Daugirdas JT, Greene T, Depner T, Gotch F, Keshaviah P, Star
RA
Estimation of double pool volume from single-pool urea
distribution volume
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1510 1996
The urea distribution volume (V) is normally calculated from pre- and
post-dialysis BUN, treatment time (t) and a predicted value for urea
clearance (K) using a single-pool urea kinetic model. This modelled V is
used to predict t needed to achieve a given Kt/V using a predicted K and to
calculate the contribution of renal function and ultrafiltration to Kt/V.
The modelled V is also compared to V calculated anthropometrically as part
of the quality assurance of the dialysis. A modelled V which is higher than
expected indicates that the expected Kt was not delivered.
Unfortunately, even if Kt is precisely delivered, the single-pool model
does not return an accurate value for V except when Kt/V(equilibrated)=1.
The error will result in a low value for V if Kt/V<1 and a high value if
Kt/V> 1. The error will also be proportional to K/V. A more realistic
two-pool model is needed to calculate the correct V.
The authors used data from the HEMO study to confirm this error. In the
HEMO study, 452 patients were treated using Kt/V=1.4. After a baseline
period about half of these were randomized to receive Kt/V = 1.0. As
predicted, V, calculated using the single-pool urea model fell by 4.6% when
Kt/V was reduced. The authors also proposed a simple empirical equation to
correct the single-pool error.
The authors have confirmed the need to recognize and correct the errors of
single-pool models. They have also demonstrated that these errors can be
corrected by a simple equation without resorting to a complex double-pool
model. This work compliments earlier work by the same authors in which a
simple correction equation was developed empirically to correct the error
in Kt/V returned by single-pool models. I was surprised that the authors
did not simply derive the V correction equation from their related Kt/V
correction equation rather than developing a new empirical solution.
(James Tattersall, M.D., United Kingdom)
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Am Soc Nephrol
Basic hemodialysis :
Adequacy, prescription, urea kinetics