Unraveling the enigmatic morphology of the atrioventricular conduction axis using hierarchical phase-contrast tomography

BACKGROUND: The 3-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 nondestructive, high-resolution 3D virtual histology. OBJECTIVES: We aimed to characterize the 3D morphology of the adult human AV conduction axis and its relationship to key anatomic landmarks using HiP-CT. METHODS: 5 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-micron 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, nonbranching bundle, and branching components. Nonbranching bundle length ranged from 2.49 to 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 4 hearts. Orthogonal sections showed a transition from interwoven cardiomyocytes in the node to parallel alignment in the insulated nonbranching bundle. A line from the anteroseptal commissure of the tricuspid valve to the medial papillary muscle reliably located the nonbranching bundle. CONCLUSION: HiP-CT provides high-resolution 3D visualization 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.