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.