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