Editor's Choice - Fluid-Structure Interaction Simulations of Aortic Dissection with Bench Validation.

INTRODUCTION: The blood flow and stresses in the flap in aortic dissections are not well understood. Validated fluid-structure interaction (FSI) simulations of the interactions between the blood flow and the flap will provide insight into the dynamics of aortic dissections and may lead to developments of novel therapeutic approaches. METHODS: A coupled, two-way blood flow and flap wall computational model was developed. The Arbitrary Lagrange-Eulerian method was used, which allowed the fluid mesh to deform. Inflow velocity waveforms from a pulse duplicator system were used in the simulations. RESULTS: The velocities for true lumen (TL) and false lumen (FL) were not significantly different between bench and simulation. The dynamics of the TL % cross-sectional area (CSA) during the cycle was similar between the bench and computational simulations, with the TL %CSA being most reduced near peak systole of the cycle. The experimental distal measurements had significantly lower velocities, likely due to the spatially heterogeneous flow distally. The conservation of mass and validity of simulations were confirmed. Additionally, regions of stress concentrations were found on the flap leading edge, towards the corners, and through the entire vessel wall. The pressure gradient across the FL results in a net force on the flap. CONCLUSION: The FSI flow velocities in the TL and the FL as well as the dynamics of the CSA during the cardiac cycle were validated by bench experiments. The validated FSI model may provide insights into aortic dissection including the stresses on the dissection flap and related flow disturbance, which may be subdued by novel therapeutic approaches. Simulations of more realistic human aortic dissections and the effects of current therapeutic approaches such as stent-graft can be developed in the future using the validated computational platform provided in the present study.

Characterization of a bioprosthetic bicuspid venous valve hemodynamics: implications for mechanism of valve dynamics.

BACKGROUND: Chronic venous insufficiency (CVI) of the lower extremities is a common clinical problem. Although bioprosthetic valves have been proposed to treat severe reflux, clinical success has been limited due to thrombosis and neointima overgrowth of the leaflets that is, in part, related to the hemodynamics of the valve. A bioprosthetic valve that mimics native valve hemodynamics is essential. METHODS: A computational model of the prosthetic valve based on realistic geometry and mechanical properties was developed to simulate the interaction of valve structure (fluid-structure interaction, FSI) with the surrounding flow. The simulation results were validated by experiments of a bioprosthetic bicuspid venous valve using particle image velocimetry (PIV) with high spatial and temporal resolution in a pulse duplicator (PD). RESULTS: Flow velocity fields surrounding the valve leaflets were calculated from PIV measurements and comparisons to the FSI simulation results were made. Both the spatial and temporal results of the simulations and experiments were in agreement. The FSI prediction of the transition point from equilibrium phase to valve-closing phase had a 7% delay compared to the PD measurements, while the PIV measurements matched the PD exactly. FSI predictions of reversed flow were within 10% compared to PD measurements. Stagnation or stasis regions were observed in both simulations and experiments. The pressure differential across the valve and associated forces on the leaflets from simulations showed the valve mechanism to be pressure driven. CONCLUSIONS: The flow velocity simulations were highly consistent with the experimental results. The FSI simulation and force analysis showed that the valve closure mechanism is pressure driven under the test conditions. FSI simulation and PIV measurements demonstrated that the flow behind the leaflet was mostly stagnant and a potential source for thrombosis. The validated FSI simulations should enable future valve design optimizations that are needed for improved clinical outcome.

Role of sinus in prosthetic venous valve.

BACKGROUND: The majority of bioprosthetic venous valves do not have a sinus pocket and, in practice, they are often placed in non-sinus segments of the veins. The aim of this study is to investigate the effect of the sinus pocket on the flow dynamics in a prosthetic valve. METHODS: A bench top in vitro experiment was set up at physiological flow conditions to simulate the flow inside a venous system. Bicuspid bioprosthetic valves with different leaflet lengths (5 and 10 mm) were tested in tubes with and without a sinus pocket and the flows around the valve were visualized by particle image velocimetry (PIV). Velocity data measurements were made and the vorticity was calculated in the with- and without-sinus set-ups. RESULTS: PIV measurements showed that vortex structure was maintained by the sinus. For the 10-mm leaflet length design with sinus, the jet width at the exit of the valve was 59% of that without sinus. For the 5-mm design with sinus, the jet width was 73% of the valve without sinus. Flow from the sinus region was entrained into the main jet observed near the exit of the sinus and altered the flow at the near wall region. CONCLUSIONS: The sinus pocket alters the flow around the valve and functions as flow regulator to smooth the flow pattern around the valve. The vortical structure inside the sinus is maintained at the valve leaflet tip during the valve cycle. For the prosthetic valve designated to be placed without a sinus, a shorter leaflet length is preferable and performs more closely to the valve with sinus.