v i t y . Comp. Biochem. P h y s i o l . 63A: 569-575. A r i e l i , R. and A. Ar. 1979. V e n t i l a t i o n o f a f o s s o r i a l mammal (Spa l a n x e h r e n b e r g i ) i n h y p o x i c and h y p e r c a p n i c c o n d i t i o n s . J . A p p l . P h y s i o l . 47: 1011-1017. B a r t e l s , H., R. S c h m e l z l e , and S. U l r i c h . 1969. Comparative s t u d i e s o f the r e s p i r a t o r y f u n c t i o n o f mammalian b l o o d V. I n s e c t i v o r a : shrew, mole, and n o n h i b e r n a t i n g and h i b e r n a t i n g hedgehog. R e s p i r . P h y s i o l . 7: 278-286. B a u d i n e t t e , R.V. 1974. P h y s i o l o g i c a l c o r r e l a t e s o f burrow gas c o n d i t i o n s i n the C a l i f o r n i a ground s q u i r r e l . Comp. Biochem. P h y s i o l . 48A: 733-743. Berkenbosch, A., J . de Goede, C.N. O l i e v i e r and H. Quanjer. 1982, S i t e s o f a c t i o n o f h a l o t h a n e on r e s p i r a t o r y p a t t e r n and v e n t i l a t o r y response t o C02 i n c a t s . A n e s t h e s i o l o g y 57: 389-398, B i g e l o w , W.G., W.K. L i n d s e y and W.F. Greenwood. 1950. Hypothermia: I t s p o s s i b l e r o l e i n c a r d i a c s u r g e r y an i n v e s t i g a t i o n o f f a c t o r s g o v e r n i n g s u r v i v a l i n dogs a t low body t e m p e r a t u r e . A n n a l s o f s u r g e r y . 132: 849-866. B i r c h a r d , G.F.,D.F. Boggs and S.M. Tenney. 1984. E f f e c t o f p e r i n a t a l h y p e r c a p n i a on the a d u l t v e n t i l a t o r y r e s p onse t o c a r b o n d i o x i d e . Resp. P h y s i o l . 57: 341-347. -126-Boggs, D.F., D.L. K i l g o r e , and G.F. B i r c h a r d . 1984. M l n i r e v i e w : r e s p i r a t o r y p h y s i o l o g y o f b u r r o w i n g mammals and b i r d s . Comp. Biochem. P h y s i o l . 77A: 1-7. B o r i s o n , H.L. 1981. C e n t r a l nervous r e s p i r a t o r y d e p r e s s a n t s -a n e s t h e t i c s , h y p n o t i c s , s e d a t i v e s and o t h e r r e s p i r a t o r y d e p r e s s a n t s . I n : R e s p i r a t o r y Pharmacology ( I n t e r n a t i o n a l Encyclopedia o_£ Pharmacology and T h e r a p e u t i c s , s e c t i o n 104). Ed: Widdicombe, J . Permagon P r e s s L t d . pp. 65-69. B r a d l e y , G.W. 1977. C o n t r o l o f the b r e a t h i n g p a t t e r n . I n R e s p i r a t o r y P h y s i o l o g y , V o l I I . Eds: J.G. Widdicombe U n i v e r s i t y Park P r e s s , B a l t i m o r e , pp. 185-217. B l i g h , J . and K.G. Johnson. 1973. G l o s s a r y o f terms f o r th e r m a l p h y s i o l o g y . J . A p p l . P h y s i o l . 35: 948-957. Chapman, R.C. and A.F. Benne t t . 1975. P h y s i o l o g i c a l c o r r e l a t e s o f b u r r o w i n g i n r o d e n t s . Comp. Biochem. P h y s i o l . 51A: 599-603. C l a u s e n , G. 1966. A c i d - b a s e b a l a n c e i n the hedgehog E r i n a c e u s Europaeus (L.) d u r i n g h i b e r n a t i o n h y p o t h e r m i a , c o o l i n g and rewarming. Arbok. Univ. Bergen.Mat.