Abstract Background Left bundle branch-optimized cardiac resynchronization therapy (LOT-CRT) is an alternative technique to conventional biventricular pacing (BiVP) and complementary to left bundle branch area pacing (LBBAP). Although it is a more complex technique than LBBAP, it achieves greater QRS narrowing and better resynchronization in most patients. Purpouse The aim of this study was to determine the predictors (echocardiographic, ECG and clinical) of greater QRS narrowing with LOT-CRT vs. LBBAP. Methods 38 consecutive patients with CRT-P indication were included in a period of 1 year. A ventricular lead was implanted in the septum until LBBAP criteria were achieved, evaluated by EGM and electrocardiographic recordings. A lead was subsequently implanted in a subsidiary vein of the coronary sinus (CS). In each patient, the QRS duration was measured during LBBAP and then with the fusion pattern from LBBAP and CS (LOT-CRT). Baseline demographic characteristics, indication for implantation, ECG, echocardiography, and NYHA class were collected in all patients. Results The mean age was 77.2 ± 6.9 years (37% women). The most common cardiomyopathy was idiopathic (55.3%) followed by ischemic cardiomyopathy (21.1%). 76.3% were in NYHA class 2 and 23.7% in NYHA class 3, with a mean LVEF of 34.1 ± 8.2%. 86.8% had a LVEDD ≥ 66mm. In 57.9% of patients the indication for implantation was HF in NYHA class ≥ 2, QRS ≥ 130ms with LBBB QRS morphology and LVEF ≤35%, while in the remaining 42.1% it was an LVEF < 50% and stimulation indication. The initial QRS was 180 ± 22 ms, 152 ± 16 ms with LBBAP and 132 ± 16 ms with LOT-CRT, with an additional QRS narrowing of 20 ± 14 ms with LOT-CRT vs. LBBAP (p < 0.001). Predictors of greater QRS narrowing with LOT-CRT vs. LBBAP were the indication for implantation due to HF in NYHA ≥ 2, QRS ≥ 130ms with LBBB QRS morphology and LVEF ≤35% (p 0.01), LVEDD ≥ 66mm (p 0.04) and advanced NYHA class (3 vs. 2) (p 0.019). Conclusion LOT-CRT significantly reduces QRS duration compared to LBBAP, favoring better resynchronization. Patients with a classic indication for CRT-P, in advanced stages of HF (NYHA class 3 and or/LVEED ≥ 66mm), are those who present greater QRS narrowing with this technique compared to LBBAP.