The Donnan effect in artificial kidney therapy.

The calculation of the effective sodium gradient in dialysis has to consider a membrane potential difference which is generally derived from the Donnan effect. Strictly this is allowed only under equilibrium conditions. This paper considered the effect of the deviation from equilibrium in haemodialysis and haemofiltration. The mathematical analysis is based on the integration of the local transport rate over the membrane area. The local transport rate is calculated from the Nernst-Planck equation using the constant field assumption. Deviation from equilibrium results in a diffusion potential across the membrane. Experimental evidence was presented for part of the theoretical results. The diffusion potential, both in haemodialysis and in haemofiltration, is too small to have any clinical significance. From the theory it follows that better tolerance of haemofiltration in comparison with haemodialysis cannot be explained by a difference in sodium transport. Calculation of the sodium transport in dialysis therapy based on the equilibrium Donnan effect is sufficiently accurate for kinetic considerations in the dialysis routine.