-N a t u r v i t e n s k . Ser. No.6., 1-11. Cooper, K.E. and W.L. V e a l e . 1986. E f f e c t s o f temperature on b r e a t h i n g . I n : Handbook o f P h y s i o l o g y ( s e c t i o n 3 ) : C o n t r o l o f b r e a t h i n g . Volume I I . Part 2.. Ed: Fishman A.P. W i l l i a m s and W i l k i n s Co. B a l t i m o r e , MD. pp. 691-702. Cragg, P.A. and D.B. D r y s d a l e . 1983. I n t e r a c t i o n o f h y p o x i a and h y p e r c a p n i a on v e n t i l a t i o n , t i d a l volume and r e s p i r a t o r y f r e q u e n c y i n the a n e s t h e t i z e d r a t s . J . P h y s i o l . 341: 477-493. C r a n s t o n , W.I., M.C. Pepper and D.N. Ross. 1955. Carbon d i o x i d e and c o n t r o l o f r e s p i r a t i o n d u r i n g h y p o t h e r m i a . J . P h y s i o l . 127: 380-389. Darden, T.R. 1972. R e s p i r a t o r y a d a p t a t i o n s o f a f o s s o r i a l mammal the p o c k e t gopher (Thomomvs B o t t a e ) . J . Comp. P h y s i o l . 78: 121-137. D a v i e s , R.O., D.V.M. M c l v e r , W. Edwards, S. L a h i r i . 1982. Halothane d e p r e s s e s the response o f c a r o t i d body chemoreceptors t o h y p o x i a and h y p e r c a p n i a i n the c a t . A n e s t h e s i o l o g y . 57:153-159. -127-Eger, E . I . , I I , L. Saidman and B. B r a n d s t a t e r . 1965a. Minimum a l v e o l a r a n e s t h e t i c c o n c e n t r a t i o n : a s t a n d a r d o f a n e s t h e t i c potency. A n e s t h e s i o l o g y 26.: 756-763. Eger, E . I . , I I , L. Saidman, L.,B. and B. B r a n d s t a t e r . 1965b. Temperature dependence o f h a l o t h a n e and c y c l o p r o p a n e a n e s t h e s i a i n dogs: c o r r e l a t i o n w i t h some t h e o r i e s o f a n e s t h e t i c a c t i o n . A n e s t h e s i o l o g y 26: 764-770. F a l e s c h i n i , R.J. and B.K. W h i t t e n . 1975. Comparative h y p o x i c t o l e r a n c e i n the S c i u r i d a e . Comp. Biochem. P h y s i o l . 52: 217-222. F a v i e r , R. and A. L a c a i s s e . 1978. S t i m u l u s oxygene de l a v e n t i l a t i o n chez l e r a t e v e i l l e , J . P h y s i o l . ( P a r i s ) 74: 411-417. Feldman, J.L. 1986. N e u r o p h y s i o l o g y o f b r e a t h i n g i n mammals. I n : Handbook o f P h v s i o l o q v - T h e Nervous System I V -I n t r i n s i c R e g u l a t o r y Systems o f the B r a i n . Ed: F.E. Bloom. Am. P h y s i o l . Soc. Bethesda. pp,463-524. F i s c h e r , B.A. and X.J. Musacchia. 1968. Responses o f hamsters to He-C>2 a t low and h i g h t e m p e r a t u r e s : i n d u c t i o n o f h y p o t h e r m i a . Am. J . P h y s i o l . 215: 1130-1136. P . J . , M.R. L e v i n e , A.L. Goncalves and S. W o o l l a r d . 1982. B a r o m e t r i c p l e t hysmogrpah: advantages and l i m i t a t i o n s i n r e c o r d i n g i n f a n t r e s p i r a t i o n . J . A p p l . P h y s i o l . , 55:1924-1931. Y.,W.R. See and Y. Honda. 1982. E f f e c t o f h a l o t h a n e a n e s t h e s i a on e n d - t i d a l PC02 and p a t t e r n o f r e s p i r a t i o n i n the r a t . P f l u g e r s . A r c h . 