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 **.