Alternating Bundle Branch Block Despite a Patent Stent in Late-Presenting Extensive Anterior STEMI.

BACKGROUND

Alternating bundle branch block (ABBB) is a rare but high-risk marker of advanced infra-Hisian disease and impending high-grade atrioventricular (AV) block in anterior ST-segment elevation myocardial infarction (STEMI).

CASE SUMMARY: A 39-year-old male smoker presented 19 hours after chest pain onset with extensive anterior STEMI and ABBB. Ostial left anterior descending artery occlusion was treated with overlapping drug-eluting stents, achieving TIMI flow grade III. Despite patent stents, he developed delayed complete AV block. Temporary transvenous pacing restored hemodynamic stability, and AV conduction recovered within 24 hours.

DISCUSSION: Septal injury, edema, microvascular obstruction, or reperfusion injury can destabilize His-Purkinje conduction despite the presence of open epicardial arteries.

TAKE-HOME MESSAGE: ABBB warrants continuous monitoring and pacing readiness, even when angiography appears reassuring.