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.