Daugirdas J, Greene T, Levin N, Maroni B, Schulman G, Star R
Modeled/anthropometric volume ratios: Effects of race, age, and
different formulas
Am Soc Nephrol
J Am Soc Nephrol (abstract)
(Sep) 7:1510 1996
In the prescription of dialysis, a value for the urea distribution
volume (V) is needed. In CAPD, V is normally calculated from the
Watson equations. In HD, V can be calculated from urea kinetic
modelling and measurements of dialysis time (t), dialyzer clearance
(K), ultrafiltration rate, pre- and post-dialysis BUN. If K is not
measured directly, an approximate value calculated from dialyzer blood
flow, dialysate flow and the dialyser's overall clearance (KoA=
theoretical K at infinite blood and dialysate flows) can be used. Any
difference between the K value used to calculate V and the actual mean
K over the entire treatment will result in a proportional error in V.
The V calculated by urea modelling in this way is normally used in
the quality assurance of dialysis. It is compared to V calculated from
the Watson equation. Higher values are assumed to be caused by
underdelivery of K or t. In both HD and CAPD, the V calculated using
the Watson equations are used as the reference. However, the Watson
equations result from an empirical linear analysis of radio-isotope
derived body water measurements and anthropometric measurements in
normal subjects. The Watson equations have never been validated in
patients with renal failure
In the HEMO study, modelled V was measured and compared with Watson V
on at least 3 occasions in 567 patients. The K values used in the
calculations were supported by direct measurements in some of the
patients. The authors analysed this data and showed that, the Watson V
was, on average, 1% higher in blacks and 6% higher than the modelled
V in nonblacks. The authors recommend an appropriate adjustment to the
Watson equations for nonblack dialysis patients.
The data presented strongly suggests that the Watson equations
overestimate V in nonblack hemodialysis patients. The authors suggest
an appropriate adjustment to the Watson equation to take this into
account.
Comment: Since the modelled V is indirect and subject to
numerous sources of error, this finding needs to be confirmed by
radioisotope dilution studies in dialysis patients. If genuine, this
overestimation may be because white dialysis patients have lower
muscle mass than the normal subjects studied by Watson (muscle has a
relatively high water content). Blacks generally have a higher muscle
mass which tends to oppose this error. Alternatively, black dialysis
patients may have worse blood access, resulting in lower effective K
and higher modelled V.
(James Tattersall, M.D., United Kingdom)
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Am Soc Nephrol
Basic hemodialysis :
Adequacy, prescription, urea kinetics