Electrocardiographic Characteristics Associated With Lead Position Variations in Left Bundle Branch Area Pacing

BACKGROUND: Physiology-based electrocardiographic (ECG) criteria are used to assess left bundle branch area pacing (LBBAP), but the impact of lead implantation site on ECG parameters remains unclear. METHODS: We analyzed 212 patients who underwent successful LBBAP and classified them into high septum (HS, n = 49), mid septum (MS, n = 119), and low septum (LS, n = 44) groups according to the polarity of leads II and III. ECG parameters, including QRS morphology in lead V1, stimulus-to-R-wave peak times in V6 and V1 (sV6RWPT, sV1RWPT), and the V6-V1 interpeak interval, were compared. The proportions meeting proposed physiology-based ECG criteria for left bundle branch (LBB) capture were also evaluated. RESULTS: The LS group demonstrated the highest prevalence of dominant R waves in V1 (qR: 38.6%, R: 4.5%, p < 0.01). It had a shorter sV6RWPT than the MS group (59 ± 9 ms, p = 0.011), with no significant difference compared with the HS group. It also demonstrated a longer sV1RWPT (105 ± 11 ms, p = 0.023) and V6-V1 interpeak interval (46 ± 10 ms, p < 0.01). Although no significant differences were observed among the three groups regarding output-dependent QRS transition or LBB potential, the LS group more frequently met the V6-V1 interpeak interval criterion (>44 ms, 86%, p < 0.01). CONCLUSIONS: In LBBAP, low septal lead placement was associated with prominent R waves in V1, shorter sV6RWPT versus the MS group, and longer V6-V1 interpeak intervals, highlighting the importance of implantation site when interpreting ECG findings.