Two-dimensional FSI simulation of closing dynamics of a tilting disc mechanical heart valve

Abstract

The fluid dynamics during valve closure resulting in high shear flows and large residence times of particles has been implicated in platelet activation and thrombus formation in mechanical heart valves. Our previous studies with bi-leaflet valves have shown that large shear stresses induced in the gap between the leaflet edge and the valve housing results in relatively high platelet activation levels whereas flow between the leaflets results in shed vortices not conducive to platelet damage. In this study we compare the result of closing dynamics of a tilting disc valve with that of a bi-leaflet valve. The two-dimensional fluid-structure interaction analysis of a tilting disc valve closure mechanics is performed with a fixed grid Cartesian mesh flow solver with local mesh refinement, and a Lagrangian particle dynamic analysis for computation of potential for platelet activation. Throughout the simulation the flow remains in the laminar regime and the flow through the gap width is marked by the development of a shear layer which separates from the leaflet downstream of the valve. Zones of re-circulation are observed in the gap between the leaflet edge and the valve housing on the major orifice region of the tilting disc valve and are seen to be migrating towards the minor orifice region. Jet flow is observed at the minor orifice region and a vortex is formed which sheds in the direction of fluid motion as observed in experiments using PIV measurements. The activation parameter computed for the tilting disc valve, at the time of closure was found to be 2.7 times greater than that of the bi-leaflet mechanical valve and was found to be in the vicinity of the minor orifice region mainly due to the migration of vortical structures from the major to the minor orifice region during the leaflet rebound of the closing phase.

Figure 1

(a): Schematic depiction of a 27 mm BSM valve in the closed position, (b): Leaflet in the fully open position with disc at 20° to the vertical rotates 70° to reach the fully closed position; (c): Schematic of the 2-D computational domain with the leaflet shown in the fully open and closed position with the disc pivoted at the position of the strut; and (d) Applied inlet ventricular pressure boundary condition. The data points in the plot refer to the time during the leaflet closure phase at which the computational results are presented in Figure 5, Figure 7, and Figure 8.

Figure 2

a) Particle distribution in the flow regime at the beginning of the computational cycle with high particle concentration at the inlet (red) and no particles present in the rest of the flow regime; b) Example of a single particle being traced over time with the green dot indicating the starting point and the red dot the end point of the particle path during the closing phase. The activation parameter is calculated cumulatively for each particle within the flow domain.

Figure 3

Comparison of angular position of the leaflets during the closing phase and the first rebound from the experimental study with the simulation results.

Figure 4

Comparison of Leaflet closing dynamics for the tilting disc and the bi-leaflet valves: (a) Angular position of the leaflet closure with time during the closing and rebound phases; (b) Leaflet angular velocity; and (c) Leaflet tip velocity.

Figure 5

Comparison of vorticity plots between the tilting disc valve and the bi-leaflet valve during the closing phase. Top panel (a, b, and c): plots for the tilting disc valve; and Bottom panel (d, e, and f): Plots for the bi-leaflet valve.

Figure 6

Plots of the shear stress, platelet concentration, and platelet activation parameter for the tilting disc valve (top panel: a, b, and c) and the bi-leaflet valve (bottom panel: d, e, and f) at the instant of closure.

Figure 7

Plots of the vorticity (left column) , shear stress (middle column), and the calculated platelet activation contours( right column): (a) First rebound phase; and (b) closure following the first rebound for the tilting disc valve. (c) First rebound phase; and (d) closure following the first rebound for the bi-leaflet valve.

Figure 8

Plots of vorticity (left column) , shear stress (middle column), and the calculated platelet activation contours (right column) : (a) second rebound phase; and (b) closure following the second rebound for the tilting disc valve. (c) during the final closure of the bi-leaflet valve.