Isochronal Apparent Dispersion at Early Activation Sites Accurately Identifies Outflow Tract Ventricular Ectopy Sites

Background: Localisation of outflow tract (OT) premature ventricular complex (PVC) sites is guided by unipolar and bipolar local activation time (LAT). However, LAT-based localisation can be inaccurate if the site is intramural or distant. Deep foci produce rapid conduction velocity (CV) if the wavefront is tangential to the surface. Aim: We evaluated whether supraphysiological CV, referred to as surface isochronal apparent dispersion (IAD) mapping, can be used to accurately differentiate right and left ventricular OT PVC origin, guiding the successful site for OT PVC ablation. Method: Left ventricular OT mapping was performed if right ventricular OT mapping demonstrated a bipolar electrogram (EGM) <20 ms. The earliest EGMs underwent analysis of the following: first deflection bipolar EGM (bipolar_earliest) to QRS, bipolar_earliest to first deflection unipolar EGM (unipolar_earliest), bipolar_earliest to unipolar −dV/dT_max, unipolar −dV/dT_max to QRS, number of early LAT breakouts, and the surface area of the earliest isochronal breakout. Polynomial CV was calculated using a custom algorithm in MATLAB using cut-offs between 1 and 100,000 cm/s and used to create IAD, referred to as apparent dispersion index. The accuracy of IAD to distinguish between successful and unsuccessful OT sites was assessed and compared with conventional EGM indices. Results: Bipolar_earliest to QRS (28.5±7.3 ms vs 17.8±5.7 ms; p<0.05) is superior to unipolar −dV/dt_max to QRS (0.4±26.4 ms vs −6.4±13.4 ms; p=0.25) in differentiating successful and unsuccessful OT PVC sites. An early isochronal breakout area of less than 1 cm² and less than two breakouts indicates a successful side (both p<0.05). Bipolar_earliest to unipolar −dV/dT_max and to unipolar_earliest were not predictive (28.1±27.7 vs 24.2±13.3 ms; p=0.97 and 6.4±7.3 vs 6.4±5.8 ms; p=0.8, respectively). IAD appears to differentiate between successful and unsuccessful sites using an apparent dispersion index cut-off of 20,000 cm/s, with an accuracy of 93.8% and area under the receiver operator characteristic of 0.95. Conclusions: IAD is a realistic two-dimensional interpretation of the three-dimensional activation mapping surface that may be associated with OT origins to guide a successful side of catheter ablation.