Prevalence and clinical relevance of left septal fascicular block following conduction system pacing.

BACKGROUND

Left bundle branch area pacing (LBBAP) has emerged as a promising physiological pacing strategy. Due to capture of either posterior or anterior fascicles there might be electrocardiograhic (ECG) changes associated with it. However, the prevalence and clinical relevance of left septal fascicular block (LSFB) following LBBAP remain poorly understood, with available data limited to isolated case reports.

METHODS

This prospective, single-center study included 137 consecutive patients who underwent LBBAP between February 2024 and March 2025. Standard 12‑lead ECGs were obtained at baseline, at 24 h and one month post-implantation, with LSFB defined by established electrocardiographic criteria. Clinical, echocardiographic, biomarker, and procedural data were collected.

RESULTS

Mean age was 67.4 ± 12.4 years. LSFB was identified in 40 patients (29 %) and was mostly common in patients with ischemic cardiomyopathy (52.5 % vs. 34.0 %, p = 0.04). Although post-implantation troponin levels showed a significant increase compared to baseline, patients with LSFB had similar level of increase in troponin compared to patients without (∆Troponin 34.3 vs 34.0, p = 0.52). Procedural success rates and pacing parameters were comparable between groups.

CONCLUSION

LSFB is a relatively common and clinically relevant finding after LBBAP, strongly associated with ischemic cardiomyopathy. These findings suggest LSFB may represent a marker of underlying ischemic vulnerability and warrant further investigation in larger multicenter cohorts.