23-i INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



18% at 20°C. Similar findings have been reported by others. Gollan was 

 imabk> to detect any A-V difference in oxygen saturation at 4°C (72). 

 As a result of the finding that as much as four volumes per cent of oxygen 

 is dissolved in the blood at 3°C he calculated that a dog could be main- 

 tained without hemoglobin at temperatures below 10°C. He subsequently 

 demonstrated that such was the case by removing the red blood corpuscles 

 and circulating plasma only (75). 



This reduction in metabolism is an over-all average and reduction in 

 some organs may be greater than others. Metabolism studies are needed 

 for organs such as the liver and kidney in which histological damage has 

 been reported following hypothermia of two and a half hours' duration 

 (see below). 



Since the brain is the critical organ in asphyxia, the fact that its 

 metabolism is depressed during hypothermia is important (67, 184). More 

 recently a linear fall in oxygen uptake during cooling has been demon- 

 strated. The rate at 26°C was found to be only one third of that at 3o°C 

 (157). An independent study reported a three-fold increase in resistance 

 to occlusion of the arterial supply to the brain in dogs at 26°C as 

 compared with those at 35°C (111). 



Heart Action. The heart, which appears to be the critical organ in 

 hypothermia, likewise exhibits a regular decrease in oxygen uptake. This 

 has been interpreted by some as a consequence of the decreased coronary 

 flow which also occurs in hypothermia (61). However, since the A-V 

 difference in the hypothermic heart is within normal limits (85, 86, 25), 

 it appears reasonable to attribute the reduced metabolism to the direct 

 action of the cold upon the myocardium. Both embryonal (62, 43) and 

 adult hearts (72, 20-22) can be cooled until all activity ceases and re- 

 warmed with the resumption of normal contractions. 



Increased irritability of cardiac muscle as the temperature falls is the 

 most serious secondary effect of hypothermia. This results in atrial fibril- 

 lation at about 29°C, followed by ventricular fibrillation at a slightly 

 lower temperature in untreated animals. In recent studies with heart- 

 lung preparations it w^as found that hypothermia reduces heart rate, 

 but over a wide range of temperatures the maximum stroke volume remains 

 high. Although work capacity decreases it remains well above the work 

 load that the heart must cope with at the same temperature. No relation- 

 ship was found between previous work load and the onset of fibrillation 



(155). 



Reduction in conductivity during cooling has been demonstrated in 

 man (58, 89) as well as in animals (142, 84). This apparently results 

 in the escape of some areas of the myocardium at low temperatures with 

 the resultant development of ectopic beats. However, another inter- 



