Increases in mass transfer-area coefficients and urea Kt/V with increasingdialysate flow rate are greater for high-flux dialyzers

The hemodialyzer mass transfer-area coefficient (K(o)A) for urea increases with increasing dialysate flow rate (Q(d))The magnitude of the increase in K(o)A varies depending on the particular dialyzer under consideration; howe ver, dialyzer properties that govern this phenomenon have not been establis hed. We hypothesized that Q(d)-dependent increases in KoAs are influenced b y the water permeability of the dialysis membrane. We evaluated in vitro th e effect of blood flow rate (Q(b)) and Q(d) on urea and creatinine KoAs for two low-flux (Polyflux 6L and 8L) and two high-flux (Polyflux 14S and 17S) dialyzers containing Polyamide S membranes with similar membrane surface a reas. Additional experiments were also performed on high-flux dialyzers con taining Polyamide S membranes with very large surface areas (Polyflux 21S a nd 24S). KoAs, calculated from the mean of blood- and dialysate-side cleara nces, were determined at zero net ultrafiltration for three different Q(b) and Q(d) combinations: Q(b) of 300 mL/min and Q(d) of 500 mL/min; Q(b) of 4 50 mL/min and Q(d) of 500 mL/min; and Q(b) of 450 mL/min and Q(d) of 800 mL /min. Urea and creatinine KoAs were independent of the Q(b) but increased w hen Q(d) was increased from 500 to 800 mL/min. These increases in both urea and creatinine KoAs were greater for high-flux than low-flux dialyzers (P < 0.0001). As expected, urea and creatinine KoAs also increased with increa sing membrane surface area. We conclude that dialysis membrane water permea bility (or flux) is a dialyzer property that influences the dependence of s mall-solute KoAs and clearance on Q(d). Whether this phenomenon is caused b y enhanced internal filtration for dialyzers containing high-flux membranes requires further study. (C) 2001 by the National Kidney Foundation, Inc.