Paroxysmal complete heart block due to bradycardia-dependent "phase 4" fascicular block in a patient with sinus node dysfunction and bifascicular block.

A 34-year-old man with ankylosing spondylitis was admitted to the hospital because of syncopal attacks and heart block. Standard ECG, telemetry and an invasive electrophysiologic examination demonstrated right bundle branch block with left anterior fascicular block, sinus node dysfunction with spontaneous slowing of the sinus rate and, as a result, complete infrahisian atrioventricular block in the remaining fascicle at sinus rates below 57 beats per minute. Infrequent supraventricular extrasystoles and abruptly terminated atrial pacing led to the same result. Atrioventricular conduction resumed after three-to-four blocked atrial impulses with successively increasing rate. Pacemaker treatment provided complete symptomatic relief.