Delayed Stimulus to V6 R-Wave Peak Time for Selective Left Bundle Branch Pacing: Characteristics and Underlying Mechanisms.

BACKGROUND: Selective left bundle branch pacing (SLBBP) preserves left ventricular synchrony via low stable capture thresholds and shortened stimulus-to-V6 R-wave peak time (S-V6 RWPT), yet delayed S-V6 RWPT (DRWPT) occurs in some recipients. This study aimed to define DRWPT's electrocardiographic and intracardiac electrogram features during SLBBP and investigate its mechanisms. METHODS: We retrospectively analyzed 368 SLBBP patients (September 2020 to November 2024, continuous monitoring), enrolled 11 with DRWPT as the study group, enrolled 33 with 1:3 matched shortened S-V6 RWPT (SRWPT) as controls, and analyzed electrocardiogram/intracardiac electrogram features with the isoelectric interval as the endpoint. RESULTS: DRWPT was identified in 3% (11 of 368) of the cohort. Baseline pacing parameters in the DRWPT group were as follows: mean QRS duration 128 ± 35.4 ms, unipolar initial myocardial threshold 1.6 ± 0.7 V, initial left bundle branch threshold 0.7 ± 0.1 V, sensed R-wave amplitude 8.0 ± 2.6 mV, and pacing impedance of 644.1 ± 157.8 Ω. Compared with controls, the DRWPT group had a significantly higher right bundle branch block incidence (63.6% vs 6.06%, P < 0.001), longer median QRS duration (138 ms [IQR 96-152] vs 96 ms [IQR 86-114], P = 0.03), longer isoelectric interval (45.0 ms [IQR 35.0-64.0] vs 31.0 ms [IQR 25.0-37.0], P = 0.005), and longer S-V6 RWPT (94 ms [IQR 81.0-122.0] vs 66 ms [IQR 63.0-72.0], P < 0.001). No significant between-group difference was found in those with stimulus-to-V6 R-wave peak time for left ventricular septal pacing (P = 0.203). CONCLUSIONS: DRWPT, linked to impaired left bundle branch conduction with prolonged isoelectric interval and S-V6 RWPT, is more prevalent in patients with native right bundle branch block. The isoelectric interval and current of injury (COI) are critical for guiding lead implantation during continuous monitoring-guided SLBBP.