Catheter ablation of atrial fibrillation may predispose patients to the development of atypical atrial flutters (AFL). We describe two cases of roof dependent AFLs that failed to terminate despite posterior wall isolation. An epicardial breakthrough involving the septopulmonary bundle is proposed. The correlation between the electrophysiological findings and the anatomical substrate is described. K E Y W O R D S atrial fibrillation, atypical flutter, catheter ablation, septopulmonary bundle 1 | INTRODUCTION Catheter or surgical ablation of atrial fibrillation (AF) may predispose patients to the development of atypical macroreentrant atrial flutters (AFLs). When macroreentry is suspected, activation mapping and entrainment maneuvers are helpful to delineate the arrhythmia circuit and the standard ablation strategyinvolves disruption of the critical isthmus by creating lines of block between two anatomical barriers. Sometimes a tachycardia fails to terminate despite an apparently correct diagnosis and a careful ablation strategy. In those cases, we must reconsider the accuracy of our initial diagnosis and rule out transition to a different circuit, but also return to the anatomy and consider the existence of anatomically distinct and separate epicardial muscle bundles, which may have a role in atrial reentrant arrhythmias.We describe two cases of roof-dependent AFLs that failed to terminate intraprocedurally despite endocardial posterior wall (PW) isolation in one and which appeared after prior PW isolation in the second. An epicardial breakthrough involving the septopulmonary bundle (SPB) was suspected. Activation/entrainment maps and the ablation approach performed in each case are presented. Additionally, the correlation between the electrophysiological (EP) findings and the anatomical substrate is described. | CASE DESCRIPTIONThe patients consented following institutional guidelines. The EP study (EPS) was conducted under general anesthesia. One decapolar catheter was positioned in the coronary sinus (CS) and another in the right atrium (RA). An 8-French phased-array intracardiac echocardiography catheter (Siemens, Mountain View, CA) was advanced into the right RA and maps were created with CARTO (Biosense Webster, Diamond Bar, CA).Access to the left atrium (LA) was achieved with double transseptal punctures with the aid of long sheaths. A high-definition multipolar Pentaray catheter (Biosense Webster) was used for mapping, and ablation was performed with a 3.5-mm-tip force sensing irrigated ablation catheter (SmartTouch DF, Biosense Webster). | CASE 1A 59-year-old male patient with a past medical history of hypertrophic cardiomyopathy presented with recurrent persistent AF despite two prior pulmonary vein (PV) isolation procedures and, more recently, ablation of a roof-dependent AFL. At the time of the repeat procedure, the patient was initially cardioverted to sinus rhythm and reconnection of both superior PVs was documented, as well as absence of block in the previous roof line. The PVs were isolated and pro...

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