Rate-related QRS morphological changes in left bundle branch pacing: A case report.

Studies have shown that a permanent left bundle branch pacing (LBBP) is feasible and may be clinically beneficial. The paced QRS morphology in a permanent LBBP typically manifests as a right bundle branch block pattern (RBBB) because the left ventricle is activated earlier than the right ventricle. We present one such case in which a small change in the pacing rate strongly influenced the LBBP QRS morphology. A 71-year-old man diagnosed with sick sinus syndrome had a dual-chamber pacemaker (LBBP) implanted. During the transition from non-selective LBBP to selective LBBP, a shrinking R' wave of the rsR' in V1 was observed. We also observed that the amplitude of the R' wave in lead V1 increased as the pacing rate increased and finally manifested as a complete RBBB. Thus, we demonstrated the impact of retrograde impulse conduction from this pacing site on the QRS complex.