VT Mapping Principles

The vast majority of sustained monomorphic ventricular tachycardia (VT) in the setting of structural heart disease is scar-based reentry. This applies to both ischemic cardiomyopathy (post-myocardial infarction scar) and non-ischemic cardiomyopathy (fibrosis from dilated CMP, sarcoidosis, ARVC, Chagas, etc.). The substrate for VT is created when surviving bundles of myocytes form conducting channels within or bordering areas of dense fibrosis, creating the anatomic framework for a reentrant circuit.

The critical isthmus is the central concept. It is a narrow, protected corridor of viable myocardium within or at the border of scar tissue through which the reentrant wavefront must pass during each cycle of VT. The circuit has several functional components: the entrance (where the wavefront enters the isthmus from the outer loop), the isthmus itself (the slow-conducting protected channel), the exit site (where the wavefront emerges into excitable myocardium, producing the QRS morphology), the inner loop, and the outer loop. Bystander regions are areas of scar with electrical activity that do not participate in the reentrant circuit — ablating bystanders will not terminate VT.

Within the border zone, electrograms with late potentials (LPs) are markers of slow conduction and potential isthmus sites. Late potentials are defined as discrete, high-frequency deflections occurring after the end of the surface QRS complex during sinus rhythm. These represent activation of slowly conducting bundles within scar and are strongly associated with VT substrates. Fractionated electrograms — signals with multiple deflections, prolonged duration (>133 ms), and low amplitude — indicate regions of disorganized, anisotropic conduction within fibrotic tissue.

In ischemic VT, scar is typically subendocardial (following coronary artery territory), with the isthmus located at the scar border zone, often along the infarct perimeter. In non-ischemic VT, scar distribution is highly variable: basal lateral and inferolateral (dilated CMP), epicardial predominance (Chagas, some dilated CMP), RV free wall and perivalvular (ARVC), or patchy septal/multifocal (sarcoidosis). This scar distribution dictates the mapping and ablation approach — non-ischemic VT more frequently requires epicardial access.

The 12-lead ECG during VT is a critical first step in localizing the exit site of the reentrant circuit before the patient reaches the EP lab. Understanding VT morphology allows the electrophysiologist to anticipate the scar location and plan the mapping strategy.

Bundle branch morphology in lead V1 is the primary discriminator. An LBBB pattern (rS or QS in V1) indicates the wavefront travels from a right-sided or septal exit leftward — the exit is on the RV side or septum. An RBBB pattern (rsR′ or dominant R in V1) indicates rightward terminal forces — the exit is on the LV free wall. The frontal plane axis further refines localization: an inferior axis (tall R in II, III, aVF) points to a superior or basal exit, while a superior axis (deep S in inferior leads) indicates an inferior or apical exit.

Precordial transition — the lead where R-wave amplitude first exceeds S-wave — helps distinguish lateral vs. medial and basal vs. apical origins. Very early transition (by V2) suggests a basal lateral exit or epicardial focus. Very late transition (V5–V6) suggests an apical or RV free wall origin. QS complexes across precordial leads suggest an anterior wall exit.

Pace-mapping is the complementary ECG technique used in the EP lab. By pacing from the catheter tip at candidate sites during sinus rhythm, the operator compares the paced QRS morphology to the clinical VT morphology across all 12 leads. A ≥11/12 lead match indicates proximity to the exit site. A perfect 12/12 match with long stimulus-to-QRS interval (>40 ms) suggests the pacing site is within the isthmus proximal to the exit, because the stimulus must traverse slow-conducting tissue before reaching excitable myocardium.

Three fundamental mapping approaches are used in VT ablation, often in combination. The choice depends on whether the VT is hemodynamically tolerable and the clinical scenario.

Activation Mapping During VT

Activation mapping is the gold standard when VT is hemodynamically stable and sustained. The operator moves the mapping catheter point-by-point across the ventricle during VT, recording local activation times relative to a fixed reference (usually the surface QRS onset). The goal is to identify the earliest activation site (the exit) and trace back through the isthmus to find mid-diastolic potentials (MDPs) — electrograms occurring in the diastolic interval of VT (between one QRS and the next). MDPs at the critical isthmus are typically low-amplitude, high-frequency signals that span the diastolic gap. The diastolic pathway can be traced by moving the catheter from the exit site back toward the entrance, showing progressively earlier diastolic signals.

