Ventricular Excitability Cycle

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Abstract
The ventricular excitability cycle of the intact dog''s heart was studied under conditions of normo-and hypothermia with and without a concomitant acidosis. Direct measurements of myocardial excitability revealed that the threshold during diastole and early in systole, at a point designated as the minor dip, are pH sensitive. The threshold at these points fell significantly in both normo- and hypothermic dogs whose pH was reduced to a value of less than 7.20. However, no threshold change occurred in those hypothermic animals who were maintained at a normal pH throughout the cooling process. The threshold at the major dip, which occurs during late systole, appeared to be temperature sensitive since a significant decrease was noted in all hypothermic dogs regardless of pH and was not observed in hypercapneic normothermic dogs. Maintenance of a normal pH during cooling is also successful in preventing the occurrence of ventricular fibrillation. Reversal of an already established acidosis at low temperature via hyperventilation does not affect the incidence of ventricular fibrillation.

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