Effect of Rate on Excitability of Dog's Ventricle

Abstract

The effect of increasing rate on the excitability of the dog ventricle is expressed in a displacement of the strength-interval curve to the left. This shift reflects a linear decrease in the duration of the total and absolute refractory periods without a change either in the relative refractory period or in the diastolic threshold. Thus, although recovery starts earlier at fast rates, restoration of complete excitability occupies a constant amt. of time. It seems, therefore, that rate affects the initiation of repolarization independently of an effect on its time course. Shortening of the Q-T interval is approx. a linear function of rate in the range 100 to 250 beats/min. and parallels the changes in total and absolute refractory periods. At all rates, the diastolic boundary of both the total refractory period and the irresponsive period bears a close relationship to the apex of the electrogram T wave. This relationship adds to the significance of the T wave with respect to the recovery of ventricular excitability and, by consequence, with respect to repolarization.