Metabolic aspects of continuous renal replacement therapies

Continuous renal replacement therapies (CRRTs) are associated with a broad pattern of additional metabolic effects beyond renal detoxification. Because of the continuous mode of therapy and the high fluid turnover usually associated with CRRTs, these side effects can become clinically relevant. With many CRRT systems currently used, heat loss is considerable, but CRRTs can also be used to modulate body temperature in hyperpyretic patients. Inappropriate glucose concentrations of some substitution fluids can result in excessive glucose intake. Most substitution and/or dialysate fluids used for CRRTs contain lactate as organic anion. In disease states with impaired lactate utilization, such as acute or chronic liver failure, and/or with increased endogenous lactate formation such as in shock states, this can result in hyperlactemia and is potentially associated with various adverse side effects. Small molecular weight substances such as amino acids or water-soluble vitamins are lost in relevant amounts. With convective clearance and the high molecular cut-off of synthetic membranes, medium-sized molecules such as hormones and cytokines are also filtered, but the pathophysiologic relevance of this observation remains to be specified. Moreover, synthetic membranes used for CRRTs have adsorptive properties for a variety of molecules, such as cytokines, complement factors, and endotoxin. Continuous blood membrane interactions cause the phenomena of bioincompatibility and a low-grade inflammatory reaction with potentially adverse consequences on protein metabolism and immunocompetence. In designing a nutritional program for a patient on CRRT, these metabolic effects--especially the loss of nutritional substrates--must be considered. Certainly, most of these side effects, such as the excessive load of lactate or the loss of nutrients, are undesirable. However, some side effects, such as the modulation of body temperature and the elimination of endotoxin and/or mediators, might be at least potentially beneficial.