Dialysis membranes in convective treatments.

Abstract

substitution fluid. No dialysis fluid is present. Thistechnique offers the advantage of excellent clearancesHaemodialysis is basically divided into two categories: for large molecules. The treatment, however, has onlylow-flux dialysis and high-flux dialysis. Low-flux dia- been applied in Europe because of the need for largelysis includes the standard haemodialysis technique in quantities of substitution fluid in commercially pre-which dialysers with low hydraulic permeability are pared bags. (ii) Haemodiafiltration is a form of therapyutilized. Blood and dialysate flow rates are at 250 and in which diffusion and convection are conveniently500 ml/min, respectively and the average duration is combined. Dialysate is circulated countercurrent to4 h. When the dialyser surface area is increased, and blood but the amount of ultrafiltration exceeds theflow rates of blood and dialysate are increased up to programmed patient weight loss and therefore the final400 and 800 ml/min, respectively, the treatment is fluid balance is achieved by the infusion in the venousdefined as high efficiency haemodialysis [1]. In this line of appropriate amounts of substitution fluid [4–6].treatment the clearance of small molecules is remark- Due to the high costs related to replacement solutions,ably increased and the treatment time generally can be new approaches have been proposed in Europe forshortened, but the clearance for middle to large molec- performing haemofiltration and haemodiafiltration [7].ules is still very low due to the permeability character- Part of the fresh dialysate is diverted from its line toistics of the membrane. the dialyser, it is filtered and it finally is used as aClassic cellulosic membranes such as cuprophan are replacement solution. These on-line treatments havetypically low-flux membranes i.e. permeability to water the advantage of reduced costs, unlimited productionis in the range of 5–6 ml/h/mmHg/m2, they are strongly of substitution fluid and no need for tons of commer-hydrophilic and remarkably thin to permit an optimal cially produced fluid in bags. On the other hand, theutilization in diffusive treatments such as microbiological and chemical purity of the on-linehaemodialysis. prepared fluid may represent a concern since no on-lineOn the other hand, classic high-flux synthetic mem- controls can be made. (iii) High-flux dialysis is abranes originally were fully hydrophobic, strongly technique in which highly permeable dialysers areasymmetric and with a thick wall. They were employed utilized in conjunction with an accurate ultrafiltrationprincipally in convective therapies such as haemofiltra- control of the dialysis machine. Because of the naturetion due to their high hydraulic permeability, i.e. of the hollow fibre dialyser, the correct water balance30–40 ml/h/mmHg/m2, and their high sieving coeffi- is achieved thanks to an internal filtration in thecients [2]. proximal part and a backfiltration in the distal part ofOnly recently has it been possible to design synthetic the filter [8]. As a consequence, some of the advantagesmembranes with reduced wall thickness and a mixed of removing larger molecules are maintained becausehydrophobic–hydrophilic structure. This has permitted of the internal convection, while the need for replace-their use in new techniques such as high-flux haemodia- ment solution is avoided by a significant amount oflysis and haemodiafiltration in which diffusion and backfiltration of fresh dialysate. In this way, someconvection are conveniently combined [3]. of the advantages of haemodiafiltration are maintainedWhen high-flux membranes are utilized, three differ- with a simpler and easier layout of the technique [8].ent approaches can be chosen: (i) haemofiltration is a Again, the dialysate purity is a key issue in thisfully convective treatment in which large amounts of treatment where bacterial products could be back-ultrafiltrate are produced and replaced by a sterile transported into the blood together with the reverseflux of water. This may affect the entire concept ofbiocompatibility of the system. Concerning this aspect,