"viscosity-plus" effect in whole blood at high shear rates

A SUSPENSION of red blood cells (RBC) in normal saline is characterized by a viscosity dependent on the haematocrit. When RBC at the same haematocrit are suspended in plasma or certain other media, the viscosity of the suspension is higher than that in saline, and the difference is much greater than can be accounted for by that between the viscosities of saline and new suspending medium. This phenomenon has been called the "viscosity-plus" effect^1,2. Although the basis of this effect is not definitely understood, it may be based in part on aggregation of RBC by components of the plasma. Blood viscosity is known to increase when aggregation increases^1-3, and aggregation, like the "viscosity-plus" effect, is enhanced as blood flow (rate of shear) decreases^1-3. Aggregated RBC in normal blood, however, are thought to disperse completely as shear rates increase to approximately 100 reciprocal seconds³. At greater shear rates, whole blood viscosity is thought by some to depend solely on the viscosity of the plasma⁴. This communication shows that blood viscosity may be disproportionately large, when related to plasma viscosity, even at shear rates as great as 30,000 reciprocal seconds. Interpretations of the "viscosity-plus" effect must take cognizance of its operation at such high shear rates.