Unveiling deceleration zones through the s3 protocol: a functional substrate mapping for ventricular tachycardia ablation

Background Functional substrate mapping with double extrastimuli (S1–S2–S3 protocol) enhances the localization of areas of conduction deceleration associated with ventricular tachycardia (VT) circuits. The correct identification and ablation of these areas may improve remarkably the outcomes of VT ablation. However, some signals may be blocked on the S3 map, while they can be found in the S1 and S2 maps. Indeed, the way deceleration zones (DZs) evolve across successive stimulation steps remains poorly characterized. Purpose To evaluate the distribution of DZs across S1, S2, and S3 functional maps, and to prove that although the S3 map identifies the greatest number of DZs, integration of S1 and S2 remains essential. Methods We retrospectively analyzed all VT ablation procedures in which the complete S3 protocol was attempted between November 2023and July 2025 at our centre. Left ventricular activation maps were sequentially constructed with S1 (drive train), S2 (ERP + 30 ms), and S3 (ERP + 50 ms) extrastimuli. All maps were acquired using the HD Grid catheter and EnSite X mapping system. The entire activation window was divided into 8 equally timed isochrones. DZs were identified on the basis of the presence of at least 3 isochrones within a 1-cm radius. The area of a DZ was then manually delineated at each level based on local activation time isochronal crowding. Each DZ was classified into one of seven possible presence combinations of functional mapping (S1, S2, S3, S1–S2, S2–S3, S1–S3, and S1–S2–S3) Results Twenty-six consecutive patients undergoing VT ablation with functional substrate mapping using the S3 protocol were included. (mean age 65 ± 12 years, 92% male). A total of 116 DZs were identified. DZs distribution was: S1 = 9, S2 = 14, S3 = 38. DZs present only in S3 were significantly more prevalent than those found exclusively in S1 or S2 (32.8% vs 7.8% and 12.1% respectively, p < 0.01). Moreover the number of DZs were in both S1–S2=6, in both S2–S3=14, in both S1–S3=12, while in S1–S2–S3=23. The distribution of shared DZs across map combinations was significantly non-uniform, with S2–S3 and S1–S2–S3 combinations occurring more frequently than S1–S2 or S1–S3 (p < 0.001). (Figure 1-2). Conclusion S3 mapping reveals the greatest number of deceleration zones, underscoring its key role in substrate characterization, while S1 and S2 remain crucial for detecting earlier and transient conduction slowing. Integrating all three levels yields a comprehensive view of conduction dynamics, potentially enhancing the precision of substrate-guided VT ablation.