Introduction
In patients undergoing cardiac surgery, atrial fibrillation (AF) can complicate the postoperative course, for instance by increasing the risk of strokes and inducing hemodynamic instability (1). Subsequently, surgical techniques of AF ablation have been developed. One of these techniques is the application of high-intensity focused ultrasound (HIFU) with the EpicorTM device (St Jude Medical, Maple Grove, MN, Figure 1A), which aims to create a box-lesion around the four pulmonary veins (PV). Although HIFU proved to be feasible, it is associated with a rather low rate of complete or near-complete box isolations resulting in successful pulmonary vein isolation (PVI) (2,3). Moreover, cardiac surgery induces myocardial fibrosis, which promotes scar-related atrial tachycardia (AT) (4). Appropriate treatment of these complex arrhythmias is possible by three-dimensional (3D) electroanatomic mapping (EAM) that helps to understand the underlying activation pattern and to identify critical targets for ablation. However, scar tissue can significantly reduce the accuracy of this method. For instance, scarred myocardial tissue can lead to a relevant delay between the onset of the local electrogram and the time to the peak amplitude, leading to inconclusive activation maps (5). Calculation of conduction velocity vectors, which can be visualized in so-called coherent maps, might help to correctly interpret the mechanisms of these arrhythmias and allow a targeted ablation.