Association Between Septal Implantation Level and Pacing Threshold Stability in Leadless Pacemaker Implantation

Background/Objective: Leadless pacemakers (LPs, Micra™, Medtronic) offer a safe alternative to traditional transvenous systems. However, optimal implantation site within the right ventricular septum (RVS) and its effect on long-term pacing threshold stability remains under debate. The aim was to evaluate the relationship between pacing site within the RVS and pacing threshold stability following leadless pacemaker implantation. Methods: We retrospectively analyzed 36 patients who underwent LP implantation at two centers between 2022 and 2023. Patients were classified into two groups based on final device position by fluoroscopy: Group A (mid or upper RVS, n = 8) and Group B (low or apical RVS, n = 28). Pacing threshold, QRS duration, and left ventricular ejection fraction (LVEF) were assessed over 6 months. Results: At the 6-month follow-up, Group A demonstrated significantly lower and more stable pacing thresholds compared to Group B (0.57 ± 0.09 mV vs. 1.55 ± 0.97 mV, p < 0.001). No significant differences were observed in QRS duration or LVEF changes between groups. Echocardiography did not reveal new-onset tricuspid regurgitation. Conclusions: Given the small sample size, particularly in the mid/high septal group, these findings should be interpreted as hypothesis-generating and require confirmation in larger prospective studies. These findings highlight the importance of careful anatomical targeting during LP implantation. Prospective studies incorporating advanced imaging are warranted to confirm these results and evaluate long-term clinical outcomes.