90 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



artificially, auricular fil)ri]lation, abnormally slow conckictioii and 2: 1 and 3: 1 A-V 

 blocks have been reported."'- " This would indicate that of all the conducting mech- 

 anisms of the heart the auriculo-ventricular junctional tissue is the one most suscep- 

 tible to the effects of coldJI 



No measurements have been made on the blood pressure of animals in hiberna- 

 tion, but measurements taken shortly after the start of arousal (see below) indicate 

 that the blood pressure during hibernation must be very low.'" The bright pink feet 

 of the hibernating hamster indicate that the peripheral circulation is dilated, so that 

 the heart is working against a low resistance. The extremely low blood pressure 

 which probably exists in the hibernating state raises a number of interesting ques- 

 tions which should be answered by future research. For example, the activity of the 

 kidney in deep hibernation and the problem of exchange of substances across the 

 capillary wall in accordance wath the Starling hypothesis deserve investigation. 



Metabolism. Both the metabolic rate and the respiratory quotient of mammals 

 in hibernation have been studied extensively. In general, it may be said that the 

 metabolic rate of animals in hibernation is between \^o and Moo of the "resting" 

 metabolic rate when the animal is in the homeothermic state. Kayser'^ has shown 

 that, within the narrow temperature range of hibernation, the metabolic rate of the 

 common dormouse increases with temperature according to Vant Hoff's law, but 

 Popovic"'* failed to find this in the European ground squirrel. 



Earlier workers reported questionably low respiratory quotients for animals in 

 hibernation but the careful work of Benedict and Lee*'^ and Kayser''* established 

 that in all mammals studied the respiratory quotient was very close to 0.7. indicat- 

 ing that the hibernator was using fat almost exclusively as the source of energy 

 during the dormant period. In accordance with this, Carpenter*" has shown that 

 protein metabolism is greatly depressed in the woodchuck during hiljernation al- 

 though it is not altered qualitatively. The measurements of liver and kidney lipids 

 in the arctic ground squirreP^ indicate that there is an increase of fat usage during 

 hibernation when compared to the usage just prior to hibernation and confirm the 

 conclusion that fat is the main source of energy during hibernation. 



Benedict and Lee*^^ measured the weight loss of woodchucks during the time the 

 animals were actually hibernating and found that it amounted to 0.2 gms. per day 

 per kilogram. Kayser^^ has shown that the weight loss of European ground sciuirrels 

 during the total period of hibernation is dependent on the length of time the animal 

 is awake during that period. A single arousal with its following period of wakeful- 

 ness uses more energy than many days of hibernation. Thus he showed that a ground 

 squirrel which hibernated for 4,126 hours used a calculated total of 70 calories. 



II A. R. Dawe and P. R. Morrison (Am. Heart J. 49: 367, 1955) found that an Arctic ground 

 squirrel showed paired beats during hibernation, and that both the FrankHn ground squirrel and 

 hedgehog had even bursts of heart beats. They also found that the T-P interval was greatly 

 lengthened when the animal was in hibernation, and concluded tliat the primary cause for the 

 slowing of tlie heart rate was the decrease in the automaticity of the SA node. Since tlie P-R 

 interval showed lengthening second only to the T-P interval, they point out that the conduction 

 time must also be slowed. The RS-T interval was slowed relatively the least, and this would 

 indicate that repolarization was least affected by the hibernating state. H. S. S. Sarajas (Wcta 

 Physiol. Scand. 32: 28, 1954) reported very similar clianges in the P-R interval with temperature 

 in hedgehogs chilled after injection of insulin, and (i. Biorck and P.. Johansson ( .'Xcta Pliysiol. 

 Scand. 34: 257, 1955) further confirm this in normal hibernating hedgehogs. 



