Lifting the curtain on ventricular tachycardia substrate with cardiac MRI: Path to enhance ablation outcomes

Catheter ablation is increasingly recognized as the cornerstone therapy for scar-mediated ventricular tachycardia (VT). Advancements in both electro-anatomical (EAM) mapping and ablation strategies have contributed to improved procedural success and reduction in VT burden, yet recurrence after ablation is common, with reported rates of 26% to 70% [1,2]. Activation and entrainment mapping are gold standard techniques but risk hemodynamic decompensation, target only inducible VTs and demand operator experience. On the other hand, EAM guided substrate approach targeting areas with abnormal voltage, local abnormal ventricular activity (LAVA) and late potentials can be time consuming and VT may still be inducible as EAM-approach can fail to identify critical isthmus, conducting channels and deeper midmyocardial or epicardial substrates. Therefore, pre-ablation preparedness and superior understanding of the substrate is valuable to improve procedural success and can be achieved with cardiac magnetic resonance imaging (CMR). CMR localizes scar and identifies midmyocardial and epicardial substrates. In addition, CMR derived color-coded pixel signal intensity (PSI) maps can delineate three-dimensional, multilayer, heterogeneous myocardial channels that survive within the scar tissue and border zones [3]. These corridors are recognized to be arrhythmogenic owing to their slow conduction properties.

In this issue of Indian Pacing and Electrophysiology, the meta-analysis by Chaunchai et al. showed pooled superiority of CMR-guided VT ablation over EAM-alone approach across ischemic and non-ischemic cohorts and across publication years [4]. This meta-analysis included 1243 patients from 18 observational studies with a mean follow-up of 18 ± 13.5 months. Both the CMR-guided (CMR integrated with EAM) and CMR-aided (CMR not integrated but utilized to aid ablation) had a ventricular arrhythmia (VA) recurrence of 25% and 26%, compared to 51% in EAM only group. VA recurrence was significantly lower in CMR-guided integration with EAM (OR 0.26, 95% CI 0.17–0.39), along with higher procedural success (OR 2.27, 95% CI 1.16–4.41), when compared to EAM alone approach. Heterogeneity was low for both outcomes (I² = 0%). The authors are credited for a well-conducted meta-analysis, PROSPERO registration and for adhering to PRIMSA guidelines [5]. Systematic methodology with risk-of-bias assessment, attempts to explain heterogeneity with meta-regression and relevant subgroup analyses are appreciated.

This observational evidence is informative and encouragingly, resonates with other published findings [[6], [7], [8]]. Yet the findings necessitate cautious interpretation due to non-randomized nature of the pooled studies, which can introduce both selection bias and small-study bias. Differences in CMR acquisition techniques, PSI processing, EAM mapping density, and ablation techniques are not accounted for and reflected in the high heterogeneity for continuous variables. Other factors that could introduce confounding and likely inflate these estimates include patient-level risk profile, scar location (epicardial or deep septal vs endocardial) and disease progression. For example, obtaining CMR may not be feasible in high-risk patients in intensive care or those with VT storm, and/or on mechanical support and notably, these patients are also at high risk of VT recurrence. In addition, the authors acknowledge the lower rates of anti-arrhythmic use, especially amiodarone, in the EAM group after ablation (22 % vs 28%) and that VA recurrence was statistically lower in those receiving amiodarone.

Nevertheless, CMR imaging is of low-risk and remains underutilized. As increasingly powerful ablative tools become available, knowledge of the substrate for targeted ablation is vital. Observational studies have shown CMR to reliably identify anatomical channels that correspond to functional slow-conduction deceleration zones, supporting its utility in pre-ablation substrate characterization [6,7,9]. Whether focused CMR-guided-only ablation approach is superior to conventional ablation is not known. Prospective randomized studies with uniform and standard imaging protocol can provide further insights. One such study, the VOYAGE study (NCT04694079), evaluating VT ablation guided or aided by CMR in comparison with EAM only approach has been completed [10]. Also, emerging data on functional VT substrate mapping such as wavefront discontinuity lines (WADL) and decremental evoked potential (DEEP) mapping is intriguing and the integration of CMR may enlighten us about the dynamic nature of VT substrate [11,12]. It remains to be determined if these modalities improve efficacy, safety, efficiency and reduce long-term VT recurrence rates when compared to conventional substrate-based approach.

In the meantime, the phrase “Give me six hours to chop down a tree and I will spend the first four sharpening the axe” remains relevant. A successful VT ablation requires pre-procedural planning. Where feasible, CMR should be obtained to inform VT substrate, to aid ablation strategy and to complement conventional electro-anatomical mapping.