Middle Molecules as Uremic Toxins: Still a Viable Hypothesis?

The dose of dialysis in hemodialysis patients: impact on nutrition

Multiple lines of evidence have indicated that the dose of hemodialysis impacts upon patient outcome. Among these outcome measures, nutrition is inextricably linked to the adequacy of the treatment. All of the methods of determining dialysis adequacy are based on assessing the removal of toxic substances retained in renal failure, the majority of which are derivatives of protein metabolism. Urea kinetics, employing urea as a surrogate for quantifying the elimination of small molecular weight nitrogenous substances, is the method that has been most thoroughly validated to date as defining a dose range for thrice-weekly hemodialysis: Both inadequate and optimal levels of hemodialysis dose have been identified by prospective, randomized clinic trials utilizing Kt/V(urea) as the index of adequacy. The impact of urea kinetics on nutritional status during thrice-weekly hemodialysis is discussed. Recently, in an attempt to improve outcome beyond that achievable with thrice-weekly hemodialysis, alternative regimens, consisting of daily treatments, have received increasing interest. In order to compare the dose of hemodialysis associated with these regimens with conventional thrice-weekly regimens in terms of removal of small molecular weight substances, standard Kt/V(urea), a parameter that combines treatment dose with treatment frequency, and thus allows for various intermittent therapies to be compared to continuous therapy, must be used. In addition, membrane flux and middle molecule removal, factors that have not yet been well defined as parameters of adequacy during thrice-weekly regimens, may be shown to be important indices with longer hemodialysis treatments, particularly daily nocturnal hemodialysis. The impact that these alternative regimens have had on nutritional status in hemodialysis patients and how they compare to conventional therapy are important considerations.

Current standards for dialysis adequacy

Multiple lines of evidence suggest that inadequately prescribed or delivered dosage of hemodialysis is associated with increased morbidity and mortality. Conversely, retrospective studies indicate that increased levels of hemodialysis reverse this trend of poor outcome. The results of the National Cooperative Dialysis Study (NCDS), a prospective and randomized trial, suggested that urea kinetic modeling was a valid method of quantifying the dose of hemodialysis delivered and also identified a level of treatment below which a number of adverse events occurred. In the ensuing years, urea kinetic modeling has been increasingly applied to quantitate dialysis. The application of the NCDS results as well as the limitations of the study are reviewed, and the modifications in applying urea kinetic modeling due to urea rebound are discussed. To assess the impact of newer membranes and higher levels of dialysis on an older hemodialysis population with more comorbid conditions than the subjects studied in the NCDS, a 5-year, multicenter, prospective and randomized trial, the HEMO Study, has recently been initiated.