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.