392: 244-250. H. 1975. E f f e c t s o f h y p o x i a and h y p e r c a p n i a on v e n t i l a t o r y p a t t e r n o f c h r o n i c c a t s b e f o r e and a f t e r vagotomy. B u l l . Europ, P h y s i o p a t h o l . Resp. 11:89-90. H. 1976. P a t t e r n s o f b r e a t h i n g d u r i n g h y p o x i a or h y p e r c a p n i a o f the awake or a n e s t h e t i z e d c a t . R e s p i r . P h y s i o l . 27: 193-206. H. and Gaudy, J.H. 1986. V e n t i l a t o r y r e c o v e r y from hypothermia i n a n e s t h e t i z e d c a t s . Resp. P h y s i o l . 64: 329-338. F l e m i n g , Fukuda, G a u t i e r , G a u t i e r , G a u t i e r , H a l l , F.G. 1965. M i n i m a l u t i l i z a b l e oxygen and the oxygen d i s s o c i a t i o n c u r v e o f b l o o d o f r o d e n t s . J . A p p l . P h y s i o l . 21: 375-378. -128-Hammel, H.T., T.J. Dawson, R.M. Abrams, and H.T. Anderson. 1968. T o t a l c a l o r i m e t r i c measurements on C i t t e l u s l a t e r a l i s i n h i b e r n a t i o n . P h y s i o l . Z o o l . 41: 341-357. Harkness, D.R., S. Roth, and P. Goldman. 1974. S t u d i e s on the re d b l o o d c e l l oxygen a f f i n i t y and 2,3 d i p h o s p h o g l y c e r i c a c i d i n the h i b e r n a t i n g woodchuck (Marmota monax). Comp. Biochem. P h y s i o l . 48: 591 -599. Hayward, J.S. 1966. Abnormal c o n c e n t r a t i o n s o f r e s p i r a t o r y gases i n r a b b i t burrows. J . Mammal. 47: 723: 74. Hegnauer, A.H. 1959. L e t h a l t e m p e r a t u r e s f o r dog and man. Ann. N. Y. Acad. S c i . 80: 315-319. Hemingway, A. 1963. S h i v e r i n g . P h y s i o l . Rev. 43: 397-422. Hervey, G.R. 1973. P h y s i o l o g i c a l changes e n c o u n t e r e d by hypot h e r m i a . P r o c . R. Soc. Med. 66. 1053-1058. H i c k e y , R.F. and J.W. S e v e r i n g h a u s . 1981. R e g u l a t i o n o f b r e a t h i n g : drug e f f e c t s . I n : R e g u l a t i o n o f B r e a t h i n g V o l . I I . Ed: T.F. Horn b e i n . M a r c e l Dekker I n c . N.Y. pp. 1251-1312. Hirshman, C.A.; McC u l l o u g h , R.E.; Cohen, P.J. and J.V, Weil.1977. D e p r e s s i o n o f h y p o x i c v e n t i l a t o r y reponse by h a l o t h a n e e n f l u r a n e and i s o f l u r a n e i n dogs. B r. J . Anaesth. £9_: 957-963. H o l l o w a y , D.A. and A.G, Heath, 1984. V e n t i l a t o r y changes i n the g o l d e n hamster, M e s o c r i c e t u s A u r a t u s , compared w i t h 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 , d u r i n g h y p e r c a p n i a and/or h y p o x i a . Comp. Biochem. P h y s i o l . 77A: 267-273. Hudson, J.W. and D.R. Deavers. 1973. M e t a b o l i s m , pulmocutaneous water l o s s and r e s p i r a t i o n o f e i g h t s p e c i e s o f ground s q u i r r e l s from d i f f e r e n t e n v i r o n m e n t s . Comp.Biochem. P h y s i o l . 45A: 69-100. Kent, K.M. and P e i r c e , E.C., I I . 1967. A c i d - b a s e c h a r a c t e r i s t i c s o f h i b e r n a t i n g a n i m a l s . J . A p p l . P h y s i o l . 23: 336-340. K i l m o r e , M.A. and H.F. Chase. 