332 



PHYSIOLOGY OF INDUCED HYPOTHERMIA 



and (presumably) tissue oxygenation thus further ehminating hypoxia as a possi- 

 ble complication in the interpretation of the data. It is apparent from figure 3 that 

 administration of the artificial gas mixture to the respired air did not materially 

 afifect the physiological status of the animals at least in respect to diastolic thresh- 

 olds, lethal temperatures, and the distribution beween asystolic and fibrillary 

 deaths (compare with figure 2). 



In contrast to the data of figures 2 and 3 are those obtained from dogs subjected 

 to the same conditions except that throughout cooling they were respired arti- 

 ficially at a rate to maintain blood pH within the range 7.3-7.55. From figure 4 it 

 may be noted that all diastolic thresholds remained above the apparently critical 

 level of 1.4 ma., even at the lowest temperatures. Death was due to asystole in all 

 cases, and at a mean temperature of 16.9° ± 1.6° C. 



This evidence that maintenance of an approximately normal pH during the 

 course of hypothermia induction will (a) diminish the frequency of ventricular 

 fibrillation, and (b) hold ventricular excitability at a normal level, suggests that a 

 common underlying mechanism is in some measure influenced by pH. It is highly 

 probable that this mechanism is concerned with myocardial electrolyte and mineral 

 balances (a subject to be discussed presently). Extreme alkalosis, too, may induce 

 modifications in ventricular excitability which lead to fibrillation. Thus in two addi- 

 tional dogs the ventilation rate was inadvertently excessive during cooling, with a 

 consequent elevation of pH to 7.8. Ventricular fibrillation terminated both experi- 

 ments above 20°. This may be compared to the experience of Osborn^° who found 

 that hypothermic dogs whose pH exceeded 7.6 during the early stages of cooling 

 were extremely susceptible to fibrillation. Thus the optimal pH range for hypo- 

 thermic dogs appears to be that of the normal, i.e., 7.3-7.55. 



Fig. 4.- 



38° 36° 34° 32° 30° 28° 26° 24° 22° 20° 



HEART TEMPERATURE (*»C) 



-Diastolic threshold vs. heart temperature in hypothermic dogs when pH is maintained 

 between 7.3 and 7.55. Thresholds above 2.73 ma. given by vertical arrows. 



