Unusual Electrocardiographic Pattern During Transvenous Pacing From the Middle Cardiac Vein

This report documents the unusual occurrence of a left bundle branch block pattern of ventricular depolarization during permanent pacing from the middle cardiac vein. All previous reports of ventricular pacing from the middle cardiac vein have described a right bundle branch block pattern of ventricular activation (dominant R‐waves in the right precordial leads), except in one case where both right and left bundle branch block patterns occurred at separate times. A high posterior infarct allowed early activation of the right ventricle from the middle cardiac vein. Undue reliance on the electrocardiogram may detract from the diagnosis of electrode malposition.