Another complication during their stay is the inability to feed themselves. Nutrition is a cornerstone in the care for the critical ill and should be started within 3 days of admission to the intensive care unit. To optimize a nutritional prescription, protein and energy targets need to be defined. Predicting formulae based on anthropometric measures and other parameters can be used to calculate the caloric need but indirect calorimetry (IC) remains the gold standard. Caloric need can be derived from Energy expenditure which is calculated with the Weir's equation using carbon dioxide (CO2) production (VCO2) and oxygen (O2) consumption (VO2). Therefore, it is underestimated if CO2 is lost through other means than the normal respiratory route. Hence one of the contra-indications of IC is CRRT.