Recurrent Syncope and Ventricular Standstill Due to Phase 4 His-Purkinje Block.

CASE SUMMARY: A 66-year-old man with prior atrial fibrillation ablation presented after nearly drowning due to abrupt syncope without prodrome. He reported recurrent monthly episodes for 2 years. Baseline electrocardiogram showed sinus rhythm with left bundle branch block; echocardiography was normal. Telemetry captured a premature atrial beat precipitating ventricular standstill consistent with a phase 4 infranodal block in a diseased His-Purkinje system. Recovery showed transient high-grade Mobitz II block with dynamic QRS changes, indicating diffuse conduction disease. Permanent pacemaker implantation was performed.

TAKE-HOME MESSAGES: Phase 4 block causes unpredictable syncope in His-Purkinje disease. Dynamic QRS changes suggest advanced conduction system involvement requiring pacing.