LBBAP produces a narrow QRS complex by directly capturing intramyocardial Purkinje fibers, according to a small mechanistic study published in JACC: Clinical Electrophysiology. The study included 13 patients whose surface ECG and intracardiac recordings did not align with standard LBBAP classifications. Several unexpected features were noted, like alternating incomplete bundle branch block patterns, variable recruitment of the left and right bundle systems, and correction of baseline right bundle branch block at low pacing outputs.

In some patients, the paced QRS complex closely matched the baseline narrow QRS observed after atrioventricular node ablation, suggesting activation through a unique conduction pathway. Intracardiac recordings further demonstrated unusually rapid activation of the right ventricular septum. Human cardiac tissue studies revealed dense Purkinje fibers embedded deep within the septal myocardium near typical LBBAP lead sites.

The findings indicate that LBBAP directly captures intramyocardial Purkinje fibers connected to both bundle branches. This mechanism could explain the counterintuitive pacing patterns observed and support the favorable electrical synchrony achieved with this approach. Recognizing the role of septal Purkinje capture may refine understanding of conduction system pacing physiology and help optimize implantation strategies.