Quantitative CT Lead Localization Reveals Proximal Left Bundle Targeting Facilitates Superior Physiologic Pacing.

BACKGROUND: Implant electrical criteria for left bundle branch (LBB) capture in conduction system pacing (CSP) cannot localize the lead along the conduction axis. OBJECTIVE: To classify CSP lead position with an observer-independent CT workflow (CARA) using two anatomic measurements - lead-to-His distance (LH) and lead-to-endocardium offset (LE) - and to test whether proximal position yields near-native paced activation. METHODS: Of 186 consecutive CSP patients, 145 had interpretable CT anatomy. Leads were classified by prespecified rule: LVSP if LE ≤ -2.5 mm; otherwise proximal (LH ≤ 25 mm) or distal (LH > 25 mm), with 25 mm referenced to left bundle histology. Primary analysis set: sinus rhythm, native QRS <115 ms, complete paired ECGs, LE > -2.5 mm (n = 46). PRIMARY ENDPOINT: |Δ frontal axis| ≤15° and |Δ QRS duration| ≤15 ms. RESULTS: Classification yielded 68 proximal, 70 distal and 7 LVSP leads. The procedural label carried no positional information (48% of physician-labeled LBBP leads proximal vs 49% overall). The composite endpoint occurred in 15/24 (62%) proximal versus 6/22 (27%) distal leads (OR 4.14, 95% CI 1.23-13.96; p = 0.021; adjusted OR 5.35, p = 0.013), consistent across sensitivity analyses. The ROC-optimal LH cut-point (24.9 mm) independently reproduced the prespecified 25 mm threshold. Position was not recoverable from the surface ECG (classification AUC 0.48). CONCLUSION: In this proof-of-concept anatomic study, a two-measurement rule classified CSP lead position objectively. Near-native paced activation requires proximal targeting (LH ≤ 25 mm), which neither the procedural label nor ECG can identify - quantitative imaging is required.