Patient-Specific Multi-Scale Model Analysis of Hemodynamics Following the Hybrid Norwood Procedure for Hypoplastic Left Heart Syndrome: Effects of Reverse Blalock-Taussig Shunt Diameter.

INTRODUCTION: The hybrid Norwood (HN) is a relatively new first stage palliative procedure for neonates with hypoplastic left heart syndrome, in which a sustainable uni-ventricular circulation is established in a less invasive manner than with the standard Norwood procedure. A computational multiscale model of the circulation following the HN procedure was used to obtain detailed hemodynamics. Implementation of a reverse-BT shunt (RBTS), a synthetic bypass from the main pulmonary to the innominate artery placed to counteract aortic arch stenosis, and its effects on local and global hemodynamics were studied.

METHODS: A post-op patient-derived anatomy of the HN procedure was utilized with varying degrees of distal arch obstruction, or stenosis, (nominal and 90% lumenal area reduction) and varying RBTS diameters (3.0, 3.5, 4.0 mm). A closed lumped parameter model (LPM) for the proximal and peripheral circulations was coupled to a 3D computational fluid dynamics (CFD) model in order to obtain converged flow fields for analysis.

RESULTS: CFD analyses of patient-derived anatomic configurations demonstrated consistent trends of vascular bed perfusion, vorticity, oscillatory shear index and wall shear stress levels. In the models with severe stenosis, implementation of the RBTS resulted in a restoration of arterial perfusion to near-nominal levels regardless of the shunt diameter. Shunt flow velocity, vorticity, and overall wall shear stress levels decreased with increasing shunt diameter, while shunt flow and systemic oxygen delivery increased with increased shunt diameter. In the absence of distal arch stenosis, large (4.0 mm) grafts may risk thrombosis due to low velocities and flow patterns.

CONCLUSION: Among the three graft sizes, the best option seems to be the 3.5 mm RBTS which provides a more organized flow similar to that of the 3.0 mm configuration with lower levels of wall shear stress. As such, in the setting of this study and for comparable HN physiologies our results suggest that: (1) the 4.0 mm shunt is a generous shunt diameter choice that may be problematic particularly when implemented prophylactically in the absence of stenosis, and (2) the 3.5 mm shunt may be a more suitable alternative since it exhibits more favorable hemodynamics at lower levels of wall shear stress.