Post-dialysis urea rebound.
This year, the most presented theme was on the post-dialysis rebound and
multi-compartment models. There was a general consensus that conventional
single-pool UKM is not a sufficiently precise tool for monitoring hemodialysis.
After dialysis there is a significant upward rebound in the urea concentration
which takes about 30 minutes to complete. By ignoring this rebound, single-pool
UKM overestimates Kt/V by up to 30%
In support of the regional blood flow model.
TO George presented date demonstrating that
patients with a lower cardiac index
(calculated by the bioimpedance method) had a significantly greater rebound.
This provides supporting evidence for the regional blood flow model proposed by
Schneditz and Daugirdas (ASAIO J 40:M667-M673, 1994). In this model, the rebound is
explained by delays in transfer of urea
into the fistula due to low blood flow in certain poorly-perfused regions of the
body. M. Germain and
C Ronco independently showed that the post-dialysis rebound
was reduced and efficiency of dialysis increased when the patients exercised
Estimating and predicting rebound.
There were numerous papers supporting the use of the Smye and Daugirdas/Schneditz
to predict and correct for the effects of the post-dialysis rebound. The
Daugirdas/Schneditz formula (ASAIO J 41:M719-M724, 1994) is derived from the regional blood
flow model and predicts the
equilibriated Kt/V (eKt/V) from the conventional single-pool arterial
Kt/V (sKt/V) using
the formula eKt/V= sKt/V-0.6.(sKt/V)/t+0.03. The Daugirdas/Schneditz
formula was shown to
predict eKt/V with reasonable precision in most patients. This shows that there
is a consistent relationship between rebound and K/V. The Smye formula uses a
third BUN measured about 40 mins into dialysis to predict the equilibrated post
-dialysis BUN. The Smye formula was particularly useful when there was access
recirculation and a greater than normal post-dialysis rebound.
Factors Influencing the precription of dialysis.
Gotch, Daugirdas and Held all presented papers demonstrating that larger
patients tend to receive less dialysis. This is because dialysis is still
prescribed relatively empirically with inadequate individualization of dialysis
so that larger patients did not receive correspondingly greater Kt.
Linkage between dialysis dose and nutrition.
Kt/V has been shown to correlate positively with nPCR in both HD and CAPD. This
may be a genuine connection as better dialysed patients have a higher Kt/V and
feel better so eat more and have a higher nPCR. However, it has been pointed out
that both Kt/V and nPCR are calculated from the same data and may correlate due
to mathematical linkage. There were a number of papers advocating both views.
Harty (237/601), who had previously advocated the mathematical linkage theory showed, in a
longitudinal study, that, as dialysis dose was deliberately decreased in PD patients,
nutritional intake fell. This
demonstrates that a component of the connection between adequacy and nutrition
is genuine. Tattersall, on the other hand, showed that, in a cross-sectional
study, there was a correlation between nPCR and Kt/V but this correlation
disappeared when the Kt/V was independently calculated using the plasma
disappearence rate of iohexol. This showed that there is an significant
Dialysis dose and outcome.
The debate centered on wether there is any advantage to the patient in
increasing Kt/V above 1.2. Data was presented by
Levin which suggested that
eKt/V of 1.0-1.2 gave optimal outcome. However, this data from the 1991 USRDS
case-mix study had relatively few patients with high Kt/V. Also, it is possible
that patients who are doing poorly may be prescribed high Kt/V, obscuring any
improved survival in this group. The prospective study now underway may settle
this debate. (James Tattersall)
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