In Silico Pace Mapping To Optimise Ventricular Tachycardia Ablation

Catheter ablation of ventricular tachycardia (VT) most frequently requires the identification of exit sites of slow-conducting diastolic isthmuses associated with infarcted tissue which are critical to sustain the reentrant arrhythmia. Pace mapping is a technique that identifies exit sites by matching the recorded 12-lead electrocardiogram (ECG) QRS of catheter-paced beats at different myocardial locations with the QRS of the clinical VT. When a high correlation between QRS morphologies is found, it is believed that the paced-beat lies at the VT exit site, helping guide the ablation. Procedures thus involve methodologically moving the catheter to multiple (accessible) locations to produce a pace map. Unfortunately the procedure is inherently limited due to the following reasons:

i) Surface maps - Most often, pacing sites are limited to the endocardium (where catheter access is easiest). Consequently, identification of VTs with an intramural or epicardial substrate can be challenging, with mis-leading and/or difficult to interpret pace maps being obtained.

ii) Low Resolution - Due to time-pressures and practical difficulties in catheter manipulation, pace map sites are often sparsely located (typically 40-60 separate sites). In complex structural VTs, with multiple possible anatomical circuits through the scar, it is often challenging to accurately identify from the map the relevant isthmus exit site responsible for the presenting clinical VT.

iii) VT Induction - Pace mapping usually necessitates VT induction during the procedure, which is time-consuming and carries inherent risk (\~80% of induced VTs in ischemic heart disease (IHD) are either non-sustained or not haemodynamically tolerated), but induction of the exact clinical VT itself is often challenging.

A non-invasive, pre-procedural approach that is capable of generating accurate, high-resolution 3-dimensional personalised pace maps that correlate with the presenting clinical VT without the need for VT induction would thus revolutionise VT ablation planning, guidance, safety and efficacy.

Two recent studies have demonstrated the utility of performing a pace map based on comparison to stored implanted cardioverter defibrillator (ICD) electrograms (EGMs) of the clinical VT. The first study initially demonstrated that stored ICD EGMs allow differentiation of the clinical VT from other VTs, and their subsequent use in pace mapping may be useful for identifying the VT exit site. More recently, it has been prospectively shown that pace mapping of non-inducible clinical VTs based on ICD EGMs is feasible and resulted in higher freedom from recurrent VT, compared to targeting only inducible VTs.

The investigators have recently published a pipeline for creating a patient-specific image-based whole torso-cardiac model to perform virtual ('in silico') pace mapping. The approach creates a high-resolution 3D pace map that successfully identifies the exit site of an induced (simulated) VT within the same model, using both simulated 12-lead ECG and ICD EGM data derived from the simulated VT episode. Here, it was demonstrated: 1) the importance of creating a dense, fully transmural (3D) pace map to allow differentiation of epicardial vs endocardial substrates; 2) that accuracy of exit site identification could be enhanced by considering multiple EGM sensing vectors, e.g. from multipolar cardiac resynchronisation devices (CRT-D) with additional epicardial electrodes. Developed on simulated data, this workflow now requires full validation with clinical data from ablation patients, as proposed here, and refinement of methodological algorithms.