Impact of design parameters on bileaflet mechanical heart valve flow dynamics

Abstract

Background and aim of the study: One significant problem encountered during surgery to implant mechanical heart valve prostheses is the propensity for thrombus formation near the valve leaflet and housing. This may be caused by the high shear stresses present in the leakage jet flows through small gaps between leaflets and the valve housing during the valve closure phase.

Methods: A two-dimensional (2D) study was undertaken to demonstrate that design changes in bileaflet mechanical valves result in notable changes in the flow-induced stresses and prediction of platelet activation. A Cartesian grid technique was used for the 2D simulation of blood flow through two models of bileaflet mechanical valves, and their flow patterns, closure characteristics and platelet activation potential were compared. A local mesh refinement algorithm allowed efficient and fast flow computations with mesh adaptation based on the gradients of the flow field in the gap between the leaflet and housing at the instant of valve closure. Leaflet motion was calculated dynamically, based on the fluid forces acting on it. Platelets were modeled and tracked as point particles by a Lagrangian particle tracking method which incorporated the hemodynamic forces on the particles.

Results: A comparison of results showed that the velocity, wall shear stress and simulated platelet activation parameter were lower in the valve model, with a smaller angle of leaflet traverse between the fully open to the fully closed position. The parameters were also affected to a lesser extent by local changes in the leaflet and housing geometry.

Conclusion: Computational simulations can be used to examine local design changes to help minimize the fluid-induced stresses that may play a key role in thrombus initiation with the implanted mechanical valves.

Figure 1

Comparison of valve geometry for: (a) Valve-1 and (b) Valve-2; (c) and (d): The leaflet edge geometry of valve-1 is sharper than that of Valve-2 shown in (d). The angle made by the valve leaflets in fully open and closed positions is also indicated in the figure. The leaflet dimensions differ slightly but not significantly from each other.

Figure 2

(a) Schematic of experimental set-up. The rectangle marked out in the figure indicates the regions where the LDV measurements are made. (b) Photograph of experimental set-up. (c) Flow visualization with a light sheet illuminating the centerline of the valve indicating the recirculation regions near the leaflets during the valve closing phase. (d) Numerical simulation results show qualitatively similar re-circulation regions.

Figure 3

Leaflet closure characteristics of Valve-1 and Valve-2: (a) angle made by leaflet with vertical axis as a function of time; (b) leaflet angular velocity as a function of time; and (c) leaflet tip velocity as a function of time.

Figure 4

Comparison of the simulation results during the closure stage for Valve-1 and Valve-2. Note that the intensity of vortices is much lower for the first valve. The valve closure angle and the time during the leaflet motion for (a), (b), and (c) are indicated in the legend.

Figure 5

Comparison of: (a) the minimum pressure; and (b) the maximum shear stress in the leaflet-housing gap region for the two valves. (c) Comparison of the computed product of platelet activation parameter and concentration at the instant of closure for the two valves. In this plot, regions with bright red indicates higher potential for platelets to be activated and dark blue represents minimal potential for the same. Larger regions of bright red for Valve-2 indicate higher potential for platelet activation compared to that for Valve-1.

Figure 6

Plots of vorticity contours (Column 1), shear stress (Column 2), and the platelet activation parameter (Column 3) of (a) Valve-1 and (b) Valve-2 at 6 ms after the instant of valve closure. In the activation parameter plots, regions with bright red indicates higher potential for platelets to be activated and dark blue represents minimal potential for the same. Larger regions of bright red for Valve-2 indicate higher potential for platelet activation compared to that for Valve-1 during the initial impact and rebound phases of valve closure.

Figure 7

Plots of vorticity contours (Column 1), shear stress (Column 2), and the platelet activation parameter (Column 3) of (a) Valve-1 and (b) Valve-2 at 12 ms after the instant of valve closure. In the activation parameter plots, regions with bright red indicates higher potential for platelets to be activated and dark blue represents minimal potential for the same. Larger regions of bright red for Valve-2 indicate higher potential for platelet activation compared to that for Valve-1 during the initial impact and rebound phases of valve closure.

Figure 8

Plots of vorticity contours (Column 1), shear stress (Column 2), and the platelet activation parameter (Column 3) of (a) Valve-1 and (b) Valve-2 at 18 ms after the instant of valve closure. In the activation parameter plots, regions with bright red indicates higher potential for platelets to be activated and dark blue represents minimal potential for the same. Larger regions of bright red for Valve-2 indicate higher potential for platelet activation compared to that for Valve-1 during the initial impact and rebound phases of valve closure.