Energy expenditure and caloric targets during continuous renal replacement therapy under regional citrate anticoagulation. A viewpoint

Background: Indirect calorimetry (IC) is the gold standard for measuring energy expenditure in critically ill patients However, continuous renal replacement therapy (CRRT) is a formal contraindication for IC use. Aims: To discuss specific issues that hamper or preclude an IC-based assessment of energy expenditure and correct caloric prescription in CRRT-treated patients. Methods: Narrative review of current literature. Results: Several relevant pitfalls for validation of IC during CRRT were identified. First, IC measures CO 2 production (VCO 2 ) and O 2 consumption to calculate resting energy expenditure (REE) with the Weir equation. VCO 2 measurements are influenced by CRRT because CO 2 is exchanged during the blood purification process. CO 2 exchange also depends on type of pre- and/or postdilution fluid(s). CO 2 dissolves in different forms with dynamic but unpredictable impact on VCO 2 . Second, the effect of immunologic activation and heat loss on REE caused by extracorporeal circulation during CRRT is poorly documented. Third, caloric prescription should be adapted to CRRT-induced in- and efflux of different nutrients. Finally, citrate, which is the preferred anticoagulant for CRRT, is a caloric source that may influence IC measurements and REE. Conclusion: Better understanding of CRRT-related processes is needed to assess REE and provide individualized nutritional therapy in this condition.