LAUSR

We read with great interest the recent article by Lambell et al. discussing nutrition therapy in critically ill patients and the role of indirect calorimetry (IC) [1]. Indirect calorimetry allows for the measurement of VO2 and VCO2 through the ventilator and is the gold standard method for measuring resting energy expenditure (REE) in critical illness when ideal test conditions are implemented [1]. Both the European (ESPEN) and American (ASPEN/ SCCM) clinical practice guidelines recommend the use of IC to measure energy expenditure [1]. At this time, there are only three randomized controlled trials (RCTs) comparing IC with formulae (25 kcal/kg/day) [1]. In all three RCTs, indirect calorimetry was feasible and energy targets were more closely met when using IC in place of fixed energy prescription [1]. While supporting the use of IC in some settings, we believe it is important to warn clinicians about a limitation of the technique, particularly when patients are under continuous renal replacement therapy (CRRT) [2]. Fifty percent of the critically ill septic and non-septic population develop acute kidney injury, and 25% require renal replacement therapy (RRT) [3]. Patients undergoing CRRT lose a substantial amount of CO, in gas form and as bicarbonate, in the effluent, making IC unreliable [4]. This is also true for IC performed in patients receiving extracorporeal membrane oxygen (ECMO), unless a mathematical correction is applied [5]. It is important that clinicians are aware not only of the indications of IC, but also of the limitations.