348 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



that blocking the sino-auricular node permitted the performance of right ventricu- 

 lotomy without ventricular fibrillation in dogs under drug-induced "artificial hiber- 

 nation."^ Knowing of this work prior to its publication, a group of us at Indiana 

 University began some studies in December 1953 which have proved informative. 

 All of these experiments were performed upon dogs anesthetized with intraven- 

 ously administered thiopental sodium, intubated, and hyperventilated with oxygen 

 by a mechanical insufflation apparatus operating from 35 to 45 times per minute. 

 All were cooled by immersion in a bath of water and cracked ice. At the completion 

 of the operative procedure they were rewarmed to a temperature of approximately 

 36 degrees in a bath of warm water. 



We found that injection of procaine into the area of the superior vena caval- 

 atrial junction (fig. 1) produced slowing of the heart, some drop in blood pressure, 

 and alterations of the electrocardiographic P wave, suggesting sino-auricular node 

 blockade." In roughly 70 per cent of the animals the P wave disappeared (fig. 2) 

 and in the remainder it showed reduction in voltage or inversion. In a series of 10 

 animals the average reduction in pulse rate was from Q to 46, the blood pressure 

 from 146/110 to 125/78, and the duration of the effect as measured by P wave 

 changes a little less than 20 minutes (table I). In 10 normothermic dogs the aver- 

 age reduction in pulse rate was from 143 to 91, the blood pressure from 174/122 to 

 165/98, and the duration of effect as measured by P wave alterations was approxi- 

 mately 10 minutes. We gained the impression that the cardiac rhythm was more 

 stable and the heart less irritable on external manipulation after such injection. In 

 the hypothermic animals the heart also appeared pinker. 



The apparent decrease in myocardial irritability proved to be correct when sub- 

 jected to the test of other manipulative procedures. In moderately hypothermic ani- 



£' 



Fig. 1. — Drawing showing area uf procaine infiltration. From Riberi, ei al. (Surgery, Nov., 1955.) 



