150 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



0.9 r 



HYPOTHERMIA 



NORMOTHERMIA 



I7.5°C 





Cycle 



Fig. 



240 220 200 180 160 140 120 100 80 60 40 20 

 HEART RATE PER MINUTE 



5.— Relative length of activity phase with decreasing heart rate at normal temperature 



and in hypothermia. 



adequate coronary blood flow ? Can the heart extract from blood at approximately 

 20° C. an adequate volume of oxygen? 



The adequacy of coronary blood flow has been demonstrated by Berne.=' He ob- 

 served a difference in the slopes of the aortic pressure curve and the coronary blood 

 flow curve. Aortic pressure decreased slightly in the early stages of hypothermia, 

 then underwent a sharper decrease beginning in the vicinity of 28° C. In contrast, 

 the coronary blood flow decreased sharply in the early stages, then at about 53° C. 

 began a more gradual decline. Figure 6 illustrates the average coronary blood flow 

 in thirteen hypothermic dogs. This graph depicts the relationship between coronary 

 blood flow and the perfusion pressure behind the flow. In these experiments the 

 perfusion pressure was that of the subclavian artery. The relatively sharp initial de- 

 crease in coronary blood flow is followed by a more gradual decrease beginning at 

 a perfusion pressure of about 80 mm. Hg. At 19° C. the magnitude of coronary 

 blood flow is approximately one- fourth that of control flow. 



If this rather drastic reduction in coronary blood flow were the prime causative 

 factor in some pathological state of cardiac function, then the experimental increase 

 of coronary blood flow should relieve that pathological state. Berne controlled the 

 volume of coronary blood flow by means of a pump perfusion system in parallel 

 with his arterial perfusion system. The results of six experiments are siunmarized 

 in table I. By raising the artificial pressure head above the normal for a given low 

 temperature, Berne was able to increase coronary blood flow from 200 to 900 per 



