Impact of Left Bundle Branch Pacing on Left Ventricular Mechanical Efficiency

Background: Left bundle branch pacing (LBBP) restores physiological ventricular activation, yet its impact on left ventricular (LV) mechanical efficiency remains incompletely understood. Objective: To compare LV mechanical efficiency during spontaneous rhythm and LBBP using myocardial work (MW) indices, and to evaluate the influence of QRS transition type. Methods: In 115 patients enrolled in the prospective TREEBEARD study (NCT06324682), MW indices (i.e., global work index (GWI), global constructive work (GCW), global wasted work (GWW), and global work efficiency (GWE)) were measured before and after LBBP. Patients were classified according to QRS transition during decremental output as Left Bundle (LB) group (nonselective to selective LBB capture) or non-LB group (nonselective capture to LV septal pacing). Results: Overall, LBBP achieved mechanical efficiency comparable to that observed during spontaneous rhythm. In the LB group, all MW indices showed strong correlations between spontaneous rhythm and LBBP. In the non-LB group, GWI (r = 0.685, p < 0.001) and GCW (r = 0.750, p < 0.001) remained strongly correlated, whereas GWE (r = 0.4224, p = 0.01) and GWW (r = 0.395, p = 0.002) showed moderate and weak correlations, respectively. Conclusion: Based on our findings, LBBP appears to preserve LV mechanical efficiency to a degree comparable to spontaneous rhythm. Direct engagement of the conduction system, reflected by a nonselective to selective LBBP QRS transition, is associated with greater concordance of MW indices.