Epicardial Course of the Septopulmonary Bundle: Anatomical Considerations and Clinical Implications for Roof Line Completion

Background: Gaps in the roof line have been ascribed to epicardial conduction using the septopulmonary bundle.

Objectives: We sought to evaluate the frequency of septopulmonary bundle bypass during roof line ablation, to describe anatomical conditions favoring this epicardial gap, and to propose an alternative strategy when present.

Methods: One hundred consecutive patients underwent atrial fibrillation ablation. A de novo roof line was created between the superior pulmonary veins. In cases of residual gaps, a floor line was created between the inferior pulmonary veins. Microtomography imaging and histological analyses of 5 human donor hearts were performed: a specific focus was made on the dome and the posterior wall.

Results: Residual gaps were more frequent in roof lines than floor lines (33% vs 15%; P = .049). Electrogram morphologies, activation sequences, and pacing maneuvers indicated an epicardial bypass of the roof line in all cases. Conduction block was obtained in 67 roof lines and 28 floor lines, resulting in a 95% success rate of linear block, without "box" isolation. Between the superior pulmonary veins, the atrial myocardium was thicker and consistently displayed adipose tissue separating the septopulmonary bundle from the septoatrial bundle.

Conclusion: Epicardial conduction across the roof line is common and requires careful electrogram analysis to detect. In such cases, a floor line can be an effective alternative strategy, with clear validation criteria. Myocardial thickness and fat interposition may explain difficulties in achieving lesion transmurality during roof line ablation.