Tachycardia-Dependent Left Bundle Branch Block Associated with Bradycardia-Dependent Variable Left Bundle Branch Block

This report describes tachycardia- and bradycardia-dependent left bundle branch block in a patient who exhibited normal intraventricular conduction with impulses intermediate in timing. During bradycardia, longer cycles were associated with increasing degrees of incomplete left bundle branch block. These clinical observations support the existence of slow Phase 4 depolarization of latent pacemaker cells as one of the mechanisms for abnormalities of conduction.