Left ventricular endocardial pacing via controlled interventricular septal lead passage as bailout after failed left bundle branch area pacing optimized CRT

Background Left bundle branch area pacing (LBBAP) and left bundle branch–optimized CRT (LOT-CRT) improve electrical resynchronization; however, a subset of patients remain non-responders or present technical failure of coronary sinus lead implantation. Fully endocardial LV pacing using standard conduction system pacing tools has not been systematically studied. Importantly, interventricular septal (IVS) lead passage implies intentional septal perforation, raising procedural safety concerns. Purpose To evaluate feasibility, safety, and clinical efficacy of LV endocardial pacing via controlled IVS lead passage using standard conduction system pacing tools (“LOT endo CRT”) as bailout therapy after ineffective LOT-CRT. Methods Between 2019–2025, 636 LBBAP procedures were performed for CRT indications. LBBAP was ineffective in 35 patients (5.5%). After additional coronary sinus lead implantation, 12 patients had ineffective or technically impossible LOT-CRT and underwent LOT endo CRT. Using a Selectra 3D delivery sheath (Biotronik) and stylet-driven active fixation lead (Solea 60 cm, 5.6F), the lead was advanced through the mid-septal IVS under continuous transthoracic echocardiographic guidance into the LV cavity and positioned at the lateral LV wall. The LV endocardial lead was connected to the LV port; the LBBAP lead to the RV port. All patients received uninterrupted therapeutic oral anticoagulants consistently with anticoagulation strategies recommended for transseptal left-sided interventions. Results Procedural success was achieved in 10/12 patients (83.3%). IVS thickness >15 mm predicted failure. Mean number of septal screwing attempts: 3.2 ± 1.7. QRS duration decreased from 171 ± 19 ms (failed LOT-CRT) to 130 ± 14 ms (LOT endo CRT), p<0.001. No acute or long-term complications—including hemopericardium, cardiac tamponade, stroke, systemic embolism, thromboembolic or device-related events, or lead dislodgement—were observed during the extended follow-up of 3.3 ± 1.6 years. LVEF improved from 26.4 ± 9.0% to 37.8 ± 7.1% at 6 months (p<0.001). Clinical response (≥5% absolute LVEF increase) occurred in 9/10 patients (90%). Conclusion In selected patients with ineffective LOT-CRT, LV endocardial pacing via controlled IVS perforation using standard LBBAP equipment is feasible and appears relatively safe under uninterrupted anticoagulation and imaging guidance. Septal thickness may predict technical success. Larger studies are warranted to define procedural safety boundaries and thromboembolic risk.