His bundle recordings in bradycardia-dependent AV block induced by premature beats.

Symptomatic bradycardia-independent atrioventricular block occurred in a patient with right bundle-branch block, left anterior hemiblock, and prolonged HV interval. The arrhythmia, triggered by a spontaneous or induced premature beats, appeared when the post-extrasystolic PP and HH intervals increased to a critical value. Reinitiation of atrioventricular conduction required the presence of ventricular escapes. Bradycardia-dependent atrioventricular block was related to either an enhanced or slightly rising slope of diastolic depolarization, or to a decrease in membrane responsiveness. The patient also, most probably, had tachycardia-dependent atrioventricular block. Both types of conduction disturbance occurred in the same part of the intraventricular conducting system, either in the low His bundle or left bundle-branch or its posteroinferior division. It is suggested that the electrophysiological study of cases with prolonged HV intervals should include procedures which can expose bradycardia-dependent atrioventricular block.