Electromechanical window as an integrative marker of remodeling and arrhythmic risk in biventricular vs. left bundle branch area pacing

Background The electromechanical window (EMW), defined as the difference between mechanical and electrical systole, is an emerging marker of arrhythmic risk and ventricular dysfunction. Although cardiac resynchronization therapy (CRT) improves mechanical synchrony, the influence of different pacing strategies on EMW is not well established. This study compared the effects of conventional biventricular (BiV) pacing and left bundle branch area pacing (LBBAP) on EMW, reverse remodeling, CRT response, and arrhythmic burden. Methods In this prospective, observational study, 80 patients with left ventricular ejection fraction (LVEF) ≤ 35% undergoing CRT received BiV pacing ( n = 40) or LBBAP ( n = 40). Echocardiography with simultaneous electrocardiography was performed at baseline and 6 months. The primary endpoint was change in EMW. Results At 6 months, LBBAP was associated with greater QRS narrowing (42 ± 15 vs. 28 ± 13 ms, p < 0.001) and EMW normalization (− 38 ± 22 to − 5 ± 18 ms vs. − 36 ± 20 to − 18 ± 19 ms; p = 0.003). Improvements in LVEF, global longitudinal strain, and left ventricular end-systolic volume were also greater with LBBAP. EMW improvement correlated with reverse remodeling and QRS narrowing. Baseline EMW predicted CRT response (AUC 0.71), while persistent EMW negativity was associated with a higher incidence of non-sustained ventricular tachycardia (45% vs. 18%, p = 0.01). Conclusions LBBAP was associated with greater EMW normalization, improved reverse remodeling, and a lower arrhythmic burden than BiV pacing. EMW may represent an integrative marker of CRT response and residual arrhythmic risk. These findings are hypothesis-generating and require validation in larger multicenter randomized studies.