A Case of Presumptive Marshall Bundle Conduction Between the Left Atrium and Left Superior Pulmonary Vein During Box Isolation

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A 65-year-old man with symptomatic persistent atrial fibrillation was referred for initial radiofrequency catheter ablation. He underwent box isolation to simultaneously isolate all four pulmonary veins (PVs) and the posterior wall of the left atrium (LA). During the box isolation with a double lasso catheter positioned in both the left superior and right superior PVs, the atrial fibrillation terminated spontaneously. After the box lesion set was created, conduction was still observed between the LA and PVs. After an additional radiofrequency energy delivery to the bottom line of the box lesion, the activation sequence of the PVs became uniform during sinus rhythm and pacing from the coronary sinus (CS), indicating that only one more conduction pathway was present. The 3-dimensional map revealed that the earliest activation site within the box lesion was located in the anterior aspect of the left superior PV during pacing from the CS, whereas that of the LA during pacing from the superior right PV within the box was on the posterolateral LA wall below the left inferior When a 2 Fr electrode catheter was inserted into the vein of Marshall (VOM) and paced within the BOX, VOM potentials preceded CS potentials. When the left superior PV anterior wall, the earliest site of excitation within the box, was energized, simultaneous posterior pulmonary vein wall isolation was completed. We experienced a case in which Marshall bundle conduction between the left atrium and the pulmonary vein was assumed.