Definitions of a Reentrant Circuit and Classification of AT
ATs are classified according to the activation pattern: focal AT, anatomical macroreentrant AT (AMAT), and non-AMAT (localized reentry). As previously reported,1 AMATs include peritricuspid flutter (PTF) dependent on the cavotricuspid isthmus (CTI), perimitral flutter (PMF) dependent on the mitral isthmus, and roof-dependent macroreentrant tachycardia (RMAT) dependent on the left atrial roof. Localized scar-related reentry was defined by atrial activation that was recorded over the entire AT CL of a localized area. The localized reentry circuit was variable in size and was composed of a scar with a conduction block and a channel of slow conduction consistent with an ”isthmus.” Focal AT was defined as atrial activity originating from a discrete site activating the surrounding tissue centrifugally (not necessarily recorded over the entire AT CL). Multiple-loop ATs were defined as those composed of more than one independent active circuit sharing one common isthmus, including dual-loop ATs and triple-loop ATs, with the entire CL included in each of the circuits.5The epicardial conduction of a CS/vein of Marshall (VOM) system-related AT was suspected if a PMF used only the CS as an epicardial conduction route and the activation map demonstrated a continuous jump over a line of complete block on the mitral isthmus between the distal CS and the left lateral LA and the postpacing interval minus tachycardia cycle length (PPI-TCL)<20 ms was confirmed at the circuit. When the PMF used the VOM (+/- CS), the activation map showed a jump between the inferolateral LA (overlying the CS) and/or the mid-to-distal CS region and the left lateral ridge or posterior base of the left atrial appendage (LAA), with >10% of the TCL missing. Entrainment mapping showed a PPI-TCL ≤ 20 ms at the VOM, the ridge left pulmonary vein (LPV)-LAA, the anterior wall of the LA, and 6 to 11 o’clock on the mitral annulus (MA) and a PPI-TCL >20 ms at the distal CS, the posterior wall of the LA and the mitral isthmus. Non-AMAT (localized reentry) around the LAA-LPVs using the CS/VOM system demonstrated >10% TCL missing around the CS/VOM distribution. A PPI-TCL<20 ms was observed at the left lateral ridge, posterior base of the LAA, inferolateral LA or the mid-to-distal CS (ostium of VOM), and a PPI-TCL >20 ms was observed in the septal MA.6,7