Urea and the electrolytes

THUS: urea clearance is essentially equal to the volume of effluent.

  • Dialytic clearance of urea is almost completely determined by the flow rate of the dialysate.
    • In ultrafiltration, it is determined by the volume of ultrafiltrate.
    • In general, urea goes wherever water goes.
    • Increasing blood flow beyond 150ml/hr does not increase the rate of urea clearance.
    • increasing the volume of "exchanges" will increase urea and electrolyte clearance
    • Increasing fluid removal will increase urea and electrolyte clearance
    • Both have about equal influence; hemofiltration and hemodialysis result in roughly the same urea clearance.

Removal of middle molecules

THUS: in CVVH middle molecule clearance is proportional to the volume of ultrafiltrate.

    • This relies more on the sieving coefficient of the membrane.
    • Convective clearance by hemofiltration is a more effective way of removing these molecules than dialysis.
    • For any given membrane, a higher rate of ultrafiltration increases the rate of middle molecule clearance.
    • You replace the ultrafiltrate with clean replacement fluid which dilutes the remaining middle molecules, and the concentration decreases.
    • In CVVHDF, not all of the effluent is ultrafiltrate (some is dialysate). However the utrafiltrate volume needs to be replaced by replacement fluid. Thus, it is possible to estimate the rate of ultrafiltration in net-neutral CVVHDF if you know the volume of replacement fluid.

References

For a definitive treatment of all of this, you ought to pay homage to the gigantic and all-encompassing "Critical Care Nephrology" by Ronco Bellomo and Kellum (2009).

The Gambro and Fresenius websites have also been an excellent source of information.