336 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



dogs succumb to fibrillation because their ventricular thresholds are lowered, and 

 the lowering is in some manner the consequence of calcium penetration into the 

 myocardium in exchange for potassium and hydrogen ions. Those acidotic hypo- 

 thermic dogs which can maintain a normal electrolyte balance across the myocardial 

 membrane succumb ultimately to the effect of temperature on metabolic processes 

 responsible for energy-mobilization and release, i.e., death is asystolic. 



A summary to this point reveals that a proportion of acidotic hypothermic dogs 

 succumb to ventricular fibrillation between 25° and 19° whereas asystole at lower 

 temperatures accounts for the remaining acidotic dogs and all of those with normal 

 pH. The characteristics which appear to distinguish the fibrillators from the others 

 are: (a) lower diastolic thresholds, (b) positive calcium and negative potassium 

 and hydrogen ion balances with respect to the myocardium, and (c) elevation of the 

 early portion of the S-T segment of the electrocardiogram. Since none of these 

 characteristics has thus far been noted in hypothermic dogs possessing normal pH 

 the conclusion that the acidotic rather than hypothermic state is the more important 

 causal agent seems justified. Not ruled out by the observations to date, however, is 

 the possibility that, although pH appears to dominate, temperature itself may be an 

 ally. This point will be discussed further. 



Exogenous calcium and fibrillation. On the premise that hypothermic spon- 

 taneous ventricular fibrillation is the consequence of (or intimately related to) the 

 myocardial penetration of calcium, and that the latter is controlled in considerable 

 degree by the systemic pH, one might conclude that the hypothermic acidotic dogs 

 would be more sensitive to an elevation of the plasma calcium level than would 

 non-acidotic dogs. To test the hypothesis, normothermic and hypothermic dogs 

 were infused with isotonic calcium chloride at the rate of 5 ml. per minute.-^ The 

 hypothermic dogs were again subdivided into acidotic and non-acidotic groups. In 

 addition to volumes (or m.eq. of calcium) infused to terminus, plasma levels and 

 pH were measured in a proportion of dogs in each group. These are given in table 

 III, together with total number of animals and the character of the terminal cardiac 

 action. 



Results show that 65 per cent of normothermic dogs terminate in ventricular 



TABLE III 

 Cardiac Sensitivity to Exogenous Calcium in Normal and Hypothermic Dogs 



Fibrillators 



AH pH Plasma 



Heart experiments Number Ca admin. , -^ , mEq./L Ca 



temp. VF/total of dogs niEq./Kg. Init. Term. init. term. 



38±1 n/17 5 |J;M 7.39 7.42 |J;i Jj 



33±1 4/4 4 / ^^ I 7.36 7.31 — — 



2<S±1 4/4 4 1 ~'^~\ 7.32 7.44 — — 



-"- '^/'^ ' {31 -« ~ {ji :a 



25±1 4/4 4.. [ ^•^'^1 7.45 7.62 - — 



^ -:0.24/ 



