Electrophysiological characteristics and possible mechanism of bipolar pacing in left bundle branch pacing

Left bundle branch pacing is a physiological pacing modality with a low and stable threshold. The electrophysiological characteristics and mechanisms of bipolar pacing remain unclear.This study aimed to assess the electrophysiological characteristics of bipolar pacing of left bundle branch pacing and to infer the mechanisms underlying each electrocardiogram and electrogram waveform morphology.A total of 65 patients who strictly met the criteria for left bundle branch capture were enrolled. The changes in the morphology of the electrocardiogram and electrogram during the threshold testing with different outputs on unipolar and bipolar pacing were recorded. The electrophysiological characteristics were then analyzed.Four distinct morphologies and 3 different types of transitions during bipolar pacing threshold testing were identified; we labeled the 4 types of morphologies as nonselective (NS)-bipolar-left bundle (LB), NS-cathodal-LB, selective (S)-cathodal-LB, and left ventricular septal-cathodal. Except left ventricular septal-cathodal, the other 3 types (NS-bipolar-LB, NS-cathodal-LB, and S-cathodal-L) had a short and constant V6 R-wave peak time (RWPT) (64.8 ± 7.7 ms vs 65.7 ± 7.8 ms vs 65.7 ± 7.3 ms). The paced QRS (P-QRS) complex was the narrowest in NS-bipolar-LB rather than in NS-cathodal-LB (118.2 ± 14.2 ms vs 133.8 ± 15.8 ms; P < .001). NS-bipolar-LB had a higher threshold than did NS-cathodal-LB (2.5 ± 1.2 V vs 0.8 ± 0.4 V; P < .001).With a higher output on bipolar pacing, NS-bipolar-LB capture had the shortest V6 RWPT, V1 RWPT, and P-QRS. S-cathodal-LB capture had the longest V1 RWPT and P-QRS complex.