Transport characteristics of different hemodialysis membranes

Previous studies have demonstrated the importance of both diffusive and convective (sieving) transport on the clearance of clinically-significant middle molecules during hemodialysis. However, there have been few quantitative analyses of the intrinsic transport properties of available dialysis membranes. In vitro experiments were performed using cellulose acetate (CA110 - Baxter), cellulose triacetate (CT100 - Baxter), polysulfone (F60 - Fresenius), and acrylonitrile copolymer (AN69 - Hospal) dialyzers. Clearance and sieving data were obtained with urea, vitamin B₁₂, and dextrans as model solutes. Simulated dialysis was done using donated human plasma. Results were analyzed using membrane transport theory. Data for the CA, CT, and AN69 dialyzers were consistent with the homogeneous structure of these membranes. The effective pore size of the AN69 membrane was 50% larger than that for the CT and 6 times larger than that for the CA. This results in much higher clearance for the AN69, particularly for large molecular weight solutes. Results for the F60 suggest a distinct 2-layer membrane structure. Quantitative estimates of the pore size of each layer were obtained using hydrodynamic analysis. The tight skin provides a large resistance to convective transport but has minimal effect on diffusion. Exposure to plasma causes a significant reduction in clearance for large solutes because of the resistance provided by a protein layer on the membrane surface. The properties of this layer depend upon the membrane and dialysis conditions. A tighter protein layer formed on the F60 membrane, causing a greater reduction in solute clearance than seen with either the AN69 or CT dialyzers. The relative reduction in clearance was also greater at high ultrafiltration rates. The implications of these results to clinical dialysis are discussed.