What is bifascicular block?
Bifascicular block means disease of two of three fascicles of the His-Purkinje system. The most common combination — accounting for 80–90% of cases — is RBBB with left anterior fascicular block. Recognition matters because the combination implies more extensive conduction system disease than either component in isolation.
ECG recognition
RBBB: RSR' in V1 (the 'M' or 'rabbit ear' pattern), wide S wave in leads I and V6, QRS >120 ms. LAFB with RBBB: left axis deviation (−45° to −90°), small Q in I and aVL, small R in II, III, and aVF. LPFB with RBBB: right axis deviation (+90° to +180°) — always a diagnosis of exclusion.
Clinical risk: progression to complete heart block
Risk of progression to complete heart block is estimated at 1–4% per year in asymptomatic patients. In symptomatic patients — presenting with syncope, pre-syncope, or unexplained dyspnoea — the risk is substantially higher and requires urgent cardiology input.
The ED decision framework
Asymptomatic, known bifascicular block, haemodynamically stable: document clearly, ensure cardiology follow-up is arranged before discharge. New bifascicular block with syncope or haemodynamic compromise: emergency cardiology review, transcutaneous pacing available at the bedside, admit for monitoring.