Related Experiment Video
Updated: Sep 21, 2026

Real-Time Cardiac Mapping with a Noninvasive Imageless Electrocardiographic Imaging System
Published on: April 11, 2025
A Curious Change in QRS Width: Timing Is Everything
David Litonjua Zepeda1, Abharika Sapru2, Ignatius Gerardo E Zarraga3
1Department of Medicine, Oregon Health & Science University, Portland, Oregon, USA.
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.
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