Substrate determinants of slow and fast ventricular tachycardia

Abstract Background Factors influencing Ventricular Tachycardia (VT) Cycle Length (CL) remain controversial. While some studies suggest that structural factors, such as total scar mass, strongly influence VTCL, recent research emphasizes the role of functional features such as outer-loop conduction velocity. However, the combined effect of VT channel characteristics and full ventricular functional and structural substrate analysis remains underexplored. We hypothesize that integrating these factors could improve the understanding of VTCL determinants, potentially allowing for better Implantable Cardioverter Defibrillator (ICD) programming. Purpose To analyse the structural and functional arrhythmogenic substrate in relation to VT cycle length in patients undergoing VT ablation. Methods We conducted a single-centre, retrospective, observational study, including 85 VT episodes from 51 patients. Structural substrate was assessed with Late Gadolinium Enhancement Magnetic Ressonance Imaging (LGE-MRI), while functional substrate was analysed using an Endocardial Isochronal Late Activation Map (ILAM) during S1 apical right ventricular pacing in patients undergoing VT ablation. Variables studied included Late Potential (LP) area, paced QRS width, Left Ventricular Activation Time (LVAT), deceleration zones (DZ) and Conduction Duration. Clinical data was also collected. CL<320 ms was defined as fast VTC and CL>320 ms as slow VTC. The mean or median for each variable was then compared using Wilcoxon rank sum test, Fisher’s exact test or Pearson’s Chi-squared test as suitable. Results Regarding functional substrate, slow VT had a wider paced QRS (ms) [201,7± 36,4 vs 179,6 ± 24,8; p 0,02], longer LVAT (ms) [213 ± 78,1 vs 165,7 ± 51,1; p<0,01], longer DZ mean duration (ms) [128,8 ± 63,9 vs 91,9 ± 55,5; p 0,01] and greater LP area (cm2) [40,7 ± 24,2 vs 20,4 ± 16,4; p<0,01]. Structurally, slow VT had more scar mass (g) [13.3 ± 7,2 vs 8,9 ± 2,9; p<0,01] and BZ mass (g) [29,6 ± 15,7 vs 18 ± 5,7; p 0,01]. VT isthmus analysis revealed that slow VT had longer duration of DZ (ms) [132,4 ± 90,7 vs 71 ± 43,3; p<0,01] and greater LP area (cm2) [37,5 ± 25,1 vs 16,6 ± 16,6; p<0,01] within the isthmus. Culprit-conduction-channels were wider (cm) in slow VT [5,4 ± 2,5 vs 4,2 ± 1,1; p 0,02]. Conclusion Our findings suggest that slow VT and fast VT exhibit significantly different functional and structural substrates, both at a full ventricular level and specifically within the isthmus. Slow VT appear to have a heavier scar burden, more BZ tissue and wider VT channels. Additionally, they present longer LVAT, DZ duration and greater LP surface. These results are promising and encourage further study on VTCL determinants, which may ultimately lead to enhanced ICD programming.