Small molecules like urea are removed by diffusion. The driving force is the concentration gradient. Along the concentration gradient the molecules move through the membrane wall, from blood into dialysate, to reach (electro-) chemical equilibrium. This therapy model is called low-flux haemodialysis, as the flow across the membrane wall is low (<10 ml/h/mmHg).
Larger molecules, particularly the so-called middle molecules (low molecular weight proteins like beta-2-microglobulin), can preferably be removed by convection across the membrane wall, achieved by a pressure gradient across the membrane. Therefore, the membrane needs to be more permeable and must have larger pores to allow passage of these larger molecules. As the water flux across the membrane wall is higher (>20 ml/h/mmHg) compared to low-flux dialysis, this therapy mode is called high-flux haemodialysis.
Haemofiltration does not use any dialysate, but instead directly infuses a physiologic solution into the patient’s blood.
This infusion is administered within the extracorporeal circuit. The excess fluid is removed from the patient via a haemofilter by applying trans-membrane pressure. The therapeutic effect is achieved, as together with the fluid also toxins and other harmful substances are removed. In most cases, haemofiltration is operated with a high-flux dialyser. Depending on the volume to be removed per time interval and molecules to be retained, different flux conditions and dialysers can be used.
As a combination of haemodialysis and haemofiltration the process of haemodiafiltration uses both diffusion and enhanced convection.
The patient’s blood is passed through a dialyser with dialysate on the other side of the membrane. Additionally, a substitution fluid is infused into the patient’s blood in the extracorporeal circuit. This infusion can be done either before (pre-dilution haemodiafiltration) or after the dialyser (post-dilution haemodiafiltration). This very efficient and effective treatment can enhance the range of removed toxins.