A Curious Change in QRS Width: Timing Is Everything.

CASE BACKGROUND: A 61-year-old man with a mechanical aortic valve developed Staphylococcus aureus bacteremia. His electrocardiogram showed sinus rhythm with atrioventricular Wenckebach and phase 4 left bundle branch block. He was subsequently found to have a peri-aortic root abscess that was likely eroding not only into the atrioventricular node, but into the left bundle fibers as well. TAKE-HOME MESSAGES: Phase 4 block is a pause-dependent conduction block that reflects underlying His-Purkinje system disease. During a critical pause, spontaneous phase 4 depolarization in a diseased His-Purkinje system allows more sodium channels to become inactivated, reducing excitability and resulting in transient conduction block.