The effect of shear rate on platelet interaction with subendothelium exposed to citrated human blood

Human blood, anticoagulated to a plasma concentration of 19.7 mM sodium citrate, was exposed to subendothelium in an annular perfusion chamber for wall shear rates ranging from 50 to 10,000 sec^-1 and exposure times of 0.5 to 40 min. Platelet adhesion increased with exposure time until complete saturation (100% surface coverage) was attained. Platelet thrombi were transiently formed under all flow conditions, reached a maximum at 5 to 10 min, and eventually disappeared by 40 min. Rates of adhesion increased with wall shear rate to 650 sec^-1; at higher values of shear no significant increase was observed. Platelet thrombi were virtually nonexistent below shear rates of 350 sec^-1, but increased continuously as shear rate was increased to 10,000 sec^-1. Blood, anticoagulated to a plasma concentration of 45 mM sodium citrate, formed no platelet thrombi for shear rates as high as 10,000 sec^-1; platelet adhesion was relatively independent of wall shear rate to 650 sec^-1 and decreased as shear was increased above 650 sec^-1. Analysis of the experimental adhesion results with classical mass transport theory suggests that platelet diffusivity increases and platelet-surface reaction rate coefficient decreases with increasing wall shear rate. This combination results in a diffusion-controlled reaction rate below 350 sec^-1 with a relatively rapid transition to a more reaction-controlled rate for wall shear rates above 800 sec^-1.