Action of hydroxyethyl starch on the flow properties of human erythrocyte suspensions

Abstract

Hydroxyethyl starch (RES) has often been used as a plasma expander, but questions still remain concerning the mechanism by which it produces changes in the rheological properties of blood and erythrocyte (RBC) suspensions under various flow conditions. The present investigation has shown that the dynamic viscosity of RES (232,000 and 565,000 daltons) solutions rises in a nonlinear fashion with increasing HES concentration, and for a given concentration of HES exhibits Newtonian behavior at shear rates between 0.15 to 124 ⁻¹. At low (< 0.9 ⁻¹) shear rates the apparent viscosity of a 40% RBC suspension increases with lower concentrations of HES because of RBC aggregation. At higher concentrations of RES, increases in suspension viscosity are due to an increase in the viscosity of the continuous phase since the RBC are largely disaggregated. At high (> 36 sec⁻¹) shear rates the relative viscosity ( $\mathit{\eta }\mathrm{/}{\mathit{\eta }}_0$ ) of RBC suspensions slowly decreases with increasing HES concentration. At low shear rates $\mathit{\eta }\mathrm{/}{\mathit{\eta }}_0$ increases and then decreases with increasing RES concentration. Evidence of the concentration-dependent effects of RES on RBC aggregation is provided not only by the viscometric analysis but also from measurements of erythrocyte sedimentation rate (ESR) and the zeta sedimentation ratio (ZSR). HES is a more potent aggregating agent in phosphate buffered saline (PBS) than it is in plasma. Polymer size has only a slight effect on the extent of RBC aggregation produced, but does have a significant effect on the concentration of polymer at which maximum aggregation occurs. The viscosity-corrected electrophoretic mobility of RBC in HES rises monotonically with the concentration of HES in the suspending medium. Decreases in the extent of RBC aggregation with increasing polymer concentrations probably result from an increase in the electrostatic repulsive forces between the cells.