Quantifying the Effect of Plasma Viscosity onIn VivoDialyzer Performance

Dialyzer manufacturers characterize performance of their devices based on measurements of clearance using crystalloid solutions. Typically,in vitrodialyzer mass transfer area coefficients for urea (K(0)A) are substantially higher than values measuredin vivo. The reason for this reduction has not been clearly determined. We hypothesized that the known effect of viscosity on reducing solute diffusivity might partially or fully account for this reduction.In vitrodialyzer clearances of urea, glucose, and lactate were measured using crystalloid solutions as well as bovine blood with varying hematocrit and plasma viscosity under a wide range of operating conditions. Viscosities of crystalloid solutions, of blood plasma, and of whole blood were measured at 37 degrees C at a shear rate of 100/s. Diffusivity and relativeK(0)Avalues (K(0)A(rel)) in eight dialyzers were computed for each solute under these different conditions. Plasma was 1.84 times more viscous compared with crystalloid solution (eta(rel)= 1.84), suggesting a correction multiplier of 1/eta(rel)= 0.54 forin vivo K(0)Arelative to thein vitrovalue. ExperimentalK(0)A(rel)at that eta(rel)was on average reduced to 52% of crystalloidin vitro K(0)Avalues. The multiplier 0.52 measured in this study is close to the multiplier 0.55 predicted for average plasma viscosities and also close to the multiplier 0.54 assumed for urea kinetic modeling to provide reasonable urea distribution volumes. The known effect of viscosity on solute diffusivity is therefore sufficient to explain the reduction in dialyzerK(0)Afor urea and glucosein vivocompared within vitromeasurements.