Entrainment Mapping

Entrainment mapping confirms whether a given site is within the reentrant circuit and, if so, where (isthmus, exit, entrance, inner loop, or outer loop). The operator paces at a cycle length 10–30 ms shorter than the VT cycle length and analyzes the post-pacing response.

Concealed entrainment (also called exact entrainment or entrainment with concealed fusion) is the diagnostic hallmark of the critical isthmus. During pacing, the QRS morphology is identical to VT (no fusion), and upon cessation of pacing, the first return beat has a post-pacing interval (PPI) equal to the VT tachycardia cycle length (TCL). Specifically, a PPI − TCL ≤ 30 ms confirms the site is within the circuit.

Substrate Mapping

Substrate mapping is the workhorse for patients with unmappable VT — VT that is hemodynamically unstable, non-sustained, or non-inducible. This approach is performed during sinus rhythm or ventricular pacing and relies on identifying the arrhythmogenic substrate rather than the active VT circuit.

The operator creates a high-density bipolar voltage map of the ventricle(s), delineating dense scar (<0.5 mV), border zone (0.5–1.5 mV), and normal tissue (>1.5 mV). Within the scar, the key targets are:

• Late potentials (LPs): Electrograms with components extending beyond the QRS — representing slow conduction through surviving myocyte bundles
• LAVA (Local Abnormal Ventricular Activities): Sharp, high-frequency signals within or at the border of scar that are distinct from the far-field ventricular electrogram. Elimination of all LAVA has been shown to reduce VT recurrence
• Conducting channels: Corridors of relatively preserved voltage (>0.5 mV) running through dense scar, visible on high-density isopotential maps. These channels correspond to the reentrant isthmus

Pace-mapping complements substrate mapping: at sites within the scar border zone, pace-mapping with a ≥11/12 lead QRS match to the clinical VT identifies the exit site. A long stimulus-to-QRS delay (>40 ms) at a good pace-map site suggests the pacing catheter is proximal to the exit within the isthmus.

Entrainment-Guided Ablation

When the VT is mappable and entrainment identifies the critical isthmus (concealed entrainment with PPI − TCL ≤ 30 ms and stimulus-to-QRS ratio 0.3–0.7), focal or linear ablation across the isthmus can terminate VT and render it non-inducible. Radiofrequency energy is delivered at the isthmus site, and successful ablation is often accompanied by VT termination during the lesion. A line of ablation lesions connecting the isthmus site to a region of dense scar or anatomic boundary (mitral annulus, aortic root) anchors the lesion and prevents conduction around it.

Substrate-Based Ablation

For unmappable VT, substrate-based approaches target the arrhythmogenic tissue identified during sinus rhythm mapping:

• LAVA elimination: Systematic ablation of all identified LAVA signals within and bordering scar. The endpoint is complete abolition of LAVA. This approach, described by Jaïs and colleagues, has demonstrated significant reduction in VT recurrence
• Scar homogenization: Extensive ablation within the entire scar border zone, eliminating all late potentials and reducing all electrograms within scar to <0.5 mV. This aggressive strategy aims to destroy all potential isthmus tissue. Di Biase et al. showed superior outcomes with homogenization compared to limited substrate ablation
• Channel blocking: Targeted ablation at the entrance or narrowest point of identified conducting channels within scar, blocking the isthmus without extensive ablation of surrounding tissue

Endo-Epicardial Approach

Non-ischemic cardiomyopathy frequently involves epicardial or midmyocardial substrate that is inaccessible from the endocardium alone. Epicardial access via subxiphoid percutaneous pericardial puncture allows mapping and ablation of the epicardial surface. This approach is particularly relevant for dilated CMP with inferolateral epicardial scar, Chagas disease, and ARVC with epicardial predominance.

Outcomes and Complications

Long-term success rates for scar-related VT ablation are 50–70% freedom from VT recurrence at 1 year for ischemic CMP, with lower rates (40–60%) for non-ischemic CMP due to more complex, midmyocardial, and epicardial substrates. Repeat procedures improve cumulative success. Major complications include pericardial tamponade (1–2%, higher with epicardial access), stroke/thromboembolism (0.5–1%), AV block (especially with septal ablation near the His–Purkinje system), vascular access complications, and death (<1% in experienced centers).