1962. E f f e c t s o f h y p e r c a p n i a and h y p o x i a d u r i n g h y p o t h e r m i a . A n e s t h e s i a and A n a l g e s i a 41: 435-441. K n i l l , R.L. and A.W. Gelb. 1978. V e n t i l a t o r y reponse t o h y p o x i a and h y p e r c a p n i a d u r i n g h a l o t h a n e s e d a t i o n and a n e s t h e s i a i n man. A n e s t h e s i o l o g y 4.9.: 244-251. -129-L a i , Y.L., Y. Tsuya and J . H i l d e b r a n d t . 1978. V e n t i l a t o r y r e s p o n s e s t o acu t e C02 exposure i n the r a t . J . A p p l . P h y s i o l . 45: 611-618. Lambertsen, C.J. 1980. Dyspnea and abnormal types o f r e s p i r a t i o n . I n : M e d i c a l P h y s i o l o g y 14th e d . , V o l . 2, Eds: V.B. M o u n t c a s t l e , S t . L o u i s . C.V. Mosby Co. pp.1828-1842. Landau, B.R. and A.R. Dawe. 1958. R e s p i r a t i o n i n the h i b e r n a t i o n o f the t h i r t e e n - l i n e d ground s q u i r r e l Am. J . P h y s i o l . 194: 75-82. L e c h n e r , A . J . 1976. R e s p i r a t o r y a d a p t a t i o n s i n b u r r o w i n g p o c k e t gophers from sea l e v e l and h i g h a l t i t u d e . J . A p p l . P h y s i o l . 41: 168-173. L e i t n e r , L.M., and A. Malan. 1973. P o s s i b l e r o l e o f the a r t e r i a l chemoreceptors i n the v e n t i l a t o r y r e s p o n s e s o f the a n e a e s t h e t i z e d marmot to changes i n i n s p i r e d 02 p a r t i a l p r e s s u r e and C02 p a r t i a l p r e s s u r e . Comp. Biochem. P h y s i o l . 45A: 953-959. Lyman,CP. and A.B. H a s t i n g s . 1951. T o t a l C02, plasma pH, and PC02 o f hamsters and 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 . Am. J . P h y s i o l . 167: 633-637. Maclean,G.S. 1981. F a c t o r s i n f l u e n c i n g the c o m p o s i t i o n o f r e s p i r a t o r y gases i n mammal burrows. Comp. Biochem. P h y s i o l . 69A: 373-380. McA r t h u r , M.D. 1986. " E f f e c t s o f changes i n i n s p i r e d gas c o m p o s i t i o n on v e n t i a l t i o n and b r e a t h i n g p a t t e r n i n awake and h i b e r n a t i o n g ground s q u i r r e l " . M.Sc. d i s s e r t a t i o n . U n i v e r s i t y o f B r i t i s h Columbia. 132p. Mc Nab, B.K. 1966. The metabolism o f f o s s o r i a l r o d e n t s : a st u d y o f convergence. E c o l o g y 47: 712-733. Milsom, W.K. and M.D. McArthur. 1987. H i b e r n a t o r s . I n : Hypoxia and C o l d . Eds: S u t t o n , J.R., Houston, C S . and G. Coates. P r a e g e r , N.Y. New York. pp.401-416. M i l i c - E m i l i , J . M. M a z z a r e l l i , J.Ph. Derenne, W.A. Whitelaw and J . Couture. 1975. A new approach t o stu d y o f c o n t r o l o f b r e a t h i n g . C l i n . Res. 23: 646A. M i l i c - E m i l i , J , and M.M. G r u n s t e l n . 1976. D r i v e and t i m i n g components o f v e n t i l a t i o n . Chest 70 Suppl.:131-133. -130-M i l i c - E m i l i , J . A.E. G r a s s i n o , and W.A. Whitelaw. 