Septal Intramyocardial Purkinje Network

Background

Despite the growing use of left bundle branch area pacing (LBBAP) to deliver conduction system pacing, the mechanism underlying the narrow QRS interval conferred by this pacing modality remains unclear.

Objectives

This study aimed to evaluate the mechanism that provides a most plausible explanation of LBBAP physiology.

Methods

A cohort of 13 patients who had surface electrocardiographic (ECG) or intracardiac recording features not explainable by either selective or nonselective LBBAP were evaluated. Unique ECG patterns and intracardiac recordings over the right interventricular septum were analyzed, as well as septal Purkinje fiber staining patterns in human cardiac tissue, to assess whether such findings can be attributed to the capture of the recently discovered intramyocardial Purkinje network.

Results

The following unexpected ECG and intracardiac recording patterns were observed during LBBAP: 1) alternating incomplete right bundle branch block and left bundle branch block in an output-independent and output-dependent fashion; 2) variable, instead of all-or-none, recruitment of both left and right bundle systems; 3) correction of baseline right bundle branch block at low outputs; 4) paced QRS axis and duration closely matching the baseline narrow QRS interval in patients who underwent atrioventricular node ablation; and 5) intracardiac recordings demonstrating rapid, apparently nonphysiological activation of the right ventricular septum. Additionally, extensive Purkinje tissue was identified deep inside the septal myocardium in the human heart near the usual location of the LBBAP lead.

Conclusions

These data suggest a potential physiological role of the intramyocardial Purkinje system. Direct capture of Purkinje fibers connected to both bundle branches to rapidly activate both ventricles could provide a unifying explanation for these counterintuitive findings.