Rate-related left bundle branch block secondary to intra-his block

A 67-year-old man who had chronic aortic valvular disease with first degree AV block and rate-related left bundle branch block on the surface electrocardiogram was studied by His bundle electrography. During conduction with narrow QRS complexes, the first degree block was AV nodal in origin. With spontaneous heart rate acceleration a QRS pattern of left bundle branch block emerged, and the His electrogram revealed split His potentials. This report adds strong support to the evolving concept that left bundle branch block, in some cases, actually may be secondary to block anatomically localized to the His bundle.