1981. Measurement and t e s t i n g o f r e s p i r a t o r y d r i v e . I n : R e g u l a t i o n o f B r e a t h i n g ( P a r t I I ) . Lung B i o l o g y i n H e a l t h and D i s e a s e V o l 17, P t l - 2 Ed: T.F. Hornbein M a r c e l Dekker I n c . Newyork. pp. 675-743. Munson, E.S. 1970. E f f e c t o f h y p t h e r m i a on a n e s t h e t i c r e q u i r m e n t i n r a t s . L a b o r a t o r y Animal Care 20: 1109-1113. M u s a c c h i a , X.J. and W.A. V o l k e r t . 1971. B l o o d gases i n h i b e r n a t i n g and a c t i v e ground s q u i r r e l s : Hb-C>2 a f f i n i t y a t 6 and 38 C. Am. J . P h y s i o l . 221: 128-130. M u s a c c h i a , X.J. and J a c o b s . 1973. h e l i u m - c o l d induced hypothermia i n White r a t . p r o c . Soc. Exp. B i o l . med. 142: 734-739. M u s a c c h i a , X.J. 1984. Comparative p h y s i o l o g i c a l and b i o c h e m i c a l a s p e c t s o f hypothermia as a model f o r h i b e r n a t i o n . C r y o b i o l o g y 21: 583-592. N a t s u i , T. 1969. R e s p i r a t o r y response to h y p o x i a w i t h h y p o c a p n i a or normocapnia and t o CO2 i n h y p o t h e r m i c dogs. Resp. P h y s i o l . 7: 188-202. N g a i , S.H., R.L. K a t z and S.E. F a h r i e . 1965. R e s p i r a t o r y e f f e c t s o f t r i c h l o r o e t h y l e n e , h a l o t h a n e and m e t h o x y f l u r a n e i n the c a t . J . Pharmacol. Exp. Ther.148: 123-130. O l s o n , J r . E.B., and J.A. Dempsey. 1978. Rat as a model f o r human l i k e v e n t i l a t o r y a d p t a t i o n to c h r o n i c h y p o x i a . J . A p p l . P h y s i o l . 44:763-769. Pappenheimer, J.R. 1977. S l e e p and r e s p i r a t i o n o f r a t s d u r i n g h y p o x i a . J , P h y s i o l . 266: 191-207. P a v l i n , E.G. and T.F. H o r n b e i n . 1986. A n e s t h e s i a and the c o n t r o l o f v e n t i l a t i o n . I n : Handbook o f P h y s i o l o g y  ( s e c t i o n 3 ) : The R e s p i r a t o r y System. V o l I I . P a r t 2. Ed: A.P. Fishman. W i l l i a m s and W i l k i n s Co. B a l t i m o r e pp.793-813. P o p o v i c , V. 1960a. P h y s i o l o g i c a l c h a r a c t e r i s t i c s o f r a t s and ground s q u i r r e l s d u r i n g p r o l o n g e d l e t h a r g i c h y p o t h e r m i a . Am. J . P h y s i o l . 199: 461-471. P o p o v i c , V. 1960b. S u r v i v a l time o f h y p o t h e r m i c w h i t e r a t s (15 °C) and ground s q u i r r e l s (10°C). Am. J . P h y s i o l . 199: 463-466. -131-P o p o v i c , V. and P o p o v i c , P. 1974. R e s p i r a t i o n . I n : H y p o t h e r m i a i n B i o l o g y and M e d i c i n e . Eds: P o p o v i c V. and P o p o v i c , P. Grune and S t r a t t o n I n c . N.Y. pp. 169-174. Rebuck, A.S. and A.S. S l u t s k y (1981) measurement o f 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 : R e g u l a t i o n o f B r e a t h i n g . P a r t I I . Ed: T.F. Hornb e i n . M a r c e l Dekker I n c . N.Y. P. 745-770. Regan, M.J. and E . I . Egar. 1966. V e n t i l a t o r y r e s p o n s e s t o h y p e r c a p n i a and h y p o x i a a t normothermia and moderte hypothermia 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 . A n e s t h e s i o l o g y 27: 624-633. Remmers, J.E. 1976. A n a l y s i s o f v e n t i l a t o r y r e s p o n s e , Chest 70 ( S u p p l . ) : 134-137. Rogers, P.D. and H. H i l l m a n . 