154 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



been observed by Hallett^^ to undergo a decrease to 22 per cent of control flow at 

 23° C. According to Page^- effective renal i)]asma flow follows variations in cardiac 

 output. 



Thus it appears that the vohune of blood flow to organs in hypotherniia is closely 

 related to the output of blood from the heart. 



With regard to blood flow in the minute peripheral vessels, Bigelow"^ examined 

 microscopically the vessels of the conjunctiva in the hypothermic dog. He observed 

 marked vascular stasis at the lower temperatures and complete cessation of flow in 

 some arterioles and veins as large as 60 micra in diameter. 



Summary. By way of summary, let us attempt to form some conclusions as to 

 the state of the cardiovascular system in profound hypothermia. Consider the dog at 

 a body temperature of 20° C. Its cardiac output is about 15 per cent of normal. 

 Systemic arterial pressure is usually 60-70 mm.Hg. The work per minute per- 

 formed by the heart is also about 15 per cent of normal. Stroke volume, on the 

 other hand, is normal. It therefore appears that the marked reduction in these func- 

 tions is referable to the extreme bradycardia which occurs at this low temperature. 



At 20° C. the body tissues are apparently supplied with an adequate amount of 

 oxygen, in view of the facts that there is no evidence of oxygen debt upon rewarm- 

 ing and that the tissues can extract more oxygen, when needed, by increasing the 

 coefficient of utilization. Therefore, at this low temperature with its reduced oxygen 

 requirements, the slowed heart rate in spite of all its physiological consequences is 

 adequate, just as at Z7° C. the control heart rate was adequate. In this somewhat 

 restricted sense, the bradycardia at the low temperature is "normal" for that 

 temperature. 



The heart in deep hypothermia is usually described by the word "depressed." If by 

 the word "depressed" we mean "slowed in rate of contraction," then obviously the 

 heart in hypothermia is "depressed." Beyond this meaning, however, the word 

 "depressed" is used inaccurately in this connection. The characteristics of a heart 

 which has been depressed (in the classical sense of the word) by drugs include : 

 (a) a more gradual contraction ; (b) a lower amplitude of contraction ; (c) a shorter 

 duration of contraction; (d) a reduced stroke volume; and (e) an elevated initial 

 intraventricular pressure.^^^ In hypothermia the last three characteristics are lacking 

 and the second, viz., a lower amplitude of contraction, is not particularly dramatic. 



REFERENCES 



1. Hook, W. E., and Stormont, R. T. : Effect of lowered body temperature on heart rate. 



blood pressure, and electrocardiogram, Am. J. Physiol. 133: 334, 1941. 



2. Hegnauer, A. H., Schriber, W. J., and Haterius, H. O. : Cardiovascular response of the dog 



to immersion hypothermia, Am. J. Physiol. 161: 455, 1950. 



3. Berne, R. M. : The effect of immersion hypothermia on coronary blood flow, Circ. Research 



2: 236, 1954. 



4. Prec, O., Rosenman, R., Braun, K., Rodbard, S., and Katz, L. N.: The cardiovascular 



efifects of acutely induced hypothermia, J. Clin. Invest. 28: 293, 1949. 



5. Bigelow, W. G., Lindsay, W. K., and Greenwood, W. F. : Hypothermia. Its possible role 



in cardiac surgery; an investigation of factors governing survival in dogs at low body 

 temperatures, Ann. Surg. 132: 849, 1950. 



6. Berne, R. M. : Myocardial function in severe hypothermia, Circ. Research 2; 90, 1954. 



