Red blood cells (RBC) are often relegated to the role of passive observer in thrombosis. However, there is a correlation between elevated hematocrit and arterial and venous thrombosis risk. RBC dictate the rheology of blood and the transport of platelet to the vessel wall, suggesting that elevated hematocrit could promotes thrombosis by promoting the accumulation and activation of platelets at the sites of vascular injury.
In models of arterial thrombosis, we found that high hematocrit promotes a more rapid thrombus growth rate and occlusion time in an animal model and a microfluidic model (in collaboration with Alisa Wolberg, UNC-Chapel Hill and Aaron Fogelson, University of Utah). This effect was purely biophysical, as elevated hematocrit did not influence coagulation as measured by fibrin content of in vivo or in vitro clots, nor hasten thrombin generation. Rather, the reduced RBC-cell free region near the vessel wall reduced the degrees of freedom of platelets as determined by direct numerical simulations. As a result, platelets are confined to orientations and experience normal forces that increase their contact time near a growing thrombus.
In the case of venous thrombosis, we have fabricated a venous valve model in which we match the in vivo hemodynamics in humans by appropriately scaling the relevant geometric and dynamic dimensionless parameters. Importantly, inertial forces dominate over viscous forces in large veins (e.g. high Reynolds numbers), which create recirculation zones in the valve sinus. Using particle image velocimetry, we find that RBC do indeed increase the accumulation of platelet sized particles in the two countercurrent vortical flows that are a characteristic of venous valves compared to the absence of RBC. These data suggest that collisions with RBC in the bulk and recirculation zones accelerate the rate that platelets cross-stream lines, and subsequently adhere to the endothelium of the valve sinus. To simulate the initiation of a venous valve thrombus, we selectively adsorb tissue factor, a protein that initiates coagulation, in the valve sinus. In experiments with plasma alone, fibrin formation is limited to the deepest recesses of the valve sinus, but cannot penetrate into the primary vortex. However, the addition of platelets leads to an accumulation of procoagulant platelets at the interface of the initial fibrin gel that supports fibrin formation that penetrates into the primary vortex.
B.L. Walton, M. Lehmann, T. Skorczewski, L.A. Holle, J.D. Beckman, J.A. Cribb, M.J. Mooberry, A.R. Wufsus, B.C. Cooley, J.W. Homeister, R. Pawlinski, M.R. Falvo, A.L. Falvo, N.S. Key, A.L. Fogelson, K.B. Neeves, A.S. Wolberg. Elevated hematocrit enhances platelet accumulation following vascular injury. Blood, 129 (2017): 2537-2546. PMID:28251913