A single-loop macroreentrant biatrial flutter identified by the Rhythmia mapping system.

We herein present a rare case of perimitral annulus (MA) counterclockwise single-loop macro-reentrant biatrial flutter utilizing Bachmann's bundle (BB), the atrial septum and the coronary sinus (CS) ostium as the critical components of the reentrant circuit, even though the left atrial anterior line was blocked. By acknowledging the interatrial conduction via the BB and the CS identified by the ultrahigh-resolution mapping result, we could understand the atrial flutter mechanism and successfully treat the patient.