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 ECG 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.