BACKGROUND: The three-dimensional (3D) architecture of the atrioventricular (AV) conduction axis and its relationship to surrounding cardiac structures remain incompletely understood. Standard histology provides cellular detail but struggles to demonstrate spatial arrangement and connectivity within the intact heart. Hierarchical Phase-Contrast Tomography (HiP-CT) overcomes this by enabling non-destructive, high-resolution 3D virtual histology. OBJECTIVES: We aimed to characterise the 3D morphology of the adult human AV conduction axis and its relationship to key anatomical landmarks using HiP-CT. METHODS: Five adult human hearts (mean donor age 81.6 years) underwent HiP-CT at the European Synchrotron Radiation Facility. Whole-heart scans were acquired at 20 microns isotropic resolution, with targeted imaging of the AV conduction axis at 6.51 microns. The axis was segmented in multiple orthogonal planes, reconstructed in 3D, and measured. RESULTS: HiP-CT distinguished the compact AV node, non-branching bundle, and branching components. Non-branching bundle length ranged from 2.49-8.80 mm (mean 6.31 mm). In all hearts, the axis branched first into a broad left bundle, followed by a cord-like right bundle branch. Additional superior septal pathways were identified in four hearts. Orthogonal sections showed a transition from interwoven cardiomyocytes in the node to parallel alignment in the insulated non-branching bundle. A line from the antero-septal commissure of the tricuspid valve to the medial papillary muscle reliably located the non-branching bundle. CONCLUSION: HiP-CT provides high-resolution 3D visualisation of the AV conduction axis in adult hearts, revealing its morphology and variability. These findings offer new insights into structure-function correlations relevant to clinical electrophysiology and interventional procedures.
Almehandi et al. (Wed,) studied this question.