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1 2022

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Diffusive solute transport in hollow fiber dialyzers is not affected by variable feed viscosity

Schneditz Daniel, Sauseng Notburga

Biocybernetics and Biomedical Engineering

2022

Vol. 42, no. 4

1112--1122

Dialyzer clearance (K) for hemodialysis is usually predicted from the mass transfer area product (K0A) provided in manufacturer data sheets without accounting for elevated feed-viscosity when treating blood. The boundary layer model for mass transport across hollow fiber membranes, however, predicts an increase in mass transfer resistance (1/K0) and a decrease in K with increasing feed-viscosity. The effect of increased feed-side viscosity relative to baseline crystalloid viscosity on small solute K and 1/K0 was therefore examined in commercial high- (HF) and low-flux (LF) dialyzers in lab-bench studies using standard dialysis equipment in the normal operating range. Homogeneous colloid solutions and bovine plasma were used to simulate the range of relative viscosities (ηrel) and oncotic pressures expected under in-vivo conditions. Internal filtration (IF) was quantified by a mathematical model to obtain diffusive transport characteristics (K’, 1/K'0). An up to 5-fold increase in ηrel caused a small increase in K and a small decrease in 1/K0 in HF, but not in LF dialyzers. After correction for a small convective contribution by IF, K’ and 1/K'0 remained constant in both LF and HF dialyzers. Diffusive transport characteristics of commercial HF and LF dialyzers are independent of variable feed-side viscosity. This suggests an insignificant contribution of the feed-side boundary layer resistance in dialyzers optimized for operation in hemodialysis. Increasing the feed-side viscosity, however, increases the convective component of dialyzer solute transport because of IF. Diffusive dialyzer clearance predicted from the dialyzer K0A is independent of elevated feed-viscosity.