Optimizing cardiac resynchronization therapy in left bundle branch block: real-time synchrony assessment using UHF-ECG

Background Despite advances in conduction system pacing (CSP) – including His Bundle Pacing (HBP) and Left Bundle Branch Area Pacing (LBBAP) -, achieving optimal cardiac resynchronization therapy (CRT) remains challenging. Ultra-high-frequency 14-lead ECG (UHF-ECG) is an investigational tool that enables real-time evaluation of electrical activation and synchrony, potentially guiding CRT implantation and optimization. Objective To characterize baseline electrical parameters and dyssynchrony patterns in patients with left bundle branch block (LBBB), and to assess acute and follow-up changes in electrical synchrony after CRT guided by UHF-ECG. Methods Patients with baseline LBBB referred for device implantation between Oct/23 and Apr/25. UHF-ECG was used intra-procedurally to quantify cardiac dyssynchrony, defined by the maximal time difference between the centers of mass of the UHF-QRS complexes (DYS16 and DYS18), as well as local depolarization duration (meanVD16 and meanVD18) across leads V1–V6 and V1–V8. QRS duration (QRSd) was automatically measured by UHF-ECG, distinguishing total QRSd from the spike (QRSdt) and pure QRSd (QRSdp). Results 77 patients (58.4% male, mean age 73.7 ± 10.3 years) were included; 59.7% received pacemakers and 40.3% defibrillators. The most prevalent comorbidities were hypertension (74%), dyslipidemia (71.4%), diabetes mellitus (37.7%), and chronic kidney disease (37.7%). Ischemic cardiomyopathy (24.7%) was the leading indication for device implantation, followed by LBBB-induced cardiomyopathy (20.8%) and new-onset LBBB post-transaortic valve implantation (TAVI) (20.8%). Mean left ventricular ejection fraction (LVEF) was 44.5 ± 14.3%. Among patients with LVEF<40%, CRT significantly improved systolic function (ΔLVEF +8.1 ± 9.0%, p<0.001). CSP was the preferred strategy (41.6% HBP, 50.6% LBBAP), only 7.8% underwent conventional coronary sinus CRT (cs-CRT). Among patients with post-TAVI LBBB, 68.8% received LBBAP and 31.3% HBP. CRT led to a significant reduction in QRSd, QRS area, electrical dyssynchrony, and depolarization duration (p<0.001). Compared with cs-CRT, CSP showed faster depolarization duration times and a non-significant trend toward narrower QRSd, lower QRS area, and improved electrical synchrony (Pictures 1 and 2). Capture thresholds (1.1±1.3V@0.49±0.02 ms) and R-wave amplitude (8.8±6.1mV) remained stable or improved over follow-up: at 4 [2-6] months (0.9±0.7V@0.44±0.1ms, p=0.251 and 11.4 ± 5.9mV, p=0.026) and at 13 [12-17] months (1.1±0.9V@0.46±0.2ms, p=0.239 and 12.3±6.3mV, p=0.085). Conclusion UHF-ECG is an innovative tool for guiding and optimizing CRT, providing real-time insights into electrical synchrony and depolarization dynamics. In patients with LBBB, CRT —particularly CSP—achieved significant improvements in QRSd, dyssynchrony, and LVEF. These findings highlight the potential of UHF-ECG to refine CRT delivery, enhance physiologic pacing precision, and support a more personalized approach to CRT.Electrical parametersUHF-ECG map