Microrheological effects of drag-reducing polymers in vitro and in vivo

Soluble long-chain polymers with a molecular mass >106 D have been found to significantly reduce resistance to turbulent flow in pipes, thereby increasing flow rate at constant pressure or decreasing pressure at constant flow rate when added to the flowing fluid at minute concentrations (Toms effect). These molecules, named drag-reducing polymers (DRPs), did not affect resistance to laminar flow in a straight tube. Although, the flow conditions associated with the Toms effect do not occur in the cardiovascular system, a number of studies demonstrated that intravenous administration of nanomolar concentrations of DRPs in experimental animals produced significant hemodynamic effects increasing tissue perfusion and decreasing peripheral vascular resistance without affecting blood viscosity and with no direct effect on the tone of the vessel wall and the intraluminal area. The DRPs were successfully applied in animal models of severe hemorrhagic shock, myocardial ischemia and other pathological conditions demonstrating improvement of impaired hemodynamics and animal conditions/survival. In vitro studies of blood flow in microchannels demonstrated that the DRPs reduced flow and phase separations which occur in the vascular system at normal physiological and pathological conditions. This paper reviews some animal and in vitro studies which employed DRPs as blood additives. Furthermore, the potential mechanisms behind the observed polymer effects on blood circulation are discussed.