Revisiting unipolar and bipolar current of injury during left bundle branch area pacing: clarifying electrophysiological signatures and diagnostic pitfalls

Introduction Left bundle branch area pacing (LBBAP) requires deep septal lead positions to capture the left bundle branch area (1). Current of injury (COI) of the lead electrodes has been proposed as a guide for septal lead deployment, yet the diagnostic value of unipolar tip and ring recordings remains to be better evaluated (2,3). Purpose This study aims to investigate the value and usefulness of monitoring COI recorded at the tip and ring unipolar electrodes and in bipolar mode during LBBAP implant procedures using an institutional prospective protocol. Methods In this prospective, single-center study, patients undergoing LBBAP were evaluated with COI monitoring using an institutional protocol. COI amplitude was measured from EGMs recorded at lead tip and ring during both sensing and pacing, and correlated with lead depth by septography. COI values were stratified according to absolute values (< 3 mV, ≥ 3 mV and < 5 mV, ≥ 5 mV and < 10 mV, and ≥ 10 mV), COI ratios (COITip/COIRing) and LBBAP capture type (LBB pacing [LBBP] vs left ventricular septal pacing [LVSP]) were analyzed after successful LBBAP. Results Successful LBBAP was achieved in 33/34 patients (97%). COITip was consistently higher than COIRing, both during sensing (13.9 ± 5.9 vs 6.3 ± 5.3 mV, p < .001) and pacing (14.8 ± 5.6 vs 7.7 ± 5.0 mV, p < .001). COITip ≥ 10 mV occurred in 74% of cases versus 24% for COIRing, while COIRing ≤ 3 mV was more frequent than COITip ≤ 3 mV (29% vs 6%, p = .039). Most patients (77%) showed tip-dominant COI ratios (>1), although subsets with ring-dominant (<1) or balanced (~1) values were also observed despite successful LBBAP. No correlation was found between COITip and lead depth (R² = .001, p = .882), while COIRing correlated modestly (R² = 0.165, p = .039) with lead implant depth. Patients with COITip ≥ 10 mV were significantly more likely to exhibit LBBP than LVSP (88% vs 12%; p = .039). Conclusion Unipolar COITip provides the most reliable and physiologically meaningful indicator of safe and effective lead deployment during LBBAP and the dynamic rise in COITip during lead advancement provides the most robust procedural marker, rather than absolute cut-offs. COIRing, and COI ratios are strongly influenced by anatomical and technical factors and may yield misleading interpretations when considered in isolation.