Clinical significance and possible mechanism of different ventricular electrogram morphology in selective left bundle branch pacing

Background Currently, splitting of electrogram (EGM) or electrocardiogram (ECG) under threshold test are used as the gold standard to assess Left bundle branch (LBB) capture in LBB area pacing. However, discrete intracardiac ventricular EGM has not been reported until now. This study aims to explore the clinical significance and possible mechanism of different pacing ventricular EGM morphologies in selective LBB pacing. Methods Only patients with evidence of selective LBB pacing (splitting of EGM under threshold test) were included. According to the differences between intrinsic and paced ventricular EGM morphologies, the participants were further divided into three groups: concordant EGM (CE) group, similar EGM (SE) group and discordant EGM (DE) group. Baseline characteristics, indications for pacing, pacing parameters, and V6 R-wave peak time were analyzed. Results 274 patients (85.6%) achieved successful selective LBB pacing. After excluding 34 LBBB patients, LBB potential was recorded in 192 (80%) of 240 patients. In patients with LBB potential, the correlation between V-V6(P) RWPT and V-V6(S) RWPT in CE group (r=0.083, P < 0.0001) and SE group (r=0.766, P < 0.0001) were strong. V-V6(S) RWPT was significantly shorter than V-V6(P) RWPT (38.14±9.42 vs. 43.68±6.72, P<0.01 ) in DE group. In patients without LBB potential, V-V6(S) RWPT was significantly shorter than V-V6 RWPT (38.14±11.60 vs. 46.15±11.81, P<0.05) in DE group. There was a strong correlation (r=0.943, P<0.0001) between V-V6 RWPT and V-V6(S) RWPT in CE group, a possible correlation (r=0.564, P=0.07) in SE group, while poor correlation (r=0.259, P=0.27) in DE group. Conclusion The continuous recording technique combined with High Pass-200 Hz filter setting was feasible and effective for confirming selective LBB pacing by discrete EGM. Concordant or similar intrinsic and pacing ventricular EGM indicated that the electric conduction shared the same pathway, while discordant intrinsic and pacing ventricular EGM indicated that the electrical stimulation is conducted through different pathway. WHAT IS KNOWN? 1. Left bundle branch (LBB) pacing is a novel physiological pacing strategy. 2. Double transition in QRS morphology during threshold testing was considered as the criteria for LBB capture, and splitting of EGM under threshold test was used as the gold standard to assess selective LBB pacing. 3. Identifying discrete local ventricular EGM is still a challenging task. WHAT THE STUDY ADDS 1. The continuous recording technique combined with High Pass-200 Hz filter setting was feasible and effective for confirming selective LBB pacing by discrete EGM. 2. Different pacing ventricular EGM morphologies compared with intrinsic EGM accounted for clinical significance and possible mechanism: concordant or similar intrinsic and pacing ventricular EGM indicated that the electric conduction shared the same pathway, while discordant intrinsic and pacing ventricular EGM indicated that the electrical stimulation is conducted through different pathway. 3. The anatomical structure of LBB and its fascicular branch was complex, which could not be adequately recorded by 12-lead ECG and EGM.