Determination of whole blood apparent viscosity: experience with a new hemorheologic technique

A new experimental technique of determining apparent blood viscosity at low rates of shear using a porous bed viscometer (PBV) was evaluated in a study of 213 blood samples from 43 mongrel dogs. Viscosity was measured over a wide range of hematocrit (Hct), 6.5-51%, and plasma fibrinogen (Fib), 11-597 mg%, levels significantly altered (P less than 0.0005) by isovolemic hemodilution. Viscosity was highly correlated with Hct and Fib (r = 0.89), which confirmed the results of previous investigations. An important finding was the ability of the PBV to detect the non-Newtonian behavior of whole blood that occurs under low shear conditions at higher Hct levels. In a subgroup of 10 animals, viscosity was also measured by a precision cone and plate viscometer (CPV) at various shear rates. Although the PBV and CPV measurements showed excellent correlation (r = 0.92), a marked nonlinear deviation of the plotted data from the regression of Hct vs viscosity measured by the PBV at Hct levels greater than or equal to 30% was noted which could not be identified by the CPV. The PBV technique affords valid measurements of the apparent viscosity of normal canine blood. Moreover, the device is apparently sensitive to the anomalous rheologic properties of whole blood attributable to shear-dependent changes in red blood cell aggregation that can become physiologically significant in low-flow states.