Anatomical substrate: unmasked epicardial bundles
The multilayer architecture of the atria was first described by Papez in 1920.3 This key finding was subsequently endorsed by renowned anatomists who gave further insight into the complexity of the atrial musculature.4-7 The breakthrough sites observed in the present work were all circumscribed to areas harboring well-identified epicardial bundles or inter-atrial connections. This strongly suggests that pseudo-focal ATs are determined by native anatomical substrates. However, acquired scar substrates may also be crucial for the genesis of pseudo-focal ATs. Hence, we consistently found a low-voltage area constituting a linear obstacle close to the breakthrough site (Figures 1-6 ). This observation confirms previous work by Pathik et al.1 Whether due to the aging process or previous ablation, this remodeling is necessary to disrupt the conduction properties of one layer, and thus reveal its electrical dissociation from another layer. Such phenomenon was recently demonstrated with the septoatrial bundle and the septopulmonary bundle, which respectively form the endocardial and the epicardial layers of the dome. The activation of the dome was compared before and after creating a roof line that blocked the endocardial layer only. While initially concealed by their synchronous activation, the electrical dissociation of the two layers was finally unmasked by the sequential activation of the protected septopulmonary bundle first, followed by the septoatrial bundle downstream.10 Besides the septopulmonary bundle, other epicardial structures may act similarly, depending on scar location: the CS-GCV and VOM bundles at the mitral isthmus, the Bachmann bundle at the anterior wall, the fossa ovalis at the septum, and the low RA at the CTI.