The characteristics of the electrocardiogram and the intracardiac electrogram in left bundle branch pacing

Introduction Left bundle branch pacing (LBBP) has recently been reported to maintain left ventricular electrical synchrony with a low and stable threshold. However, the electrocardiogram (ECG) definitions of LBBP have not been well established. We report four cases to show the characteristics of the ECG and the intracardiac electrogram (EGM) in LBBP. Methods and Results Four patients, two with an atrioventricular block (AVB) and two with left bundle branch block (LBBB), were included in the study. LBBP was performed and the ECGs and EGMs were collected and compared at different pacing outputs. Selective LBBP (S-LBBP) was defined as only capturing the LBB with a typical RBBB morphology and a discrete component between the pacing stimulus and ventricular activation in the EGMs. While nonselective LBBP (NS-LBBP) captured both the LBB and the local myocardium, resulting in a narrow right bundle branch block (RBBB) morphology without the discrete component. The left bundle branch (LBB) potential was recorded in two cases during narrow QRS complex or LBBB correction by selective His bundle pacing and SLBBP (n = 3) was achieved. A constant and shortest stimulus to left ventricular activation time (LVAT) in LBBP was obtained at different pacing outputs. Conclusion The ECG and EGM characteristics of LBBP can be summarized as: 1) RBBB pattern; 2) usually with the LBB potential; 3) SLBBP with specific ECG changes and a discrete component in EGM; and 4) with a constant and shortest stimulus to LVAT at different pacing outputs. Further studies are necessary to confirm these observations.