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. Becaus e of the continuous mode of therapy and the high fluid turnover usually ass ociated 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 hyperpyrectic patients. Inapp ropriate glucose concentrations of some substitution fluids can result in e xcessive glucose intake. Most substitution and/or dialysate fluids used for CRRTs contain lactate as organic anion. In disease states with impaired la ctate utilization, such as acute or chronic liver failure, and/or with incr eased endogenous lactate formation such as in shock states, this can result in hyperlactemia and is potentially associated with various adverse side e ffects. Small molecular weight substances such as amino acids or water-solu ble vitamins are lost in relevant amounts. With convective clearance and th e high molecular cut-off of synthetic membranes, medium-sized molecules suc h as hormones and cytokines are also filtered, but the pathophysiologic rel evance of this observation remains to be specified. Moreover, synthetic mem branes used for CRRTs have adsorptive properties for a variety of molecules , such as cytokines, complement factors, and endotoxin. Continuous blood me mbrane interactions cause the phenomena of bioincompatibility and a low-gra de inflammatory reaction with potentially adverse consequences on protein m etabolism and immunocompetence. In designing a nutritional program for a pa tient on CRRT, these metabolic effects-especially the loss of nutritional s ubstrates-must be considered. Certainly, most of these side effects, such a s 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 potentiall y beneficial.