Effect of ventricular premature beats on idioventricular pacemaker activity

Abstract

The effect of ventricular premature beats on idioventricular pacemaker activity was studied in open-chest dog hearts with a surgically induced block in the His bundle. While the ventricle was paced by basic stimuli at a given rate, the pacing was interrupted for about 2.5 seconds following every twelfth basic beat (V 1) in order to obtain the interval between V 1 and the first escape beat (V e) or the basic escape interval (V 1V e). Ventricular premature beats (V 2) were then introduced at various coupling intervals (V 1V 2) and the effect of these premature beats on the postextrasystolic escape interval (V 1V e) was observed. The plot of V 2V e against V 1V 2 intervals showed that the V 2V e interval was shortest at shorter V 1V 2 intervals, and it increased gradually with the increase in V 1V 2 intervals. The V 2V e intervals at shorter coupling intervals were much shorter than the basic escape interval (V 1V e), indicating enhanced automaticity after early premature beats. The V 2V e at longer coupling intervals were much longer than the basic escape intervals, indicating suppressed automaticity after late premature beats. The similar response to ventricular premature beats was noted during spontaneous idioventricular rhythm. The suppression was more pronounced at faster pacing rates and following two successive premature beats, probably due to the phenomenon of overdrive suppression. The same phenomena of altered automaticity after premature beats could be observed under the influence of ouabain, epinephrine, lidocaine, and propranolol, although these agents either decreased or increased the average escape intervals. The results may explain the clinically observed alteration of the idioventricular pacemaker rate following ventricular premature beats.