Electrical and mechanical impacts of left posterior, anterior and proksimal fascicular trunk capture during left bundle branch area pacing: a single center observational study

Background Left bundle branch area pacing (LBBAP) can capture different components of the left conduction system, including the left bundle trunk, posterior fascicle, or anterior fascicle. Anatomical studies suggest that the posterior fascicle supplies a broader portion of the left ventricular Purkinje network, potentially enabling more homogeneous activation when selectively captured. Whether different fascicular targets during LBBAP result in distinct electrical or mechanical responses remains unclear. Purpose To compare electrical, functional, and clinical outcomes among patients exhibiting paced patterns consistent with trunk, posterior fascicular, or anterior fascicular capture. Methods Seventy-six consecutive patients (mean age 70±10, male n:43) undergoing LBBAP in 2 years in ourcenter were prospectively analyzed. The pacing indication was atrioventricular block in 60% and heart failure in 40%. Based on paced QRS axis and morphology, patients were categorized as likely capture of the left bundle trunk (15%), posterior fascicle (60%), or anterior fascicle (25%). Echocardiography, global longitudinal strain (GLS), right-heart function, and NYHA class were evaluated at baseline and at one-month follow-up. Results Posterior fascicular capture produced the narrowest paced QRS complexes compared with trunk and anterior fascicular capture (118 ± 9 ms vs 128 ± 8 ms and 131 ± 10 ms; p < 0.001). At one month, improvement in left ventricular ejection fraction (LVEF) was greatest in the posterior fascicular group (ΔLVEF +12.1%) compared with trunk (+7.4%) and anterior fascicular pacing (+6.0%; p = 0.01). GLS recovery was likewise most pronounced with posterior fascicular capture (ΔGLS 3.2% vs 1.8% and 1.1%; p = 0.01). Right-heart functional gains (TAPSE) and symptomatic improvement (NYHA class reduction) followed a similar pattern. Trunk pacing demonstrated relatively greater improvement in septal strain, while posterior fascicular pacing produced the most homogeneous global strain pattern. (Table) Conclusion Different fascicular targets during LBBAP are associated with distinct electrophysiological and mechanical responses. Posterior fascicular capture which is anatomically the largest and most widely distributed segment of the left conduction system, produced the narrowest paced QRS complexes and the most pronounced early improvement in systolic and global strain parameters, suggesting more homogeneous ventricular activation. These findings support systematic exploration of fascicle-specific pacing strategies within LBBAP.