THE CIRCULATION DURING REW ARMING 



HENRY SWAN 



I should like to discuss the state of the circulation during the period of rewarm- 

 ing and the first three hours thereafter. On the basis of clinical and laboratory expe- 

 rience, it is our opinion that acute circulatory insufficiency exists during at least a 

 part of this time. The experiments I wish to present involve mild hypothermia. Dogs 

 were surface-cooled to 30° C. (rectal), maintained thus for one hour, rewarmed to 

 normal temperature and studied thereafter each hour for the next three hours. 



A typical protocol is shown in figure 1, where it can be seen that femoral arterial 

 pressure is low following cooling. During rewarming there is a rise in pressure but 

 control values are not marked by the end of three hours. Cardiac rate post-warming 

 is elevated, as is total peripheral resistance. Following warming total oxygen con- 

 sumption and A-V oxygen difference tend to be elevated. Cardiac index is below 

 normal at this time. 



So here we have a dog with a rapid heart rate, a low blood pressure in spite of the 

 high peripheral resistance, v^ho has not been able to raise his cardiac output, and yet 

 has a high oxygen consumption. He is alive only because, as Dr. D'Amato pointed 

 out, he is able to extract a lot more oxygen from the blood. I maintain that, at this 

 stage, the animal has an insufficient circulation for his needs. 



If the ratio between cardiac output and oxygen consumption during cooling and 

 during the cool state is plotted, there is found a linear relationship (fig. 2). The line 

 is the same for the control normothermic period and the cool state. In other words 

 the normothermia and the hypothermia plots fall pretty much on the same line. 

 Under these conditions we feel that the circulation is adequate. 



To contrast with that, figure 3 shows the slant of the line in the post-hypothermic 

 state, in which the cardiac output has failed to maintain pace. 



Figure 4 illustrates the oxygen consumption of the left ventricle as measured by 

 A-V oxygen differences across the coronary circulation. The oxygen consumption 

 decreases and even three hours later it has not returned to the previous level. 



Finally, the efficiency of the left ventricle falls off during the cool state (fig. 4). 



It is not proper, therefore, to state that if an animal is cooled and rewarmed, he 

 returns to physiologic normality. I think it is more proper to state that for a period 

 of at least three hours after mild hypothermia in a dog (whose respiration has been 

 unsupported) there is a state of circulatory insufficiency. 



DISCUSSION 



Dr. R. 0. Hciinbeckcr: We have noted complications during rewarming which 

 were similar to those described by Dr. Swan. We believe that these are due to too 

 rapid warming. As the result of excess surface heat peripheral arteriolar dilation 

 results, peripheral resistance is lowered and a shock-like state develops. Removal 

 of some of the surface heat must be a part of therapy. 



Dr. Jacob Fine: In Dr. Friedman's experiments on cooled dogs subjected to hem- 

 orrhagic shock the effects of morphine and ether were pretty well dissipated before 



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