ASN95 Hot Topic-- Urea Kinetics

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 during dialysis.

Estimating and predicting rebound. There were numerous papers supporting the use of the Smye and Daugirdas/Schneditz formulae 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 artefactual component.

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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