Platelet motion near a vessel wall or thrombus surface in two-dimensional whole blood simulations

Abstract

Computational simulations using a two-dimensional lattice-Boltzmann immersed boundary method were conducted to investigate the motion of platelets near a vessel wall and close to an intravascular thrombus. Physiological volume fractions of deformable red blood cells and rigid platelet-size elliptic particles were studied under arteriolar flow conditions. Tumbling of platelets in the red-blood-cell depleted zone near the vessel walls was strongly influenced by nearby red blood cells. The thickness of the red-blood-cell depleted zone was greatly reduced near a thrombus, and platelets in this zone were pushed close to the surface of the thrombus to distances that would facilitate their cohesion to it. The distance, nature, and duration of close platelet-thrombus encounters were influenced by the porosity of the thrombus. The strong influence on platelet-thrombus encounters of red-blood-cell motion and thrombus porosity must be taken into account to understand the dynamics of platelet attachment to a growing thrombus.

Figure 1

Simulation of whole blood flow at 40% Hct and 1100 s⁻¹ wall shear rate. After 600 ms of flow, the platelets are distributed preferentially near the wall.

Figure 2

Platelet distribution histograms inside the RBC-DZ after 600 ms. Distance is measured from the wall to the platelet’s center of mass. Bin width is 250 nm.

Figure 3

Fluid velocity and shear rate across the channel for 20% (red, dash), 40% (blue, solid), and 0% (black, dot-dash) Hct cases. Shear rate ∂u/∂y is approximated with a finite-difference. Values of shear rate and fluid velocity are then averaged over the length of the domain and over 10 ms of time.

Figure 4

(Top) Distances from the wall for four different platelet trajectories. In three trajectories (solid, dashed, and dash-dot) the platelets are tumbling, and in one (dotted) the platelet is sliding. (Bottom) Histogram of time intervals that tumbling platelets spend within 350 nm of the wall.

Figure 5

Tumbling period versus distance from the wall for shear rate 1100 s⁻¹. (Asterisks) Data from whole blood simulations at Hct 40%; (circles) shear flow simulations without RBCs.

Figure 6

(A) Region 1 (magenta) contains locations within 250 nm of the thrombus. Region 2 (blue) and Region 3 (gray) contain locations between 250 and 750 nm, and 750 and 1750 nm, from the thrombus, respectively. (B) Superimposed snapshots (3 ms apart) of platelets moving past a thrombus.

Figure 7

Histogram of the residence times of platelets within 250 nm of a thrombus of porosity 0.46.

Figure 8

Histograms showing platelet distance from the thrombus as they cross the specified lines around the leading edge and top of the thrombus. Observe that platelets are most often closest to the thrombus near its upstream corner.

Figure 9

Orientation of the platelets within 250 nm of the thrombus. The value θ is the angle the platelet makes with respect to the direction normal to the thrombus’s surface.

Figure 10

Widths of the RBC-DZs over a thrombus with porosity 0.21 (top) and 0.67 (bottom). (Shading) Time-averaged hematocrit.