424 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



rate of body cooling is not uniform. The right auricle and right ventricle are cooled 

 more rapidly than the rest of the body. Consequently, the SA node, which is highly 

 susceptible to cold, is depressed. Cardiac output is decreased. Ventricular arrhythmia 

 is more likely to appear. This objection would not appear to be valid when a venous 

 arterial method of cooling is used. Gollan^''' "^' "^ has cooled dogs down to five 

 degrees in 40 minutes ; and Peirce,^* to twenty degrees centigrade in 20 minutes 

 without ill efifects. If the arterial cannula is inserted high in the arch of the aorta, 

 cooling is essentially uniform and can be carried out rapidly. If the cannula is 

 inserted into the femoral artery, cooling proceeds more rapidly in the lower ex- 

 tremities. By careful localization of the arterial cannula, therefore, either uniform 

 or selective cooling can be carried out. As Peirce has suggested, selective cooling 

 of the heart and brain may be the ideal approach. 



All studies of hypothermia have shown a rise of three or four hundred per cent 

 in oxygen consumption when the animal is shivering. Since one of the primary aims 

 of hypothermia is to decrease the metabolic rate, the avoidance of shivering would 

 appear to be a prime requisite of any method of cooling. Shivering has been avoided 

 by: (1) using deep anesthesia, (2) adding to a mild anesthesia one of the curare 

 drugs, and (3) using Thorazine and Phenergan, the French lytic cocktail. ^^' *°"*^ 

 Jung and his associates^** established that shivering stimuli arise from receptors in 

 cold skin. External cooling applies the cooling agent directly to the skin and, thus, 

 a strong stimulus to shivering is promoted. Blood stream cooling avoids this reflex 

 and shivering is decreased. As stated by Ross,-^ "By direct cooling of hypothalamus, 

 adrenals and thyroid, this method aims at circumventing the highly efficient and 

 coordinating reactions of the body to the surface application of cold." Furthermore, 

 Ross has pointed out that shivering can be avoided completely if the skin is kept 

 relatively warm. Thus, blood stream cooling would seem to offer a safer means of 

 controlling shivering than the use of high doses of anesthetic agent or curare. 

 In contrast to external cooling, there is no definite evidence of injury to nerves, 

 fat or skin from blood stream cooling. Such injury might be difficult to detect. 

 Damage to the constituents of blood, however, is more frequent with a simple 

 extracorporeal circulation than when hypothermia is added. 



Perhaps the most important advantage of vein-to-artery blood stream cooling is 

 that at low temperatures it permits easier control of coronary perfusion rate, sys- 

 temic pressure, and systemic blood flow. If the patient does develop a dangerous 

 complication, it is possible to maintain life l)y maintaining an adequate coronary 

 and cerebral blood flow until the complication is corrected. Brock is not concerned 

 when ventricular fibrillation develops during cardiac occlusion. He feels that 

 fibrillation may even be advantageous from the point of view of avoiding air 

 embolisms and providing a quiet heart. Senning,'' Juvenelle, Lind-^ and Wegelius 

 have induced fibrillation to achieve a quiescent heart. Crafoord^^ found it difficult 

 to maintain deliberately induced ventricular fibrillation while using a pump oxygena- 

 tor. Brock permits the ventricular fibrillation to continue until the procedure on 

 the heart has been completed and then defibrillates the heart with electrical shock 

 following a period of cardiac massage and intracardiac adrenalin. He makes no ef- 

 fort to warm the patient until defibrillation has been acconii)lished and may con- 

 tinue decreasing body temperature if ventricular fibrillation appears l)efure the pa- 

 tient has reached a sufficiently low temperature. 



