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Alternating Bundle Branch Block — EECC MCQ

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HardArrhythmia & ElectrophysiologyAlternating Bundle Branch BlockEECC

A 75-year-old man presents with pre-syncope and a 12-lead ECG showing alternating LBBB and RBBB on consecutive beats. What does this finding indicate?

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Correct answer: B — Alternating bundle branch block (bilateral bundle branch disease) indicates severe infranodal conduction disease with imminent risk of complete heart block — permanent pacemaker implantation is indicated (Class I)

Alternating bundle branch block (ABBB) — LBBB on one beat followed by RBBB on the next — is a rare but pathognomonic finding indicating bilateral bundle branch disease with disease in both the left and right bundle branches. It represents infranodal conduction disease with a very high risk of progression to complete heart block (often within hours to days). The 2021 ESC Pacing Guidelines classify ABBB as a Class I indication for permanent pacemaker implantation regardless of symptoms, as the risk of asystole is imminent. A dual-chamber pacemaker (DDD) is recommended. ABBB should not be confused with rate-dependent bundle branch block (which occurs at faster rates and resolves at slower rates).

Reference: ESC (2021): Guidelines on Cardiac Pacing and CRT