Roof-dependent Atrial Flutter Changed into Single-loop Biatrial Flutter During Ablation.

A single-loop biatrial flutter is an uncommon form of atypical atrial flutter, and it can occur with septal or anterior line ablation in the left atrium (LA). We report a case with a roof-dependent atrial flutter that changed into a single-loop biatrial flutter during roof-line ablation. The activation entered the right atrium (RA) at the septum/fossa ovalis and coronary sinus ostium, exited the RA likely via the Bachmann's bundle and/or septopulmonary bundle, and entered the LA posterior to the roof line. The biatrial flutter was terminated with linear ablation between the right and left inferior pulmonary veins. RA mapping and biatrial flutter should be considered if roof-dependent atrial flutter slowed down during the roof-line ablation without termination.