Main Findings
Convertibility of the FCB regions occurred in 5 of 8 patients in our study, suggesting that it may not be an uncommon electrophysiological feature in ATs, especially scar-related ATs. This substrate species could be described as exhibiting DPs or fractionated and complex potentials and showed a conduction block through this area in one AT but not in another AT or in sinus rhythm. In 8 of 14 ATs, the FCB regions were the main obstacle and the center of the reentrant circuit (such as in Fig. 1 AT1, Fig. 2 AT2). Other FCB regions may participate in reorganizing the conduction pathway maintaining ATs (such as in Fig. 2 AT3, Fig. 3 AT2). A specific FCB region acts as an obstacle only for its corresponding AT and does not seem to play a substantial role in other ATs. In severe native scars or progressed scars related to myocardium disease, the electrophysiology coupled with the complex substrate has not been summarized systematically. The FCB region is occasionally described in atrial-related arrhythmia, and although it is thought to play an important role in some types of arrhythmia, there has been no method of analyzing this mechanism and definitively determining its role.4 Our study is the first to describe the convertibility of FCB regions and their important role in maintaining some stable ATs. The instability of the FCB region may be the reason for the high recurrence of related ATs, but this possibility has been shown in only one specific AT and is not easy to identify with conventional mapping procedures. The potential FCB region described in our study may account for another mechanism of multiple ATs.