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Research Papers

Two-Dimensional Simulation of Flow and Platelet Dynamics in the Hinge Region of a Mechanical Heart Valve

[+] Author and Article Information
V. Govindarajan

Department of Biomedical Engineering, College of Engineering, The University of lowa, 1402 SC, lowa City, IA 52242

H. S. Udaykumar

Department of Mechanical and Industrial Engineering, The University of lowa, lowa City, IA 52242; IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, IA 52242

K. B. Chandran1

Department of Biomedical Engineering, College of Engineering, The University of lowa, 1402 SC, lowa City, IA 52242; IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, IA 52242chandran@engineering.uiowa.edu

1

Corresponding author.

J Biomech Eng 131(3), 031002 (Dec 31, 2008) (12 pages) doi:10.1115/1.3005158 History: Revised August 14, 2008; Received October 22, 2008; Published December 31, 2008

The hinge region of a mechanical bileaflet valve is implicated in blood damage and initiation of thrombus formation. Detailed fluid dynamic analysis in the complex geometry of the hinge region during the closing phase of the bileaflet valve is the focus of this study to understand the effect of fluid-induced stresses on the activation of platelets. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a two-dimensional geometry of the hinge region of a bileaflet mechanical valve. Use of local mesh refinement algorithm provides mesh adaptation based on the gradients of flow in the constricted geometry of the hinge. Leaflet motion is specified from the fluid-structure interaction analysis of the leaflet dynamics during the closing phase from a previous study, which focused on the fluid mechanics at the gap between the leaflet edges and the valve housing. A Lagrangian particle tracking method is used to model and track the platelets and to compute the magnitude of the shear stress on the platelets as they pass through the hinge region. Results show that there is a boundary layer separation in the gaps between the leaflet ear and the constricted hinge geometry. Separated shear layers roll up into vortical structures that lead to high residence times combined with exposure to high-shear stresses for particles in the hinge region. Particles are preferentially entrained into this recirculation zone, presenting the possibility of platelet activation, aggregation, and initiation of thrombi.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 2

(a) Schematic of the hinge geometry used in the analysis with dimensions and the leaflet ear position at 32deg and the fully closed position at 63.8deg. Also indicated are the applied boundary conditions. (b) Angular velocity of the leaflet starting at 32deg moving toward closure. (c) Time varying pressure drop at the vicinity of the hinge region specified as pressure boundary condition. (d) Plot of the Cartesian grid with local mesh refinement on the hinge geometry. The mesh is adapted according to the gradients of flow for efficiency and accuracy. (e) Comparison of velocity profiles at an instant in the lower gap width with the base mesh and the finer mesh density.

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Figure 3

(a) Velocity, (b) shear stress, (c) vorticity contour plots in the hinge region based on the static analysis with the leaflet ear in the fully closed position, and (d) platelet activation parameter calculated during static analysis. Jetlike flow and high bulk shear stresses through the gap width, as well as the development of strong vortices that are advected away from the leaflet edge, can be observed.

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Figure 5

The velocity, fluid shear stress (FSS), and platelet concentration distribution at cross sections 1–3 within the gap width with the rectangular geometry of the leaflet ear. The horizontal axis indicates the distance from the valve housing (lower) edge to the leaflet ear (top) edge in reference to the figure within the gap width (GW) in the schematic of the geometry (top panel).

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Figure 6

Flow field with contours of existing geometry and the ellipsoidal geometry: (a) velocity contour showing the leakage jet from the hinge region, (b) shear stress contour during the late stage of valve closure where the shear stress reaches the maximum value, (c) particle concentration, and (d) activation parameter at the time of closure.

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Figure 7

Activation parameter during various stages of valve closure of the two geometries. The thick line shows the activation parameter of the existing geometry and the dashed line shows that of the ellipsoidal geometry.

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Figure 8

The product of activation parameter and the concentration of platelets at the end stage of closure for (a) existing geometry and (b) elliptical geometry. It can be inferred that the number of platelets getting trapped in the region of high activation is less in the elliptical geometry than in the existing geometry.

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Figure 1

(a) Photograph of a bileaflet valve. (b) Three-dimensional model of 1∕4 of a bileaflet valve with a cutting plane from which the two-dimensional model was extracted. (c) Zoomed in view of the hinge region. (d) Resultant two-dimensional model that was used in this study.

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Figure 4

Plots of (a) velocity (m/s), (b) fluid shear stress (Pa), (c) concentration, and (d) simulated platelet activation (Pa s) at various stages of the closing phase in the dynamic analysis of flow in the hinge region

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