Introduction
The adverse effects of His bundle pacing (HBP) have been realized, including the higher capture thresholds, risks of acute loss of capture, early battery depletion, and lower R-wave amplitudes1. Some studies have reported that the left bundle branch pacing (LBBP) has lower capture thresholds, higher success rate and stability, and larger R-wave amplitudes 2,3. Therefore, LBBP has emerged as a novel technique that can directly capture the left bundle branch (LBB) or LBB area by a transventricular septal approach 4. Different from HBP, LBBP induced a right bundle branch block (RBBB) morphology while shortening the stimulus-peak left ventricular activation time (stim-LVAT), which represents the negative effect of LBBP on right ventricular activation5. However, there is only limited information about LBBP performed on patients with RBBB. In practice, we observe that the QRS complex is narrower than the intrinsic one in almost all patients with RBBB after implantation of LBBP (Figure 1). In this article, we describe the changes in the QRS complex in patients with RBBB after LBBP and discuss the possible mechanisms for this situation.