Mechanisms of solute and fluid removal in hemodiafiltration

Prescribing therapeutic conditions for online predilution hemodiafiltration (HDF) with fixed total dialysate flow rate Q(Dtotal) is not straightforward, since the increase in the substitution fl ow rate Q(S) is compensated by the decrease in the net dialysate flow rate Q(Dnet).

Clearances of various solutes under online predilution HDF were clinically evaluated with fixed Q(Dtotal) (= 520 ml/min) divided into Q(Dnet) and Q(S). Three polysulfone membrane dialyzers and 5 polyester polymer alloy membrane dialyzers were chosen to measure sieving coefficients (SC) for albumin in vitro at 37 degrees C to predict when the albumin loss is greatest during clinical treatment.

Clearances of small solutes such as urea and creatinine increased in vivo with the increase in blood flow. These values, however, slightly but steadily decreased with the increase in Q(S) because the increase in Q(S) decreased Q(Dnet). Clearances of beta2-microglobulin and alpha1-microglobuin increased with the increase in Q(S) and decreased with the increase in Q(Dnet), because clearances of larger solutes were more strongly dependent on ultrafiltration than on diffusion. The SC for albumin in vitro showed a peak at the beginning of the experiment in those membranes with large proportions of polyvinylpyrroridone (PVP), which may lead to large amounts of albumin loss at the beginning of the treatment.

Dialysis prescription in online predilution HDF in terms of maximizing clearance for the solute of interest may be different for each target solute. The amount of albumin loss may be closely related to the amount of PVP included in the membrane.