Two-Dimensional Dynamic Simulation of Platelet Activation During Mechanical Heart Valve Closure

Abstract

A major drawback in the operation of mechanical heart valve prostheses is thrombus formation in the near valve region. Detailed flow analysis in this region during the valve closure phase is of interest in understanding the relationship between shear stress and platelet activation. A fixed-grid Cartesian mesh flow solver is used to simulate the blood flow through a bi-leaflet mechanical valve employing a two-dimensional geometry of the leaflet with a pivot point representing the hinge region. A local mesh refinement algorithm allows efficient and fast flow computations with mesh adaptation based on the gradients of the flow field in the leaflet-housing gap at the instant of valve closure. Leaflet motion is calculated dynamically based on the fluid forces acting on it employing a fluid-structure interaction algorithm. Platelets are modeled and tracked as point particles by a Lagrangian particle tracking method which incorporates the hemodynamic forces on the particles. A platelet activation model is included to predict regions which are prone to platelet activation. Closure time of the leaflet is validated against experimental studies. Results show that the orientation of the jet flow through the gap between the housing and the leaflet causes the boundary layer from the valve housing to be drawn in by the shear layer separating from the leaflet. The interaction between the separating shear layers is seen to cause a region of intensely rotating flow with high shear stress and high residence time of particles leading to high likelihood of platelet activation in that region.

Mechanical Valve Fluid Dynamics and Thrombus Initiation

“…It can clarify how bulk flow within the valve orifice mechanically affects microscale flow within the complex hinge structures [83] with a level of detail that cannot be reached even with advanced experimental flow-measuring techniques. In addition, multiscale CFD models will also aid in computing the actual load imparted on the blood cells, as well as in investigating the mechanical interaction between red blood cells and platelets [108].…”

“…In addition, multiscale CFD models will also aid in computing the actual load imparted on the blood cells, as well as in investigating the mechanical interaction between red blood cells and platelets [108]. This is particularly important in regions where the dimensions of the blood cells are comparable to the size of the flow geometry [83] and where non-Newtonian effects should be taken into account in the governing equations. Furthermore, knowledge of shear and pressure forces and resulting deformation is desirable to improve our insight into blood cell damage.…”

“…These elevated shear regions potentially are sites for platelet activation and (sub)lethal blood damage. On the other hand, platelet activation may as well be the result of an imbalance of the homeostatic system due to chronic blood damage [16,83]. When activated platelets enter regions of flow stagnation and recirculation with long residence times adjacent to the valves, deposition and aggregation of damaged blood elements is promoted, leading to thrombus formation and subsequent emboli [84].…”

“…At the same time, multiscale simulations would be convenient to improve hinge mechanisms for better washout to reduce the problem of thrombus formation [106]. Krishnan et al [83] used a 2D FSI model with a Cartesian grid meshing technique to depict the detailed flow dynamics through the small gaps between the leaflets and the housing and its effect on platelet activation during valve closure and rebound. The large disparity in cell size between the hinge region and the valve orifice was accounted for with a local mesh refinement algorithm.…”

Algorithms for Fluid–Structure Interaction

“…The previous equations may appear to represent a rather limited setting for fluidstructure interaction problems, but in reality encompass a wide range of biologically relevant FSI problems. Examples include the deformation of cells [11][12][13], the pumping action of the heart [14], deformation of blood vessels [15], the operation of heart valves [16,17], locomotion of cells due to flagellar motion [18,19], and a host of others. The previous system of equations also provides a platform to examine general concepts and challenges associated with solving coupled fluid-structure interaction problems.…”