Haemodiafiltration-optimal efficiency and safety

Abstract

Haemodiafiltration (HDF) is the blood purification therapy of choice for those who want significant removal of uraemic solutes beyond the traditional range of small molecules. Combining diffusive and convective solute transport, a HDF treatment comprises the largest number of variables among blood purification therapies, and it is important to understand how they interact in order to optimize the therapy. This review discusses the parameters that determine the efficiency of HDF and how they can be controlled in the different forms of HDF and 'HDF-like' therapies practised today. The key to safe and effective HDF therapy is to have access to large volumes of high-quality fluids. Starting with ultrapure dialysis fluid, on-line preparation of a sterile, non-pyrogenic substitution solution can be made an integral part of the treatment, and we describe the necessary conditions for this. On-line HDF can provide the largest removal of the widest range of solutes among available dialysis therapies, and the potential clinical benefits of this are within practical reach for the increasing number of patients dialysed with high-flux membranes and ultrapure dialysis fluid.

Fig. 1

Sieving curves for low-flux and high-flux dialysis membranes and human glomerular basement membrane. The molecular size for which the sieving coefficient = 0.1 is the cut-off of the membrane.

Fig. 2

Clearance in postdilution HDF for urea (MW = 60), vitamin B₁₂ (MW = 1 355) and inulin (MW = 5 000) illustrating the impact of increasing convection (dark areas) on diffusion (light areas) at a blood flow rate of 300 ml/min and increasing ultrafiltration rate. Reprinted from [14] with permission.

Fig. 3

Flow diagrams for different forms of haemodiafiltration (HDF) at typical operating conditions (blood flow rate 300 ml/min, dialysis fluid flow rate 500 ml/min and weight loss 10 ml/min) showing possible convective removal.

Fig. 4

Process steps in the preparation of fluids for dialysis starting with tap water and resulting in sterile, non-pyrogenic substitution fluid for on-line, convective therapies. (RO = reverse osmosis, CFU = colony-forming units, EU = endotoxin units, SAL = sterility assurance level).