doi:10.1007/s10840-026-02288-9engSales-Belles, ClaraMincholé, AnaCabrera-Ramos, MercedesMelero-Polo, JorgePueyo, EstherRamos Maqueda, JavierReal-time assessment of inter and intraventricular synchrony in cardiac resynchronization therapy with left bundle branch pacing using ECGi and low-frequency QRS analysisART-2026-148970Introduction: Left bundle branch pacing (LBBP) is an alternative to conventional biventricular pacing for cardiac resynchronization therapy (CRT). Methods: A 76-year-old male was referred for CRT via left bundle branch pacing (LBBP). We present the real-time characterization of the transition from intrinsic left bundle branch block (LBBB) to sequential pacing modalities: right ventricular septal pacing (RVSP), left ventricular septal pacing (LVSP), and finally LBBP. Epicardial activation maps from electrocardiographic imaging (ECGi) and low-frequency (LF) QRS analysis were used to provide complementary insights into interand intraventricular synchrony. Results: LVSP, LBBP, and anodal capture improved activation compared with baseline, with LBBP achieving the greatest improvements in interventricular and LV intraventricular synchrony despite higher RV dispersion. Conclusion: These findings highlight ECGi and LF QRS as valuable tools to guide CRT.2026http://zaguan.unizar.es/record/17046110.1007/s10840-026-02288-9http://zaguan.unizar.es/record/170461oai:zaguan.unizar.es:170461info:eu-repo/grantAgreement/ES/AEI/PID2021-128972OA-I00info:eu-repo/grantAgreement/ES/DGA/LMP 94-21info:eu-repo/grantAgreement/ES/DGA/T39-23Rinfo:eu-repo/grantAgreement/ES/MICINN/AEI/CNS2022-135899info:eu-repo/grantAgreement/ES/MICINN/PID2022-140556OB-I00info:eu-repo/grantAgreement/ES/MICINN/RYC2019-027420-Iinfo:eu-repo/grantAgreement/EUR/MICINN/TED2021-130459B-I00JOURNAL OF INTERVENTIONAL CARDIAC ELECTROPHYSIOLOGY (2026), [6 pp.]byhttps://creativecommons.org/licenses/by/4.0/deed.esinfo:eu-repo/semantics/openAccess