Discussion
Catheter-based PVI has emerged as a cornerstone in AF therapy and improves clinical outcomes compared to medical treatment in selected patients (6). However, roughly 50% of all patients experience recurrence of AF within 2 years (7). Thus, new techniques of AF ablation have been investigated in the past, such as the minimal-invasive epicardial HIFU electroablation treatment. First studies reported rather high efficacy rates, with 83-85% freedom from atrial arrhythmias 6 months after treatment (8,9). In contrast, the recurrence rate of atrial arrhythmias was found to be much higher in more recent studies (2,3,10). Furthermore, rate of complete or near-complete box isolation resulting in successful PVI 6 months after HIFU AF ablation was less than 40% (2). Accordingly, in this case, multiple gaps have been found along the EpicorTM ablation line, which resulted in the occurrence of a focal AT and persistent electrical connection of the PV. Activation mapping of these complex scar-related AT can be challenging because of difficulties in the accurate annotation of multicomponent electrograms, the correct differentiation between active and passive diastolic activity, and the dependency on a window of interest with arbitrarily defined early and late activation (11). Consequently, a new activation mapping algorithm has been developed that identifies and highlights zones of slow or no conduction, and calculates the most coherent pattern of activation (4). In the present case, the application of this new technical feature helped to interpret the mechanism of the AT and to identify the critical site for ablation.