Ultra-high-frequency ECG quantifies divergent electrical phenotypes after left bundle branch area pacing:...

Left bundle branch area pacing (LBBAP) is increasingly used to achieve physiologic ventricular activation. However, electrical "success" may differ by baseline substrate: resynchronization in left bundle branch block (LBBB) versus preservation of synchrony in intrinsically narrow QRS. Ultra-high-frequency ECG (UHF-ECG) provides quantitative indices of ventricular activation timing and dispersion beyond QRS duration.

We report a contrastive two-case series undergoing dual-chamber pacing with an LBBAP lead. Pre- and post-implant UHF-ECG was analyzed using ventricular depolarization maps and dyssynchrony indices, including ventricular electrical delay (VED) and mean ventricular dispersion (MeanVD). Conventional ECG timing metrics were assessed in parallel (QRS duration when available, R-wave peak time [RWPT], and inter-peak intervals). Global RWPT was calculated as the sum of RWPT in lead I and lateral precordial lead (V5/6 or V6, depending on availability).

In Case 1 (baseline complete LBBB), LBBAP resulted in a resynchronization phenotype with QRS duration reduction from 201.6 ms to 137.6 ms, VED16 improvement from +48 ms to -5 ms, and MeanVD16 reduction from 85 ms to 66 ms. RWPT(V5/6) was 90.0 ms and RWPT(I) 96.8 ms, yielding a global RWPT of 186.8 ms. In Case 2 (baseline narrow QRS), UHF-ECG demonstrated preservation of synchrony with stable VED18 (-3 ms pre- and post-implant) and low MeanVD18 (25 ms to 24 ms), alongside RWPT(V6) 87.6 ms and RWPT(I) 56.0 ms (global RWPT 143.6 ms).

UHF-ECG provided reproducible, quantitative descriptions in both cases and was demonstrative of two clinically relevant response patterns after LBBAP (resynchronization in baseline LBBB and preservation of low dyssynchrony in baseline narrow QRS). These findings are descriptive and warrant confirmation in larger cohorts.