Bankhead MM, Toto RD, Star RA
Accuracy of urea removal estimated by kinetic models
Kidney Int
(Sep) 48:785-793 1995
Dialysate-side urea kinetics is complex with currently
available technology, but some previous work has suggested that
dialysate side measurements are essential because blood sided
measurements of Kt/V or solute reduction index (SRI) cannot be done
reliably. Star et al compared 4 blood sided methods of urea kinetics
(which do not involve any delayed post-dialysis blood samples) with
total dialysate collection (which also involves measurement of the 1-hr
post-dialysis BUN). Each of 8 patients were studied on 4 separate
occasions. The four blood-sided methods were: formal single-pool UKM, a
formal 2-pool (ECF-ICF) model (based on 45 and 90 min intradialytic
samples), the Smye method (based on either a 45 min or a 90 min
intradialytic sample), and a rate equation recently described by
Daugirdas and Schneditz (ASAIO '95), which estimates dKt/V based on K/V,
and requires only a pre and postdialysis sample. The principal
findings were, that both the Smye method and the rate equation
accurately predicted SRI. As the rate equation requires only a pre and
post-dialysis sample, it has certain advantages over the Smye method.
The implications of the study are, that perhaps total dialysate
collection or dialysate sampling is not required in the majority of
patients to accurately assess SRI.
Additional comments: Based on its good predictive power in both
the study by Star and in analysis of pre-pilot and pilot NIH HEMO Study
data (see ASN '95 abstract by Daugirdas et al), the rate equation is
being used to target Kt/V in the full scale NIH HEMO trial. The rate
equation may not work for patients taking vasodilators, esp. hydralazine
or minoxidil or in patients with abnormally high or low cardiac output
values (see ASN '95 abstract by TO George et al). With regard to the
Smye method, the intradialytic and postdialysis samples should be
drawn using exactly the same technique; i.e, if the 1 hr sample is
drawn at full blood flow and the post sample using a 15 sec slow flow
technique, results will not be accurate in patients with access
recirculation (AR). If both samples are drawn at full blood flow in
patients with AR, the Smye method may still give erroneous results when
the degree of AR is not constant throughout dialysis (see Pflederer et
al, Kidney Int. Sep '95, 48:832-837). When utilizing the Smye method,
it is best to wait 15 sec (or 2 min) after slowing the pump to 50 ml/min
when taking both the intradialytic and post-dialysis sample.
(Daugirdas)
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Basic hemodialysis :
Adequacy, prescription, urea kinetics