Alternation of sinoatrial block and left bundle branch block

A case of alternating second degree sinoatrial block and left bundle branch block is described. Electrophysiologic evaluation of this patient revealed impaired sinoatrial, atrioventricular and intraventricular conduction (panconductional defect). Alternation of intraventricular conduction disturbances with sinoatrial block can be explained by the tachycardia-dependent (phase-3-dependent) occurrence of the left bundle branch block.