Fehmi Keçe

Chapter 8 174 As ablation lines have to be connected to anatomical barriers to prevent scar-related reentry, the roof line of a box lesion is connected to the superior ostia of the superior veins and the posterior line of a box lesion is connected to the inferior ostia of the inferior PV’s. However, the anatomical posterior LA is not limited to the area between the veins but extends more caudally towards the coronary sinus (14, 26). In the current study, patients with a small box surface ratio had a decreased arrhythmia-free survival compared to patients with a large box surface ratio, while box lesion width, height and surface area, total LA surface area and LA volume were not predictive. A possible explanation is the extent of isolation of the posterior wall which shares the same embryologic origin with that of the pulmonary veins, containing substrate for AF maintenance. Although the ratio of the isolated box lesion surface area and the total LA surface was calculated, box lesion surface area as a ratio of total left atrial posterior wall surface area could be superior to sustain our hypothesis. However, as the borders of the posterior wall of the LA are not well defined in the literature we did not adopt this parameter. A second explanation of our findings may be that an increase in left atrial size outside the area between the pulmonary veins will also decrease the box surface ratio. It may be hypothesized that enlargement of the left atrium will be more distinct outside the box lesion while the box lesion itself may be more resistant to dilation, as this area is bounded by the pulmonary veins. Therefore, the combination of anatomical variation and left atrial dilation outside the box lesion may explain why box surface ratio was predictive of outcome while box lesion length, width and surface area were not. It remains to be proven that the positive influence of a large box lesion is dependent on substrate modification of the LA posterior wall and not on extensive atrial debulking per se. Pre-procedural visualization of a small posterior LA box as a ratio of left atrial surface could be an important factor to predict failure in patients in whom a box lesion is considered. 8.4.3 Clinical implications The box lesion surface area and total left atrial surface area can be measured during the procedure irrespective from prior imaging. In concordance with the fact that the AF substrate in the LA posterior wall is not confined to the area between the PV’s, it may be hypothesized that ablation of a relatively larger box lesion is beneficial. This may support a decision to increase the size of the box lesion, e.g. extending it inferiorly below the level of the PV’s towards the coronary sinus, especially in patients with a relatively small anatomical box lesion. This hypothesis needs to be proven in further studies, however. Concordantly, Di Biase et al. described PVI together with an extensive box lesion extended down to the coronary sinus and to the left sided atrial septum in patients with persistent