Syncope in a patient with right bundle branch block and left posterior fascicular block

ECG: Complete right bundle branch block pattern (QRS duration > 120 ms, rR’ or RsR’ pattern in V1-V2, delayed intrinsicoid deflection in V1, wide S wave in V5-V6, repolarization disorders in V1-V3) associated with a left posterior fascicular block (marked right axis, rS pattern in lead I and qR pattern in leads II, III);

Comments: This patient presents a bifascicular block with association between complete right bundle branch block and left posterior fascicular block. This form is much less common than the right bundle branch block + left anterior fascicular block association and certainly much more worrisome. Indeed, the risk of evolution toward a complete atrioventricular block is increased, the left anterior hemibranch, the only one ensuring conduction, being the most vulnerable. This is therefore an indication for pacemaker implantation especially when the patient is symptomatic and when other plausible causes of syncope have been eliminated.

It is difficult to differentiate right block + left posterior fascicular block and right block + right ventricular hypertrophy, a right axis deviation being observed in both cases. The absence of clinical or ultrasound evidence indicative of right ventricular hypertrophy thus suggests in this instance the diagnosis of a bifascicular block.

Take-home message: A major right axis deviation in a patient with a right bundle branch block may reflect the presence of right ventricular hypertrophy or associated left posterior fascicular block. The presence of a qR pattern in leads II, III is rather in favor of an associated left posterior fascicular block.