20 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



reduce the acidosis but was unal)le to reduce the mortality rate and frequency of 

 cardiac irregularities. 



ACID-BASE BALANCE IN HYPOTHERMIA 



The pH of the blood has received considerable attention in investigations con- 

 cerned with the physiology of hypothermia. It is well known that blood cooled 

 in vitro becomes more alkaline.^*'' ^' However, the response is not as simple for 

 in vivo blood which is under the influence of respiratory and urinary functions. 

 The results obtained by various investigators are complicated by the type of artificial 

 respiration, or lack of it, employed in their experiments. Thus hyperventilation, 

 hypoventilation and spontaneous respiration each result in relatively different pH 

 changes during progressive hypothermia. In addition, states of artificial respiration 

 which, under conditions of normal body temperatures, may represent hyperventila- 

 tion, hypoventilation or normal ventilation, may no longer represent these states 

 when the body temperature is progressively reduced. For example, a normal ventila- 

 tion rate for an anesthetized dog at a rectal temperature of 38° C. is approximately 

 20 per minute. If an animal is artificially respired at this rate throughout hypother- 

 mia the ventilation is no longer normal when the rectal temperature has been reduced 

 to about 25 "" C. where the 'normal' respiratory rate is approximately 2-3 per minute 

 and the rate of COo production profoundly reduced.- At the lower rectal tempera- 

 ture the animal is being hyperventilated. Furthermore, in the example cited, during 

 the reduction of rectal temperature the ventilation of the animal is progressively 

 changing so that there is a progressively increasing hyperventilation. In the case of 

 the hyperventilated control, at low body temperature the hyperventilation would be 

 relatively greater and hypoventilation at normal body temperatures could represent 

 'normal' or hyperventilation at low rectal temperatures. It would appear from these 

 considerations that the physiological adjustments which the animal makes, in the 

 acid-base balance of its blood, to reduced body temperatures are best studied in the 

 spontaneously respiring animal. This does not deny the value of studies in which 

 controlled ventilation has been employed. Much valuable information has been ob- 

 tained from these data concerning the adjustments made by the animal itself, but 

 more particularly concerning the adjustments which can be made for the animal to 

 protect it against various adverse states. 



That a definite decrease in blood pH occurs during hypothermia under conditions 

 of spontaneous respiration has been well estaljlished by several investigators.^''' -"' ■*^- 

 42, 45, 58 j^ i-,^g i^een pointed out that in animals in which shivering is allowed to 

 occur during the first stages of hypothermia, there is an initial hyperventilation and 

 consequent respiratory alkalosis. This is followed by a decreased pH to acidotic 

 levels as the body temperature falls and shivering and pulmonary ventilation are 

 progressively reduced. ^^ By hyperventilating dogs and thereby increasing the blood 

 pH, previous to and during hypothermia. Swan et al.'^^ found it possible to reduce 

 considerably the incidence of ventricular fibrillation after occlusion of the circula- 

 tion for 15 minutes. Osborn*^ also observed that the maintenance of a relatively high 

 arterial pH reduced the incidence of hypothermic ventricular fibrillation. As men- 

 tioned previously, he also found a close association between low serum bicarbonate 

 and the onset of severe electrocardiographic changes or ventricular fibrillation. He 



