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Abstract: SA-PO566

Is the Donnan Equilibrium Still Intact in High-Volume Hemodiafiltration?

Session Information

Category: Dialysis

  • 801 Dialysis: Hemodialysis and Frequent Dialysis

Authors

  • Gross, Malte, Technische Hochschule Ulm, Ulm, Baden-Württemberg, Germany
  • Gagel, Alfred J., Fresenius Medical Care Deutschland GmbH, Schweinfurt, Bayern, Germany
  • Maierhofer, Andreas, Fresenius Medical Care Deutschland GmbH, Schweinfurt, Bayern, Germany
Background

High convective flow in high-volume hemodiafiltration has raised concerns that intradialytic electrolyte balance might be changed compared to standard HD, requiring adaptions of dialysate electrolyte prescription.
The presence of charged proteins in the blood which are too large to penetrate the dialyzer membrane cause an asymmetric small ion distribution governed by the Gibbs-Donnan equilibrium (GDe).
Derivations of GDe in most cases ignore the hydrodynamic drag from transmembrane fluid flow. We studied the effect of this flow by numerical simulations in an extended model.

Methods

The Nernst-Planck equation was extended by an advective term taking into account the hydrodynamic transmembrane flow as an additional driving force for ion transport. For the simplest case of two oppositely charged monovalent permeable ions a set of differential equations was derived. This allowed for a numerical calculation of permeable ion concentrations on both sides of the membrane depending on the transmembrane flow speed. The deviation of ion concentrations from their equilibrium values was analyzed for different ion species and concentrations for a large range of transmembrane flow rates.

Results

Even in the case of a non-zero convective flow, equilibrium concentrations are already reached a few nm from the inner membrane boundary, thus still within the membrane pores (cf. Fig. 1). These concentrations differ slightly from what is expected from classical Gibbs-Donnan equilibrium. However, in a typical HDF treatment, where transmembrane flow speed is less than 10-5 ms-1 this deviation is less than 6 10-5 mmol/L.

Conclusion

Even in high volume hemodiafiltration the transmembrane flow has negligible influence on the equilibrium electrolyte concentrations.

Na+ and Cl- concentrations in filtrate (cNa and cCl, resp.) at transmembrane flow speed vf = 0 (dashed lines) and vf = 10 ms-1 (solid lines). cf and c(0)f: equilibrium concentrations at vf > 0 and vf = 0, resp.; Δcf = cf - c(0)f. Gibbs-Donnan ratio rD = 0. 95.
Insert: Δcf as a function of vf; shaded region: range of flow speeds in medical applications.

Funding

  • Commercial Support – Fresenius Medical Care Deutschland GmbH