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Leypoldt JK, Cheung AK, Agodoa LY, Daugirdas JT, Greene T, Keshaviah PR

Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates. The Hemodialysis (HEMO) Study

Kidney Int (Jun) 51:2013-2018 1997

When estimating dialyzer clearance at a given blood and dialysate flow rate (Qb and Qd, respectively), one uses an equation that incorporates these variables and the KoA, which is the mass transfer-area coefficient of the dialyzer. The KoA, in effect, is the maximum attainable clearance of a given dialyzer at infinite blood and dialyzer flow rates. You skiers out there can think of the KoA as the vertical drop of a ski hill. Now, you as a skier may not go to the top of the hill on every run, but when all is said and done, you will get more skiing done in Colorado than in areas that pass for ski hills in Cleveland.

The formula that computes dialyzer clearance (Kd) from KoA, Qb, and Qd can also be run backwards; if you know a Kd at a given Qb and Qd, you can compute the KoA. The formula also assumes that, for a given dialyzer, the KoA is a constant, as it depends only on the permeability of the membrane to a solute (Ko) x the surface area.

Most dialyzer manufacturers don't publish the KoA values of their dialyzers, although this trend is changing. Instead, they give out a plethora of Kd values for various values of Qb and Qd.

In the course of the HEMO trial, we needed some starting point for dialyzer clearance in our urea kinetic computations. There is, of course, a potential problem in computing in vivo clearances from in vitro clearances, but we also wanted to just confirm the in vitro clearances given by the manufacturers. Accordingly, samples from usually at least 5 lots of dialyzers were shipped to Utah, where Drs. Leypoldt and Cheung performed in vitro urea clearance measurements, using crystalloid on both sides of the dialyzer, but measuring solute concentrations on both blood and dialysate sides.

The findings were rather surprising. Two main findings emerged. (1) The manufacturer's estimates of KoA were often an overestimate (by about 10%), and (2) whereas KoA for a given dialyzer was constant at various levels of Qb, as predicted by urea kinetic analyses, when KoA was measured at Qd rates of 500 and 800 ml/min, the KoA INCREASED when Qd was notched up to 800 ml/min! In effect, the dialyzer seemed to increase its surface area at the higher dialylsate flow rate. This effect was substantial and present with a variety of makes and models. One interpretation of this is, that the dialysate gets better penetration into the fiber bundle at higher flow rates, or that dialysate turbulence is increased at higher flows. The results confirm those previously reported by Hoenich et al..

The practical implication of these findings is, that going from a dialysate flow of 500 to 800 ml/min will allow more shortening of dialysis time (to maintain the same Kt/V) than predicted by urea kinetic equations which do not take this effect into account. (John T. Daugirdas, M.D., University of Illinois at Chicago)

The abstract of this paper is available from the National Library of Medicine's PubMed site: click here .