Paired filtration-dialysis (PFD): a separate convective-diffusive system for extracorporeal blood purification in uraemic patients.

Abstract

In haemodiafiltration (HDF) mass transfer of solutes through a membrane depends on both convection and diffusion. Diffusion has major importance in removing low-molecular weight solutes which would require a greater filtration flow if only convection were employed. Convection is essential for mass transfer of high-molecular weight solutes, since diffusive membrane permeability markedly decreases as molecular weight increases. The total clearance efficacy of HDF is less than the sum of the two single procedures since diffusion and convection do not take place subsequently but simultaneously and are mutually affected (1). In fact, simultaneous diffusion and convection with the same membrane determine convective clearance as low as contemporary diffusive clearance is high (2–4). The following equation, allowing calculation of convective clearance (Kconv), explains this fact.