Pseudo-focal AT analysis
A pseudo-focal AT was diagnosed in the mapped atrium when all the following criteria were met: (1) centrifugal propagation during high-density mapping; (2) a missing TCL of > 10%; (3) a breakthrough site > 10 mm remote from a collision site; (4) a post-pacing interval (PPI), after entrainment pacing with pacing cycle length of 10-20 ms shorter than TCL, ≤ 30 ms longer than TCL at ≥ 3 different sites of the considered atrium, including the breakthrough site and a diametrically opposite site. The latter point made the candidate AT compatible with a macroreentry, defined as a large circuit running along four walls of the mapped atrium. Hence, the three types of macroreentries included in this work were perimitral, roof-dependent, or CTI-dependent flutters. Of note, an estimated conduction velocity of the epicardial substrate was calculated by dividing the distance from the breakthrough site to the collision site by the missing TCL.
Several epicardial layers have been described in robust anatomical studies.3-7 On this basis, we inferred the bypass of pseudo-focal ATs as follows: (1) CS-GCV bundle if the breakthrough was close to the mitral annulus either at the lateral or posterior vestibule, with a PPI-TCL ≤ 30 ms from the GCV to the CS when entrainment pacing was performed; (2) vein of Marshall (VOM) bundle if the breakthrough was along the LA ridge, with a PPI-TCL ≤ 30 ms at the CS but > 30 ms at the GCV when entrainment pacing was performed; (3) Bachmann bundle if the breakthrough was at the LA anterior wall or at the anterior edge of the left atrial appendage, and associated with a collision site between the left superior pulmonary vein and the mitral annulus; (4) septopulmonary bundle if the breakthrough was at the LA anterior wall or at the dome between the bilateral pulmonary veins, and associated with a collision site at the roof; (5) fossa ovalis if the breakthrough was at the LA septum close to the transseptal puncture, with PPIs compatible with a perimitral flutter using the RA septum (i.e., bi-atrial flutter); and (6) low RA if the breakthrough was at the inferior portion of the RA lateral wall or septum, and associated with a collision site at the CTI.