Revisiting the Link Between Substrate and Ventricular Tachycardia Rate: The Critical Role of Conduction Isthmus Structure and Function

Background Ventricular tachycardia (VT) rate determines hemodynamic tolerance and informs risk stratification and ablation strategy. Although global substrate metrics have been associated with VT dynamics, the relative contribution of local conduction-isthmus properties remains incompletely defined. Objective We compared the predictive value of global versus isthmus-specific substrate features on VT rate. Methods In a prospective cohort, 62 patients undergoing ablation for scar-related VT underwent high-density electroanatomical mapping and late gadolinium-enhanced cardiac magnetic resonance. Functional metrics included total activation time (TAT), deceleration zone (DZ) duration/area, and late potential (LP) area; structural metrics included scar/border-zone mass and conduction channel (CC) length, protected volume and 2D architecture. The primary outcome was fast VT defined as >180 bpm. Results We analysed 107 VT episodes, and 39% (n=42) were classified as fast. Only isthmus-specific features independently predicted fast VT: DZ duration<110 ms (aOR 21.16, p<0.001) and CC length <3cm (aOR 10.18, p=0.02). These thresholds showed strong discrimination- AUC 0.88 for DZ duration <110 ms and 0.89 for CC length <30 mm; LP area <20 cm 2 yielded AUC 0.79—each with ≥80% global accuracy. Structural–functional coupling was observed: CC length correlated with DZ duration (ρ=0.54), DZ area (ρ=0.35), and LP area (ρ=0.55), all p≤0.01. Compared with ICM, NICM had faster VT and a more compact isthmus; however, aetiology was not an independent predictor and did not modify isthmus–VT rate associations. Conclusions Conduction isthmus properties predominate as determinants of VT rate. Quantitative characterization of VT-related conducting segments may support procedural planning in structural heart disease.