1970. I n c r e a s e d r e c o v e r y o f r a t s ' r e s p i r a t i o n f o l l o w i n g p r o f o u n d hypothermia. J . A p p l . P h y s i o l . 29: 58-63. R o s e n f e l d , J.B. 1963. A c i d - b a s e and e l e c t r o l y t e d i s t u r b a n c e s i n h y p o t h e r m i a . Am. J . C a r d i o l . 12: 678-682, R u i z , A.V. 1975. Carbon d i o x i d e response c u r v e s d u r i n g h y p o t h e r m i a . P f u l g e r s A r c h . 358: 125-133. S a l z a n o , J . and F.G. H a l l . 1960. E f f e c t o f hypothermia on v e n t i l a t o r y r e s ponse t o c a r b o n d i o x i d e i n h a l a t i o n and c a r b o n d i o x i d e i n f u s i o n i n dogs. J . A p p l . P h y s i o l . 15: 397-401. S a l z a n o , J . and F.G. H a l l . 1961. E f f e c t o f hypothermia on r e f l e x a c t i v i t y i n the a n e s t h e t i z e d dogs. P r o c . Soc. Expt. B i o l . Med. 106: 199-202. S c h l e n k e r , E. H. and C. F. H e r r e i d . 1981. The e f f e c t o f low l e v e l s o f c a r b o n d i o x i d e on metabolism o f Mus  musculus. Comp. Biochem. P h y s i o l . 68A.: 673-676. S c h l e n k e r , E.H. 1985. V e n t i l a t i o n and metabolism o f the d j u n g a r i a n hamster (Phdopus sungorus) and the a l b i n o mouse. Comp. Biochem. P h y s i o l . , 82A: 293-295. S e v e r i n g h a u s , J.W. and M. S t u p f f e l . 1955. R e s p i r a t o r y dead space f o l l o w i n g a t r o p i n e i n man and a t r o p i n e , v a g a l or g a n g l i o n i c b l o c k a d e and hyp o t h e r m i a i n dogs. J . A p p l . P h y s i o l . 8: 81-87. S e v e r i n g h a u s , J.W. 1963. R e s p i r a t i o n and Hypothermia. Ann. N. Y. Acad. S c i . 109: 384-394. - 1 3 2 -Sodipo, J.O. and D.C. Lee 1971. Comparison o f v e n t i l a t i o n r e s p o n s e s t o h y p e r c a p n i a a t normothermia and hypothermia d u r i n g h a l o t h a n e a n e s t h e s i a . Canad. Anaesth. Soc. J . 18: 426-433. S t a h l , W.R. 1967. S c a l i n g o f r e s p i r a t o r y v a r i a b l e s i n mammals. J . A p p l . P h y s i o l . 22: 453- 460. S t e f f e n , J.M. and X.J. Musacchia.1985 . G l u c o c o r t i c o i d and h y p o t h e r m i c i n d u c t i o n and s u r v i v a l i n the r a t . C r y o b i o l o g y 22: 385-391. S t u d i e r , E.H. and J.w. P r o c t e r . 1971. R e s p i r a t o r y gases i n burrows o f Spemophilus t r i d e c e m l i n e a t u s . J.Mammal. 52: 631- 633. Tenney, S.M. and D. B a r t l e t t , J r . 1981. Some c o m p a r a t i v e a s p e c t s o f c o n t r o l o f b r e a t h i n g . I n : R e g u l a t i o n o f  B r e a t h i n g . P a r t I . V o l 17. Ed. J.F. H o r n b e i n . N.Y. Newyork. M a r c e l Dekker, pp.67-101. Tenney, S.M. and D.F. Boggs. 