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.