The supernormal phase of intraventricular conduction: normalization of intraventricular conduction of premature atrial beats.

This report presents the analysis of a case in which numerous atrial extrasystoles, associated with normal intraventricular conduction, occurred during sinus rhythm conducted with right bundle branch block. The wide overlap of coupling intervals of normally conducted extrasystoles and other extrasystoles with right bundle branch block seemed prima facie to negate the possibility of supernormality. Rearrangement of these intervals, however, to allow for the timing of right bundle branch depolarization with respect to the ensuing QRS complex, resulted in a clear cut separation of all normalized complexes from those with right bundle branch block. All normalized extrasystoles were localized in a zone very early in the cycle. This finding, in the presence of atrial extrasystoles with normal P-R intervals, strongly supports the mechanism of supernormality in this case. Alternate explanations do not appear to be tenable.