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

This report describes tachyeardia- and bradycardia-dependent left bundle branch block in a patient who exhibited normal intraventricular conduction with impulses intermediate in timing. During bradyeardia, longer cycles were associated with increasing degrees of incomplete left bundle branch block. These clinical observations support the existence of slow Phase 4 depolar-ization of latent pacemaker cells as one of the mechanisms for abnormalities of conduction. Additional Indexing Words: Aberrant conduction Refractory period REFRACTORINESS of conducting tissue is intimately related to recovery time. When a pathological process causes unequal refractoriness of the bundle branches (or fascicles) relatively early impulses will be conducted along one branch only, resulting in the well-known entity of tachycardia-dependent intermittent bundle branch block.1-6 Recently, a rarer form of aberration has received emphasis. In these cases, relatively late impulses are conducted abnormally but normal intraventricular conduction occurs with earlier impulses (bradycardia-dependent bundle branch block).7-21 Rarely both tachycardia- and bradyear-dia-dependent bundle branch block (or hemiblock) occur in the same patient." ' 20, 21 This report describes tachycardia- and bradyear-dia-dependent bundle branch block (or hemiblock) a patient who exhibited normal intraventricula