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

Symptomatic bradycardia dependent atrioventricular block occurred in a patient with right bundle branch block, left anterior hemiblock, and prolonged HV interval. The arrhythmia, triggered by spontaneous or induced premature beats, appeared when the postextrasystolic 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 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.