What is the mechanism of narrow paced QRS duration during left bundle branch area pacing? A case report

Background Although left bundle branch area pacing (LBBAP) can capture the His-Purkinje conduction system and create a narrower paced QRS duration, its mechanism has not been investigated. In this case report, ventricular activation patterns were evaluated using three-dimensional electroanatomical mapping during LBBAP and right ventricular septal pacing (RVSP). Case summary An 81-year-old woman with sick sinus syndrome received LBBAP, followed 4 months later with atrial fibrillation ablation. We compared ventricular activation patterns during RVSP and LBBAP using a three-dimensional electro-anatomical mapping system. Paced QRS durations during RVSP and LBBAP were 163 ms and 115 ms, respectively. The activation pattern and the total left ventricular (LV) activation time were similar during RVSP and LBBAP (86 and 73 ms, respectively), despite the conduction system capture during LBBAP. The stimulus interval to the latest LV activation point during RVSP was 117 ms, and transseptal conduction time was 31 ms (117 - 86 ms). Discussion Although LBBAP could capture the His-Purkinje conduction system, neither ventricular activation patterns nor total activation time changed dramatically. The mechanism of narrower paced QRS duration during LBBAP compared to that during RVSP can be attributable to passing over the slow transseptal conduction.