440 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



tend to accumulate as body temperature falls, since detoxification mechanisms are 

 depressed. Animal studies suggest that ventricular fibrillation is less common with 

 one type of anesthesia than another. This has not been established for man. Gen- 

 eral anesthesia and the administration of a muscle relaxant are generally used dur- 

 ing surface cooling to minimize shivering and vasoconstriction. The ideal depth of 

 anesthesia is unknown. Whether chlorpromazine, other phenothiazine derivatives, 

 Arfonad, or various ganglionic blocking or adrenolytic drugs should be part of the 

 anesthetic regimen remains uncertain. Various beneficial effects including increased 

 rate of cooling, protection against ventricular fibrillation and decreased cardiac 

 work-load have been attributed to use of some of these substances, but controlled 

 data are lacking. 



Type of respiration. The majority of those reporting indicate a preference for 

 hyperventilation techniques in an effort to achieve a mild degree of respiratory 

 alkalosis. Disagreement with this has been voiced by a few. If hyperventilation is 

 accomplished by increased pressure in the airway, a comparison of the hypotensive 

 effects of intermittent positive pressure and alternating positive-negative pressures 

 should be made. 



Use of pumps and oxygenators. When direct cooling of blood is the method 

 for producing hypothermia, one must consider the advisability of incorporating a 

 pump and/or oxygenator in the circuit. Should well-maintained perfusion of the 

 coronary vascular bed prove of value in reducing the incidence of ventricular fibril- 

 lation, the incorporation of such devices may be essential. 



Rewarming. There is little agreement on the best method or on the optimal 

 rate of rewarming a patient. The circulatory hazards during rewarming have been 

 stressed by some and have proved of little significance to others. Overshoot of body 

 temperature to above normal values probaljly represents the same sort of response 

 reported as downward drift, only in the opposite direction. 



Apparatus. Attention should be directed towards standardization and simplifica- 

 tion of the technical details involved in hypothermia. Complex, costly apparatus 

 should be avoided if possible. The ability to change body temperature while opera- 

 tion is in progress is also desirable. 



Differential versus whole body cooling. If a single tissue such as the Ijrain or 

 liver is to be operated upon and interruption or reduction of the blood supply is 

 necessary, localized cooling of the involved region would seem preferal)le to gen- 

 eralized body cooling. Various methods of producing differential cooling have been 

 reported. The technical aspects should be simplified and refined. 



