Ventricular Excitability and Refractoriness in the Hypothermic Dog

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Abstract
Measurements of ventricular exctiability through the entire cardiac cycle of the dog under progressive hypothermia show that the ventricular refractory period (measured as absolute, total or functional refractory period) is greatly prolonged. This effect is not secondary to the changes in heart rate since normothermic animals with surgical A-V block having heart rates similar to those obtained under hypothermia do not show any great prolongation in the ventricular refractory period. At heart temperatures ranging from 38° to 23°C there is no significant alteration in the diastolic excitability of the ventricle. The rate of recovery of diastolic excitability and the response latency were greatly prolonged as reflected in the marked increase in the duration of the relative and functional refractory periods. It is suggested that the increased susceptibility of the hypothermic myocardium to ventricular fibrillation may be related to the observed changes in the rate of recovery of excitability.