Effects of a reduced inner diameter of hollow fibers in hemodialyzers

Background. The clearance of middle molecules in high-flux hemodialyzers is due to the higher contribution of convection in the overall solute transpo rt. Although net filtration can be maintained low by the machine control, i nternal filtration in the proximal parr of the dialyzer remains high. The f inal fluid balance is achieved by significant amounts of backfiltration in the distal part of the dialyzer. To increase further middle molecule cleara nce (MMK), hemodiafiltration has been used. This technique, however, requir es complex machines and large amounts of substitution fluid. We present a n ovel solution to increase the convective transport of middle molecules in h igh Aux dialyzers without the need for substitution fluids. In particular, high-flux dialyzers with a reduced hollow fiber diameter are compared with standard dialyzers in terms of internal filtration and solute clearances. Methods. Hemodialyzers with 175 mu inner diameter polysulfone fibers were c ompared with standard 200 mu polysulfone hollow fiber dialyzers. The study was carried out in vitro using a previously published method to measure int ernal filtration and backfiltration rates. The method is based on the detec tion by a gamma camera of segmental variations in concentration along the l ength of the dialyzer of a nondiffusable Tc99-labeled marker molecule injec ted in the blood in vitro circuit. At the same time, pressures were detecte d in the blood and dialysate compartment. The system was operated at zero n et filtration maintaining volumetrically constant both dialysate and blood circuits. In vivo clearances were also measured for solutes with different molecular weight. Results. The pressure drop in the blood compartment at 300 mL/min of blood flow passed from 112 to 159 mm Hg. At the same blood flow, the internal fil tration-backfiltration rates increased from 23.1 to 48.2 mL/min. This resul ted in a significant increase of in vivo in clearances of vitamin Bi, and i nulin of more than 30%. Urea, creatinine, and phosphate clearance did not d isplay any change. Conclusions. A reduction of the inner diameter of the hollow fibers in high -flux dialyzers may result in a significant increase of the blood compartme nt resistance. In turn, this results in increased rates of internal filtrat ion and backfiltration. The practical effect in clinical dialysis is demons trated on middle molecules. While, in fact, the clearances for small solute s such as urea and creatinine are not affected, the clearances of larger so lutes such as vitamin B-12 Or inulin increase significantly (P < 0.01).