Unusual tachycardia-bradycardia syndrome during atrial fibrillation: what is the mechanism?

A 60-year-old man with a history of remote myocarditis and residual mild left ventricular dysfunction presented with syncope. Before admission, the patient was not taking any regular medications. Medical history was otherwise unremarkable. Examination at presentation was unremarkable, apart from slow atrial fibrillation (AF). ECG on presentation demonstrated AF with slow ventricular response and a complete left bundle-branch block (Figure 1). Inpatient telemetry demonstrated episodes of nonsustained ventricular tachycardia (VT). Cardiac catheterization with coronary angiography was normal. Echocardiogram demonstrated mild left ventricular dysfunction (ejection fraction 50%) but was otherwise unremarkable. Figure 1. At baseline, the patient presented with persistent atrial fibrillation with complete left bundle-branch block and premature ventricular complexes. Twelve-lead ECG at a sweep speed of 25 mm/s. An electrophysiological study showed a hisioventricular interval of 75 ms during AF at a rate of 75 beats per minute. No spontaneous pauses or infranodal phase 4 atrioventricular (AV) block was spontaneously observed. Programmed stimulation from the right ventricular apex was unremarkable at a 600-ms cycle length. At a 400-ms cycle length, a sustained monomorphic VT at 165 beats per minute with left bundle-branch block morphology and an inferior axis (Figure 2) was induced with a single extrastimulus (S2=270 ms). After 35 seconds, the VT spontaneously terminated. Despite the persistence of AF, the termination of VT was followed by a prolonged complete infrahisian AV block (Figure 3). After 8.5 seconds of ventricular asystole, AV conduction resumed. In addition, spontaneous premature ventricular complexes were observed (Figure 4). Those originating from the left anterior septum were associated with transient infrahisian AV block (Figure 4A). In contrast, those originating from the right ventricular apex were not associated with AV block, despite an identical coupling interval (Figure 4B). A single-chamber cardioverter-defibrillator was successfully implanted, and the patient subsequently remained asymptomatic. Figure 2. A sustained left bundle-branch block morphology ventricular …