Electrocardiographic morphology during left bundle branch area pacing: Characteristics, underlying mechanisms, and clinical implications

Aims: In this study, we investigated the characteristics and underlying mechanisms of the electrocardiographic (ECG) morphology during left bundle branch area pacing (LBBAP), which have not been systematically described.

Methods: Patients with indications for permanent cardiac pacing underwent LBBAP attempts. The ECGs of patients with confirmed left bundle branch (LBB) capture were compared with those of individuals with right bundle branch block (RBBB) on 12-lead ECG. Intracardiac electrograms recorded during implantation were analyzed in all patients who underwent pacing.

Results: LBBAP was successfully achieved in 87.5% (56/64) of patients. The QRS morphologies in lead V₁ during LBBAP, which typically demonstrated Qr (60.7%), qR (19.6%), rSR' (7.1%), or QS (12.5%) patterns, differed from those of native RBBB, which featured rsR' (57.5%), M shape (23.7%), or monophasic R patterns (18.7%). The terminal R' wave duration in lead V₁ was significantly shorter during LBBAP than during native RBBB (51 ± 12 ms vs 85 ± 19 ms, p < 0.001). LBB potentials were recorded in 66.1% (37/56) of the LBBAP patients. No significant differences in ECG characteristics were found between LBBAP with and without recorded LBB potentials. The presence of bundle branch block during LBBAP significantly prolonged QRS duration, R wave peak time, and terminal R' wave duration in lead V₁ .

Conclusion: LBBAP-ECG patterns are characterized by a shorter terminal R' wave duration in lead V₁ compared with that of native RBBB configurations. Bundle branch conduction integrity has an impact on ECG characteristics during LBBAP.