Relation of QRS Duration to Response to Cardiac Resynchronization Therapy in Patients With Left Bundle Branch Block.

Left ventricular (LV) dyssynchrony (LVdys) is a necessary condition for successful cardiac resynchronization therapy (CRT). Despite left bundle branch block (LBBB) representing a reliable surrogate of LVdys, not all LBBB patients will respond to CRT. Our aim was to investigate the relation between QRS duration and LVdys in patients with LBBB who underwent CRT. We retrospectively studied 165 patients with LBBB who underwent CRT implantation according to the current guidelines. A 6-month reduction of LV end-systolic volume ≥15% identified responders to CRT. Baseline LVdys was defined as the delay between peak systolic velocities of the interventricular septum and lateral wall assessed by color-coded tissue Doppler imaging. Baseline characteristics of responders (61%) and nonresponders (39%) were comparable except for larger QRS complex (172 ± 24 vs 160 ± 16 ms, p <0.001) and lower degree of LVdys (46 ± 42 vs 72 ± 31 ms, p <0.001) in nonresponders. Receiver-operating characteristic curve analysis demonstrated that an optimal cut-off value of 3 for the ratio of QRS duration and LVdys (QRS/LVdys) yielded a sensitivity of 66% and specificity of 80% to predict nonresponsiveness to CRT; QRS/LVdys >3 remained an independent predictor at multivariate analysis. In patients with nonischemic origin of cardiomyopathy, the linear regression analysis documented a significant inverse relation between QRS duration and LVdys, as dyssynchrony progressively decreased as QRS widening increased (p = 0.006). This was not evident in patients with ischemic origin. In conclusion, in LBBB patients with nonischemic origin and marked QRS widening, the absence of LVdys may account for a lower response to CRT compared with patients with intermediate QRS widening.