Effects of a reduced inner diameter of hollow fibers in hemodialyzers

Effects of a reduced inner diameter of hollow fibers in hemodia- crease of the blood compartment resistance. In turn, this results lyzers. in increased rates of internal filtration and backfiltration. The Background. The clearance of middle molecules in high-flux practical effect in clinical dialysis is demonstrated on middle hemodialyzers is due to the higher contribution of convection in molecules. While, in fact, the clearances for small solutes such the overall solute transport. Although net filtration can be maintained low by the machine control, internal filtration in as urea and creatinine are not affected, the clearances of larger the proximal part of the dialyzer remains high. The final fluid solutes such as vitamin B12 or inulin increase significantly balance is achieved by significant amounts of backfiltration (P , 0.01). in the distal part of the dialyzer. To increase further middle molecule clearance (MMK), hemodiafiltration has been used. This technique, however, requires complex machines and large Several lines of evidence demonstrate the importance amounts of substitution fluid. We present a novel solution to increase the convective transport of middle molecules in high of middle molecule removal in hemodialysis (1). The flux dialyzers without the need for substitution fluids. In partic- use of high-flux membranes and the increased use of ular, high-flux dialyzers with a reduced hollow fiber diameter convective techniques have permitted improvements in are compared with standard dialyzers in terms of internal fil- the efficiency of hemodialysis, leading to better removal tration and solute clearances. of solutes in the middle molecular weight range (2, 3). Methods. Hemodialyzers with 175 m inner diameter polysul- fone fibers were compared with standard 200 m polysulfone The improvement achieved with synthetic membranes hollow fiber dialyzers. The study was carried out in vitro using is mainly due to their higher hydraulic permeability and a previously published method to measure internal filtration their increased sieving capacity compared with classic and backfiltration rates. The method is based on the detection cellulose membranes. These properties result in higher by a g camera of segmental variations in concentration along middle molecule clearance. This clearance improvement the length of the dialyzer of a nondiffusable Tc99-labeled marker molecule injected in the blood in vitro circuit. At the is due to larger amounts of ultrafiltration per treatment same time, pressures were detected in the blood and dialysate and a more important contribution of convection to the compartment. The system was operated at zero net filtration overall transport process (4). maintaining volumetrically constant both dialysate and blood Synthetic high-flux membranes are used both in Eu- circuits. In vivo clearances were also measured for solutes with rope and the United States, with different modes of different molecular weight. Results. The pressure drop in the blood compartment at 300 application (5, 6). In Europe, high-flux dialysis and hemo- mL/min of blood flow passed from 112 to 159 mm Hg. At diafiltration (HFD and HDF) are used as treatment mo- the same blood flow, the internal filtration-backfiltration rates dalities for chronic renal failure. Conventional HDF uses increased from 23.1 to 48.2 mL/min. This resulted in a signifi- large convective transport with ultrafiltration rates of cant increase of in vivo in clearances of vitamin B12 and inulin of more than 30%. Urea, creatinine, and phosphate clearance more than 70 mL/min. Since the ultrafiltration rate ex- did not display any change. ceeds the rate of desired weight loss in the patient, sterile Conclusions. A reduction of the inner diameter of the hol- replacement fluid must be administered. Net ultrafiltra- tion rate in the patient will be equal to the difference between total ultrafiltration rate and reinfusion rate. To