1986. Comparative mamalian r e s p i r a t o r y c o n t r o l . I n : Hand Book o f P h y s i o l o g y  ( s e c t i o n 3 ) . V o l I I . P a r t 2. Ed: A.P. Fishman. W i l l i a m and W i l k i n s Co. B a l t i m o r e , MD. pp.833-856. T e r z i o g l u , M., F. E m i r o g l u , N. Gokhan and F.O. 1961. The r e s p i r a t o r y response t o h y p o x i a and v a r i a t i o n s i n a r t e r i a l gas t e n s i o n s and a c i d - b a s e b a l a n c e o f normal dogs i n h y p o t h e r m i a . A r c h . I n t . P h y s i o l . B i o c h i m . 69:161-176. V o l k e r t , W. A. and X.J. Musacchia. 1976. Hypothermic i n d u c t i o n and s u r v i v a l i n hamsters: the r o l e o f temperature a c c l i m a t i z a t i o n and an a n e s t h e t i c . C r y o b i o l o g y 13: 361-367. Walker, B.R., E.M. Adams, and N.F. V o e l k e l . 1985. V e n t i l a t o r y r e s p o n s e s o f hamsters and r a t s t o h y p o x i a and h y p e r c a p n i a . J . A p p l . P h y s i o l . 59: 1955-1960. Webb, C.L., 1987. "Aspects o f the c o n t r o l o f b r e a t h i n g 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 " . M.Sc. d i s s e r a t i o n . U n i v e r s i t y o f B r i t i s h C o lumbia, 182p. Weiskopf, R. B., L.W. Raymond and J.W. S e v e r i n g h a u s . 1974. E f f e c t s o f h a l o t h a n e on c a n i n e r e s p i r a t o r y r e s p o n s e s to h y p o x i a w i t h and w i t h o u t h y p e r c a r b i a . A n e s t h e s i o l o g y . 41: 350-360. W i t h e r s , P.C. 1978. Models o f d i f f u s i o n - m e d i a t e d gas exchange i n a n i m a l burrows. Am. N a t u r a l i s t . 112: 1101-1112. Z a r , J.H. 1974. Comparing s i m p l e l i n e a r e q u a t i o n s . I n : B i o s t a t i s t i c a l A n a l y s i s . P r e n t i c e - H a l l I n c . N.J. pp. 292-295. -133-Appendix I ; Minute v e n t i l a t i o n ( V g ) , t i d a l volume ( V f ) and b r e a t h i n g f r e q u e n c y d u r i n g p r o g r e s s i v e hypothermia i n W i s t a r r a t s a n e s t h e t i z e d w i t h b o l u s i n j e c t i o n s o f Somnotol ( A ) and I n a c t i n (•) and under i n h a l a t i o n o f MAC H alothane ( • ). A l l v a l u e s r e p r e s e n t means +/- s t a n d a r d d e v i a t i o n (n=7-5). For dosage and s o u r c e o f a n e s t h e t i c s see below: Somnotol (Sodium P e n t o b a r b i t a l ) M.T.C. P h a r m a c e u t i c a l s , Cambridge, O n t a r i o . Dosage: b o l u s 7mg/100g body weight i . p . H a l o t h a n e ( F l u o t h a n e ) A y e r s t L a b o r a t o r i e s , M o n t r e a l , Quebec. Dosage: by i n h a l a t i o n 1% a t normothermia reduced g r a d u a l l y t o 0.6% a t 27 aC. I n a c t i n (5 e t h y l - 5 ( l m e t h y l - p r o p y l ) 2 - t h i o b a r b i t u r a t e ) Byk-Gulden, K o n s t a n z , West Germany. Dosage: b o l u s 12mg/100g i . p . A t r o p i n e s u l p h a t e (16mg/100g) was g i v e n as an a d j u n c t to Somnotol and I n a c t i n a n e s t h e s i a t o reduce b r o n c h i a l s e c r e t i o n s . -134-: i o I i l • i i i • i i i t i 25 27 29 31 33 35 37 Temperature (°C) 

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