Vector field heterogeneity as a novel omnipolar mapping metric for functional substrate characterization in scar-related ventricular tachycardias
Authors: Tonko J B , Ruiperez-Campillo S , Cabrero Vidal G , Rooney C , Ehnesh M , Millet Roig J , Castells F , Lambiase P 
Published: October 2024

Abstract

Background
Identification of functionally critical sites to target for ablation in scar-related VT during substrate mapping remains challenging. Vector Field Heterogeneity (VFH) is a novel omnipolar metric to quantify locally heterogeneous propagation patterns.

Objective
Assess the diagnostic value of VFH for characterizing abnormal propagation patterns during ventricular substrate mapping and compare between sites of VT isthmus, low voltage bystander areas (LVA) and normal voltage areas (NVA).

Methods
High-density substrate maps and VT activation maps acquired with a 16-pole grid catheter in patients with scar-related VT were retrospectively reviewed. Based on the VT activation maps with either entrainment and/or VT termination with RF ablation to confirm critical sites, the correlating substrate maps were segmented into sites corresponding to isthmus-site, LVA (omnipolar voltage &lt;1.5mV) and NVA (&gt;1.5mV). To compute the VFH, contact mapping data including all EGMs of each grid-catheter acquisition (16 unipols) was exported and processed offline. From each of the 4×4 electrode array acquisitions, omnipolar-derived vector maps of the directions of electrical propagation for recorded sites were estimated using an orientation-independent cross-clique configuration of 4 adjacent unipols, respectively, resulting in 9 vectors with a spatial resolution of 1.4mm2 each. The angle between each vector and horizontal bipole was assessed and compared between neighbouring vectors using finite differences to derive a VFH value per 2x2 EGM clique as a quantitative metric of local heterogeneity ranging from 0 (perfect planar wave) to 1 (maximal disorganisation - "chaos"). The VFH metric was estimated for each segment of the substrate map and compared against each other.

Results
Nine patients with scar related VT were included (100% male, age 62±20y, ischemic 66.7%, LVEF 36±16%). On average 3.5 VTs (1-8) were induced per case. Avg. substrate map point count was 3825±3190. Computed median VFH metric at sites corresponding to VT isthmus was 0.58±0.30, LVA bystander 0.47±0.39, NVA sites 0.20±0.34. VFH at isthmus-sites was statistically significant higher (i.e. more disorganised) than at LVA bystander sites (p&lt;0.01). VFH in NVA was significantly lower (i.e. more organised) compared to isthmus and LVA sites (p&lt;0.01, respectively). Median omnipolar voltage at isthmus sites was 0.16±0.27mV compared to LVA bystander 0.22±0.36mV (p=0.0816) and NVA 2.97±3.38mV (p&lt;0.01).

Conclusion
VFH is a promising novel omnipolar mapping metric for electrical substrate characterisation in scar related VTs. VFH mapping identified a quantifiable and significant increase in electrical heterogeneity at sites corresponding to the critical isthmus compared to LVA bystander and normal voltage sites. VFH may provide new insights in the arrhythmogenicity of scar tissue and support more specific functional mapping strategies to identify targets for ablation without the need to induce VT.