Ablation strategy: targeting anatomical isthmus
The correct diagnosis of pseudo-focal ATs is critical because it
conditions the ablation strategy and success. If misdiagnosed as a focal
AT, the breakthrough site will be ablated in a macroreentrant AT with
centrifugal propagation. Since ”epi-endo” connections were usually
broad, we observed a poor termination rate (17%) with a progressive
shift of the endocardial emergence as the patchy lesion expands.
From this respect, entrainment pacing maneuvers shift the focus from the
breakthrough to the circuit itself and thus clarify the ablation
strategy. Instead of a tailored but challenging ablation of the
endocardial emergence, a systematic block at a predefined anatomical
isthmus seems more sensible. Moreover, a line of block adds the
advantage of a clear endpoint. Two key factors may determine the best
candidate isthmus: (1) the ability to transect an epicardial bypass by
direct elimination; or (2) the ability to avoid an epicardial bypass by
upstream or downstream ablation. For perimitral flutters, the CS-GCV and
VOM bundles are now readily eliminated by endovascular ablation and
retrograde ethanol infusion, making the mitral isthmus an optimal target
for the linear block.12,13 For roof-dependent
flutters, the septopulmonary bundle obstacle can often be avoided at the
lower portion of the dome, making the floor line an alternative target
when the roof line is not blocked.10 For CTI-dependent
flutters, ablation inside the CS drainage system has been described in
cases of low RA bridges, but creating a line at a more lateral or septal
portion of the CTI proved efficient in the majority of cases in our
experience. However, these anatomical isthmuses may not always be
transected with conventional ablation.14 Nayak et al.
recently showed that epicardial access could sometimes be
necessary.2 This approach requires specific training
and a dedicated environment with surgical backup.