Tachycardia-dependent second degree AV-block in a patient with right bundle branch block.

A 55-year old male patient, with dizzy spells during everyday activity and a complete right bundle branch block as the sole electrocardiographic abnormality, reproducibly demonstrated tachycardia-dependent Mobitz Type II- and 2:1 second degree atrioventricular block. An electrophysiologic study revealed a provocable block within the distal portion of the bundle of His without evidence of a split His potential. Because of the truly tachycardia-dependent AV-block, beta-blocker medication was initiated to prevent high sinus rates during everyday activity. This therapy abolished symptoms totally.