The changing face of dialyzer membranes and dialyzers

The key goals for dialysis treatments are to prevent the progressive accumulation of waste products of metabolism and volume overload. Traditionally uremic solutes have been classified according to molecular weight and termed small, middle sized, and large solutes. Solute clearance during dialysis sessions will potentially be by diffusion, convection and adsorption. Dialyzer membranes act as a semi-permeable membrane restricting solute removal predominantly by size. Small molecules move faster than large molecules, so small solutes are readily removed by diffusion. Increasing the size of the pores in the membrane will potentially allow middle and larger sized solutes to pass through the dialyzer membrane, although in practice there is a limit to increasing pore sizes to prevent the loss of albumin and other important proteins. Differences in membrane surface and charge will influence protein absorption. The removal of fluid during dialysis depends in part on the hydraulic permeability of the membrane. Combining higher hydraulic permeability and larger sized pores increases convective clearance with solutes moving across the membrane with the water movement. Depending upon dialyzer design, higher hydrostatic pressure as blood enters the dialyzer leads to a variable amount of internal diafiltration, so improving the clearance of middle sized solutes. Although the dialyzer membrane plays a key role in solute clearance, the design of the casing and header also play a role in directing the countercurrent blood and dialysate flows to maximize the surface area available for diffusive and convective clearances.