Mechanism of syncope in patients with bundle branch block and negative electrophysiological test

Background-In patients with syncope and bundle branch block (BBB), syncope is suspected to be attributable to a paroxysmal atrioventricular (AV) block , but little is known of its mechanism when electrophysiological study is n egative. Methods and Results-We applied an implantable loop recorder in 52 patients with BBB and negative conventional workup. During a follow-up of 3 to 15 mo nths, syncope recurred in 22 patients (42%), the event being documented in 19 patients after a median of 48 days. The most frequent finding, recorded in 17 patients, was one or more prolonged asystolic pause mainly attributab le to AV block; the remaining 2 patients had normal sinus rhythm or sinus t achycardia. The onset of the bradycardic episodes was always sudden but was sometimes preceded by ventricular premature beats. The median duration of the arrhythmic event was 47 seconds. An additional 3 patients developed non syncopal persistent III-degree AV block, and 2 patients had presyncope attr ibutable to AV block with asystole. No patients suffered injury attributabl e to syncopal relapse. Conclusions-In patients with BBB and negative electrophysiological study, m ost syncopal recurrences have a homogeneous mechanism that is characterized by prolonged asystolic pauses, mainly attributable to sudden-onset paroxys mal AV block.