Internal filtration, filtration fraction, and blood flow resistance in high- and low-flux dialyzers.

It was the aim to examine the fluid flow in blood and dialysate compartments of highly permeable hollow fiber dialyzers where internal filtration contributes to solute removal but where excessive filtration bears a risk of cell activation and damage. Flow characteristics of high- (HF) and low-flux (LF) dialyzers were studied in lab-bench experiments using whole bovine blood. Measurements obtained under different operating conditions and under zero net ultrafiltration were compared to theoretical calculations obtained from a mathematical model. Experimental resistances in the blood compartment were within ±2% of those calculated from the model when dialysate was used as a test fluid. With whole blood, the experimental resistances in the blood compartment were only 81.8 ± 2.8% and 83.7 ± 4.3% of those calculated for the LF and HF dialyzer, respectively. Surprisingly, measured blood flow resistance slightly but significantly decreased with increasing flow rate (p < 0.001). Mathematical modeling confirmed this decrease both in LF and HF dialyzers which was accompanied by a concomitant decrease in internal filtration fraction, while overall internal filtration increased. The increase in internal filtration when increasing blood flow is associated with a beneficial reduction in internal filtration fraction. Concerns of increased hemoconcentration when increasing blood flow therefore appear to be unwarranted.