Internal filtration--advantage in haemodialysis?

Internal filtration is defined as the total water flux across the membrane within the closed blood and dialysate compartment of a dialyser. The aim of our study was to increase convection by increasing the amount of internal filtration and to study the effect on elimination of high and low molecular weight uraemic toxins during regular haemodialysis. Three high-flux polysulfone dialysers with identical type of membrane (Fresenius Sps600) and same number of fibres but with different inner diameters () and therefore different surface areas [] (175 microns [0.55 m2], 200 microns [0.65 m2] and 250 microns [0.79 m2]) were tested in routine haemodialysis sessions (10 patients). At a blood flow of 250 ml/min and a dialysate flow of 500 ml/min hydrostatic pressures at dialysate-in/outlet (PD(in), PD(out) and blood-in/outlet (PBin, PBout), mTMP, clearances (Cl) and mass balances of creatinine, urea, phosphate and beta-2-microglobulin (beta 2-M) at 30 and 180 min were measured at net ultrafiltration rate of 0 ml/min. Clearances of all three dialysers were matched for small toxins (urea: 180 +/- 4.83 ml/min). The 175 microns diameter dialyser achieved a significantly better removal of beta 2M (Cl beta 2-M 57.4 +/- 9.43 ml/min) than the two other filters (200 microns: 29.9 +/- 8.4 ml/min; 250 microns: 11.1 +/- 6.79 ml/min) although the surface area of the 175 microns filter was smallest. Hydrostatic pressure analysis revealed that at the same median transmembrane pressures (mTMP) internal filtration increased with decreasing inner fibre diameter. Longitudinal flow resistance and thereby local transmembrane pressure difference increased with narrowing the inner fibre diameter.