What is a substrate-based approach for managing ventricular tachycardia (VT) in patients with a history of heart disease, such as coronary artery disease (CAD), cardiomyopathy, or previous myocardial infarction (MI)?

Substrate-Based Approach for Ventricular Tachycardia

Substrate-based ablation targets the anatomical and electrophysiological abnormalities in myocardial scar tissue during sinus or paced rhythm, rather than requiring VT induction, and is now the preferred strategy for patients with unstable, non-inducible, or poorly tolerated VT in structural heart disease. 1, 2

Core Concept and Rationale

The substrate-based approach evolved from the recognition that most patients with scar-related VT present with unstable arrhythmias that cannot be safely mapped during active tachycardia 2. This strategy identifies and ablates the critical slow conduction zones within myocardial scar that form potential re-entry circuits, without needing to induce the clinical VT 2, 3.

The SMASH-VT trial demonstrated that substrate-guided ablation targeting abnormal ventricular potentials during sinus rhythm reduced VT episodes from 33% to 12% and appropriate ICD shocks from 31% to 9% over 23 months in post-MI patients. 1

Three Main Substrate Mapping Strategies

1. Targeting Abnormal Electrograms

This approach identifies and ablates areas displaying abnormal electrical characteristics 3, 4:

• Late potentials: Electrograms occurring after the QRS complex, indicating slow conduction 2
• Local abnormal ventricular activity (LAVA): Sharp, high-frequency potentials distinct from the far-field ventricular signal 2
• Fractionated electrograms: Multi-component signals with low amplitude and long duration 5
• Low-voltage areas: Typically defined as bipolar voltage ≤1.5 mV (dense scar ≤0.5 mV) 5

Scar homogenization eliminates all abnormal electrograms within the low-voltage zone, creating extensive ablation lesions to eliminate potential VT circuits. 2

2. Anatomical Channel Targeting (Scar Dechanneling)

This strategy identifies conducting channels of viable myocardium within or between areas of scar 3, 4:

• Map the scar borders and internal architecture using voltage mapping 4
• Identify narrow isthmuses or corridors of conducting tissue between dense scar regions 2
• Create linear ablation lesions to interrupt these channels and connect areas of dense scar 2
• The goal is to eliminate potential pathways for re-entrant circuits by creating complete conduction block across channels. 3

3. Functional Substrate Mapping

This approach uses physiologic maneuvers to identify areas of slow or decremental conduction 3, 4:

• Pace mapping: Compare paced QRS morphology to clinical VT to identify exit sites 4
• Entrainment mapping: When VT is tolerated, identify the critical isthmus by demonstrating concealed fusion 5
• Decremental conduction: Use programmed stimulation to reveal areas with rate-dependent conduction slowing 3

Practical Implementation Algorithm

Step 1: Define the Scar Substrate

• Perform high-density electroanatomical mapping during sinus or paced rhythm 4
• Create voltage maps identifying normal myocardium (>1.5 mV), border zone (0.5-1.5 mV), and dense scar (<0.5 mV) 5
• Use bipolar voltage with standardized electrode size and contact force 5

Step 2: Identify Ablation Targets

Choose based on VT characteristics and hemodynamic tolerance 2, 3:

For hemodynamically unstable or non-inducible VT:

• Target all late potentials and LAVA within the scar 2
• Perform scar dechanneling by creating linear lesions across conducting channels 2
• Consider complete scar homogenization if limited targets identified 2

For tolerated VT with specific morphology:

• Perform activation and entrainment mapping during VT 1
• Target the critical isthmus identified by entrainment 5
• Add substrate modification of surrounding abnormal electrograms 3

Step 3: Procedural Endpoints

• Elimination of all targeted abnormal electrograms 2
• Non-inducibility of any sustained monomorphic VT with aggressive programmed stimulation 1
• Complete electrical isolation of scar regions if core isolation technique used 2

Evidence Supporting Substrate-Based Ablation

The VTACH trial showed that substrate-based ablation increased VT-free survival from 29% to 47% over 24 months compared to ICD alone, and reduced appropriate ICD shocks from 3.4 to 0.6 per patient per year. 1

The Euro-VT study reported 81% acute success with substrate mapping approaches and 51% freedom from recurrent VT 1. These outcomes are superior to targeting only mappable VTs, particularly in patients with multiple VT morphologies or unstable arrhythmias 3.

Critical Caveats and Limitations

Current substrate mapping relies on assumptions that may not be valid: 5

• Low voltage does not always indicate inexcitable scar—viable myocytes can exist in low-voltage regions 5
• Fixed anatomical barriers may not accurately predict functional re-entry circuits 5
• Electrode size, spacing, and contact force significantly affect voltage measurements and must be standardized 5

Outcomes vary by underlying etiology: Patients with post-MI scar have better ablation outcomes than those with non-ischemic cardiomyopathy 1, 6. The extent of scar burden (>5% of LV mass) predicts worse outcomes regardless of LVEF 6.

ICD implantation remains mandatory in patients undergoing catheter ablation who meet eligibility criteria for primary or secondary prevention, as ablation reduces but does not eliminate arrhythmia risk 1.

Integration with Other Therapies

Substrate-based ablation should be combined with optimal medical therapy 1:

• Beta-blockers remain first-line for primary prevention 7
• Amiodarone or catheter ablation are both Class I recommendations for recurrent ICD shocks due to sustained VT 1
• Urgent catheter ablation is Class I for incessant VT or electrical storm 1, 7

Aggressive treatment of heart failure and myocardial ischemia is mandatory, as these conditions perpetuate the arrhythmogenic substrate 1.