Conclusions
The potential FCB region is common in ATs with over two types of reentrant circuits, especially in scar-related localized reentry. The occurrence of this substrate may be due to the native scars or progressed scars of myocardium disease. This substrate species could exhibit DPs or fractionated and complex potentials and showed conduction block through this area in one AT but not in another AT or in sinus rhythm. The FCB region maintains reentrant circuits as an obstacle, and the changefulness of this substrate may be one of the important reasons for the high recurrence of related ATs.