The Heterogeneous Nature of Atrioventricular Conduction Tissues in Tetralogy of Fallot demonstrated by Hierarchical Phase-contrast Tomography – redefining the anatomic substrate

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Abstract Objectives Postoperative arrhythmias are frequent after Tetralogy of Fallot (ToF) repair, yet anatomic substrate and preventive strategies remain poorly defined. Using hierarchical phase-contrast tomography (HiP-CT) the atrioventricular conduction system in pediatric ToF specimens was investigated non-destructively and in 3D. Methods Eighteen whole-heart specimens (11 ToF, 7 controls) were imaged at the European Synchrotron, ESRF. Segmentation and 3D renderings demonstrated gross morphology. Morphology, size, depth and course of the non-branching bundle and right bundle branch (RBB) were quantified using custom computational pipelines. Segmentations were visualized in VheaRts, a Unity3D-based XR platform. Results The ToF conduction system was more draped than in controls, bilaterally spanning the septum, resembling early embryonic architecture. The RBB was variable in origin, course and morphologic structure with significantly smaller indexed cross-sectional area in native ToF versus controls (0.05 ± 0.20 vs 0.50 ± 0.20 mm², p = 0.005). One anomalous fasciculo-ventricular connection and six dead-end tracts were found. Regions at surgical risk included the posterior-inferior margin of the ventricular septal defect (VSD) and the septal crest along its nadir. The superior margin of the VSD at its intersection with the aortic root was free of conduction tissue. The HiP-CT to VR pipeline enabled interactive 3D visualization of conduction pathways relative to key structures. Conclusions This first pediatric cardiac HiP-CT series reveals a broader anomalous conduction complex in ToF, including variable RBB origin and hypoplasia, providing insight into preoperative vulnerability, arrhythmia, and surgical risk. Abstract Figure Figure 7: Graphical Abstract

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