It is generally accepted that the uremic toxins cover a wide molecular weight range. Removal of higher molecular weight solutes necessitates the use of high-flux dialyzers, since low-flux dialyzer have essentially no capacity to remove solutes the size of small proteins. The most common high-flux membranes have some form of polysulfone as their major component. However, there are differences in the composition and structure of various high-flux “polysulfone” membranes, which may be associated with significant differences in performance. It is not yet clear if the differences in performance arise from differences in the polymer composition of the membrane or from differences in membrane morphology.
Removal of large molecular weight solutes by high-flux membranes is enhanced when the treatment includes convection, either as the sole means of solute removal (hemofiltration) or in combination with diffusion (haemodiafiltration). Convection provides more removal of large molecules than diffusion because the sieving coefficient decreases more gradually with increasing molecular size than does the mass transfer coefficient. Convective therapies are performed most efficiently when replacement solution is prepared on-line. Equipment to perform on-line therapies is widely available. In the absence of purpose-design equipment, the advantages of convection may be partially realized by maximizing internal filtration and back-filtration in the dialyzer. Internal filtration and back filtration occur when membranes of high water permeability are used with a volume control system, because the low transmembrane pressure required for fluid removal and the counter-current flow of blood and dialysate in a dialyzer create local pressure gradients favoring filtration from blood to dialysate at the blood inlet end of the dialyzer. Internal filtration can be increased by using smaller diameters fibers to increase the pressure at the blood inlet.