282 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



but to vary with pH as well as with tein])erature. Although the oxyt^eu dissociation 

 curve shifts to the left at low temperatures, this has not been shown to result in 

 tissue anoxia. The now common use of hyperventilation raises the pH and further 

 decreases the dissociation of O:. from hemoglobin and more careful observation of 

 the possibility of cerebral and myocardial ischemic effects is needed. Particularly 

 in the brain this may be of importance since the concomitant decrease in arterial 

 PCO2 will diminish cerebral blood tlow. 



One report suggests that spontaneous ventricular fibrillation occurs less fre- 

 quently if 5% COo is used in place of no CO2 for artificial respiration. And one 

 author claims that spontaneous ventilation was not associated with a higher in- 

 cidence of fibrillation than other workers found using controlled respiration, al- 

 though the majority of reports strongly implicate the depressed spontaneous 

 ventilation as disposing to fibrillation. It has been shown that the respiratory cen- 

 ter at 25-27° C. responds to increased pCOi. but this response should not be 

 termed "normal" for two reasons: (1 ) the response of the control dogs to CO., at 

 37° C. was only j of the "normal" expected response. (2) The response at 25- 

 27° C. was given as a percentage of the already doubly depressed ventilation at 

 that temperature. Similarly, it seems incorrect to state that low pH and high pCO^ 

 are an attempt to compensate for the depressant effect of temperature on the 

 respiratory center — when actually they are the result of this depression. 



The use of high concentrations of CO2 to induce hypothermia in rodents has 

 been amply confirmed. These non-hibernators appear usualh' to develop cardiac 

 arrest rather than ventricular fibrillation if confined in a cold closed bottle where 

 CO2 builds up to about 16%. On the other hand one group suggests that the 

 optimal pH in hypothermia is 7 .5. Thus no agreement seems possible at the pres- 

 ent time about the proper values of pCOo, pH and CO2 content during hypo- 

 thermia. The difficulty is partly that some workers merely cool and rewarm un- 

 operated animals, where others are concerned with cardiac operations during the 

 cold state. The awake poikilothermic dog is reported to keep a nearly constant 

 arterial pH down to 30° C. pCOa (calculated from the data) in the only animal 

 reported (Keller, fig. 13) is also very nearly constant, being 26 mm. at 38° C. and 

 32 mm. at 30° C. It is hoped that more respiratory studies will be done on these 

 interesting poikilothermic animals. 



Lung function during hypothermia is little altered, b^arlier reports suggesting a 

 limitation of diffusion for oxygen or CO2 appear to be incorrect. Distribution and 

 diffusion functions are well maintained as low as 20° C. in dogs, and the only major 

 change in the lung appears to be a dilatation of the anatomic dead space. The 

 occurrence of pulmonary edema in the warming period has been noted occasionally 

 and needs investigation. It might especially Ije antici]:)ated after surgical correction 

 of right to left shunts or pulmonary stenosis. 



Renal. A primary effect of hypothermia appears to be depression of distal 

 tubular excretion and reabsorption. While sodium and water excretion are unim- 

 paired, potassium excretion is reported by most workers to fall during hypo- 

 thermia. Antidiuretic hormone is not ca])able of inhibiting the water loss. Uoth 

 blood flow and filtration rate return only about 5 to normal immediatelv on re- 

 w^arming, but are normal within 24 hrs. A tem]:)erature of 25-27° C. is reixirted to 



