158 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



heart rate at Z7° varied from 128 to 85, while at 28° the rate was 100 to 61. These 

 data suggest that there is a slight reduction in contractility of the myocardium at 

 reduced temperatures, hut that the reduction in heart rate in this state allows the 

 heart to deliver a stroke work per unit of filling pressure equal to that of the nor- 

 mothermic state. In experiments in which a greater reduction in rate occurred in the 

 cold heart, the ventricular function curve was higher at 28° than at 1)7° . Considera- 

 tion of the output and pressure plots shows the very considerahle increase in these 

 values which could be achieved in the cold heart as well as in the heart at V7° . 



An increased efficiency of the heart during hypothermia is suggested by figure 3 

 where a reduced amount of left main coronary flow per unit of left ventricular 

 minute work is present at lower work loads. The range of aortic pressure was nearly 

 identical for the three runs. A marked increase in efficiency of the normothermic 

 animal occurs by simply reducing heart rate, and it is possible that this change in 

 efficiency in hypothermia may be due solely to a reduced heart rate. 



REFERENCES 



1. Sarnoff, S. J., and Berglund, E. : Ventricular function. I. Starling's law of the heart 

 studied by means of simultaneous right and left ventricular function curves in tlie dog, 

 Circulation 9; 706, 1954. 



L.CK 



Fig. 3. — Relation of left ventricular minute work (L.V.M.W.) in kilogram meters to left main 



coronary flow (L.C.F.) in cc./min 



