High viscosity plasma expanders: Volume restitution fluids for lowering the transfusion trigger.

Hemorheological studies lead to the axiom that high plasma viscosity is detrimental and that it is beneficial to lower blood viscosity, a precept embodied in the practice of hemodilution, where improved perfusion is attributed to the lowering of blood viscosity. Hemodilution is limited by the transfusion trigger, hemoglobin content of blood of about 7-8 g/dl, which indicates when further volume replacements must restore oxygen carrying capacity with red blood cells (RBC). However, oxygen consumption and delivery are not compromised upon passing this landmark. The reduced blood viscosity does not transmit adequate pressure to the capillaries, causing functional capillary density (FCD) to decrease, jeopardizing organ function through the inadequate extraction of products of metabolism from the tissue by the capillaries. Studies in hemorrhagic shock show that survival is primarily determined by the maintenance of FCD and secondarily by tissue oxygenation. FCD is maintained as hematocrit is reduced beyond the transfusion trigger by increasing plasma viscosity, which transmits systemic pressure to the capillaries and induces vasodilatation through the increased shear stress dependent release of vasodilators. Consequently the transfusion trigger is also a "viscosity trigger" indicating when blood and plasma viscosity are too low. In this condition increasing plasma viscosity is beneficial and extends the transfusion trigger reducing the use of blood transfusions.