Roof-dependent Macro-reentrant Left Atrial Flutter Masked by Probable Epicardial Conduction via the Septopulmonary Bundle.

We encountered a 77-year-old woman who had a roof-dependent macro-reentrant left atrial (LA) flutter (LAFL), masked by probable epicardial conduction via the septopulmonary bundle (SPB). The SPB, an epicardial connection in the LA, may contribute to the maintenance and complexity of LAFL. Thus, when evaluating LAFL, physicians should consider the potential involvement of the SPB. Detailed observation using a three-dimensional mapping and comprehensive electrophysiological pacing maneuvers is crucial for identifying the re-entry circuit of LAFL.