2.3. Mapping, Ablation strategy, and the induction protocol
Firstly, substrate mapping was performed during sinus rhythm (SR) or ventricular pacing. The areas of abnormal electrograms, such as fractionated or late potentials, were tagged to denote the type of electrogram, and pace mapping was performed in those areas. Pace mapping was also performed at the presumed isthmus and exit regions of the clinical VT. The QRS morphology and stimulus to QRS (St-QRS) interval were evaluated. At the area of interest, a pace map was performed at maximum output (1.0ms, 20V), and the output was decreased until pacing could no longer capture the myocardium to evaluate the functional pace map response.
Secondly, programmed ventricular stimulation was delivered with up to double extra-stimuli at two different basic cycle lengths (CLs) (600 and 400 milliseconds) from at least two sites (the right ventricular apex, outflow tract, or left ventricle). If the induced VT was hemodynamically tolerated, activation mapping was acquired, and entrainment pacing was delivered if possible. In cases of hemodynamically unstable VT, the tachycardias were interrupted by overdrive pacing or direct current (DC) defibrillator. When VT was not induced, the induced VT was hemodynamically unstable, or the VT was not sustained, pace mapping was performed and compared to the clinical VT morphology to confirm the VT exit.
The VT isthmus was defined as sites where mid-diastolic potentials (MDPs) during the VT were present, and the RF ablation terminated the VT or pace mapping showed multiple exit sites (MESs).11RF ablation was applied at the critical isthmus of the target VT during VT, based on the findings from activation, entrainment, and pace mapping. For hemodynamically unstable VTs, RF ablation was applied at sites with abnormal electrograms, a longer St-QRS, and the presence of a functional pace map response. Fundamentally, all the clinical VTs were targeted in all cases. The non-clinical VTs were also targeted in cases with electrical storms.
Finally, programmed ventricular stimulation was delivered with up to double extra-stimuli at two different basic CLs from at least two sites, down to 200ms or until the refractory period.