The Septopulmonary Bundle Revisited as an Integrated 3-Dimensional Myocardial Region With Implications for Atrial Ablation.

BACKGROUND: The septopulmonary bundle has been classically described as a discrete muscular structure involved in left atrial activation. However, current anatomical evidence supports reinterpreting it not as an isolated tract, but as part of a complex 3-dimensional myocardial architecture. DISCUSSION: Rather than a true "bundle," it is better understood as a "septopulmonary region," reflecting a variable preferential alignment of working atrial cardiomyocytes within epicardial layers, with overlap and interdigitation with deeper subendocardial fibers of variable thickness. This organization underlies marked anisotropy and contributes to conduction heterogeneity across the posterior left atrium. High-resolution mapping demonstrates epicardial-endocardial dissociation and preferential conduction along longitudinal fiber orientation, supporting intramural or epicardial conduction pathways. These features may explain incomplete lesion sets during endocardial ablation and persistent conduction across the posterior wall. TAKE-HOME MESSAGE: The septopulmonary "bundle" is more accurately a complex atrial region whose 3-dimensional architecture critically determines conduction patterns and ablation outcome.