Does the AVNeo valve reduce wall stress on the aortic wall? A cardiac magnetic resonance analysis with 4D-flow for the evaluation of aortic valve replacement with the Ozaki technique.

OBJECTIVES: Aortic valve neocuspidalization aims to replace the 3 aortic cusps with autologous pericardium pre-treated with glutaraldehyde, and it is a surgical alternative to the classical aortic valve replacement (AVR). Image-based patient-specific computational fluid dynamics allows the derivation of shear stress on the aortic wall [wall shear stress (WSS)]. Previous studies support a potential link between increased WSS and histological alterations of the aortic wall. The aim of this study is to compare the WSS of the ascending aorta in patients undergoing aortic valve neocuspidalization versus AVR with biological prostheses. METHODS: This is a prospective nonrandomized clinical trial. Each patient underwent a 4D-flow cardiac magnetic resonance scan after surgery, which informed patient-specific computational fluid dynamics models to evaluate WSS at the ascending aortic wall. The adjusted variables were calculated by summing the residuals obtained from a multivariate linear model (with ejection fraction and left ventricle outflow tract-aorta angle as covariates) to the mean of the variables. RESULTS: Ten patients treated with aortic valve neocuspidalization were enrolled and compared with 10 AVR patients. The aortic valve neocuspidalization group showed a significantly lower WSS in the outer curvature segments of the proximal and distal ascending aorta as compared to AVR patients (P = 0.0179 and 0.0412, respectively). WSS levels remained significantly lower along the outer curvature of the proximal aorta in the aortic valve neocuspidalization population, even after adjusting the WSS for the ejection fraction and the left ventricle outflow tract-aorta angle [2.44 Pa (2.17-3.01) vs 1.94 Pa (1.72-2.01), P = 0.02]. CONCLUSIONS: Aortic valve neocuspidalization hemodynamical features are potentially associated with a lower WSS in the ascending aorta as compared to commercially available bioprosthetic valves.

Analysis of Turbulence Effects in a Patient-Specific Aorta with Aortic Valve Stenosis.

Blood flow in the aorta is often assumed laminar, however aortic valve pathologies may induce transition to turbulence and our understanding of turbulence effects is incomplete. The aim of the study was to provide a detailed analysis of turbulence effects in aortic valve stenosis (AVS). METHODS: Large-eddy simulation (LES) of flow through a patient-specific aorta with AVS was conducted. Magnetic resonance imaging (MRI) was performed and used for geometric reconstruction and patient-specific boundary conditions. Computed velocity field was compared with 4D flow MRI to check qualitative and quantitative consistency. The effect of turbulence was evaluated in terms of fluctuating kinetic energy, turbulence-related wall shear stress (WSS) and energy loss. RESULTS: Our analysis suggested that turbulence was induced by a combination of a high velocity jet impinging on the arterial wall and a dilated ascending aorta which provided sufficient space for turbulence to develop. Turbulent WSS contributed to 40% of the total WSS in the ascending aorta and 38% in the entire aorta. Viscous and turbulent irreversible energy losses accounted for 3.9 and 2.7% of the total stroke work, respectively. CONCLUSIONS: This study demonstrates the importance of turbulence in assessing aortic haemodynamics in a patient with AVS. Neglecting the turbulent contribution to WSS could potentially result in a significant underestimation of the total WSS. Further work is warranted to extend the analysis to more AVS cases and patients with other aortic valve diseases.

The effect of turbulence on transitional flow in the FDA's benchmark nozzle model using large-eddy simulation.

The Food and Drug Administration's (FDA) benchmark nozzle model has been studied extensively both experimentally and computationally. Although considerable efforts have been made on validations of a variety of numerical models against available experimental data, the transitional flow cases are still not fully resolved, especially with regards to detailed comparison of predicted turbulence quantities with experimental measurements. This study aims to fill this gap by conducting large-eddy simulations (LES) of flow through the FDA's benchmark model, at a transitional Reynolds number of 2000. Numerical results are compared to previous interlaboratory experimental results, with an emphasis on turbulence characteristics. Our results show that the LES methodology can accurately capture laminar quantities throughout the model. In the pre-jet breakdown region, predicted turbulence quantities are generally larger than high resolution experimental data acquired with laser Doppler velocimetry. In the jet breakdown regions, where maximum Reynolds stresses occur, Reynolds shear stresses show excellent agreement. Differences of up to 4% and 20% are observed near the jet core in the axial and radial normal Reynolds stresses, respectively. Comparisons between viscous and Reynolds shear stresses show that peak viscous shear stresses occur in the nozzle throat reaching a value of 18 Pa in the boundary layer, whilst peak Reynolds shear stresses occur in the jet breakdown region reaching a maximum value of 87 Pa. Our results highlight the importance in considering both laminar and turbulent contributions towards shear stresses and that neglecting the turbulence effect can significantly underestimate the total shear force exerted on the fluid.