Alternating bundle branch block and infra-Hisian Mobitz II block: an educational case of rapidly progressive His-Purkinje disease.

Background: Disease of the cardiac conduction system may arise in the atrioventricular node or distal His-Purkinje network, with important differences in mechanism, prognosis, and management. Alternating bundle branch block (ABBB) is a rare, high-risk electrocardiographic finding reflecting advanced bilateral His-Purkinje involvement, particularly when accompanied by atrioventricular block. Full serial documentation of its progression with intracardiac confirmation during the same clinical course appears exceptionally uncommon. Case summary: We present an educational case of rapidly progressive His-Purkinje disease in which serial electrocardiograms documented evolution from early fascicular disease to bifascicular involvement, left bundle branch block, and ultimately ABBB with superimposed infra-Hisian Mobitz type II block over less than 8 months. Evaluation did not identify an obvious reversible or structural cause. Electrophysiology study provided intracardiac correlation of the surface electrocardiogram findings, directly capturing ABBB with superimposed infra-Hisian Mobitz type II block and confirming advanced His-Purkinje system disease. Given this high-risk conduction phenotype, cardiac resynchronization therapy pacemaker implantation was performed, with subsequent symptom resolution. Discussion: The principal value of this educational case lies not simply in the presence of ABBB, but in the unusually complete chronology with which rapidly progressive His-Purkinje disease was documented and mechanistically confirmed. This case highlights the importance of recognizing ABBB as a Class I pacing indication, irrespective of symptoms, and illustrates the challenge of completing etiologic evaluation, including cardiac magnetic resonance imaging, when urgent device implantation is required.