How to interpret and apply the results of indirect calorimetry studies: A case-based tutorial

Evidence is growing that the individual adjustment of energy targets guided by indirect calorimetry (IC) can improve outcome. With the development of a new generation of devices that are easier to use and rapid, it appears important to share knowledge and expertise that may be used to individualize nutrition care. Despite the focus of this tutorial being on one contemporary device, the principles of IC apply across existing devices and can assist tailoring the nutrition prescription and in assessing response to nutrition therapy. The present tutorial addresses its clinical application in intubated mechanically ventilated and spontaneously breathing adult patients (canopy), i.e. it covers the range from critical illness to outpatients.

The cases that are presented show how the measured energy expenditure (mEE), and the respiratory quotient (RQ), i.e. the ratio of expired CO₂ to consumed O₂, should be applied in different cases, to adapt and individualize nutrition prescription, as it is a good marker of over- or underfeeding at the different stages of disease. The RQ also informs about the patient's body's capacity to use different substrates: the variations of RQ indicating the metabolic changes revealing insufficient or excessive feeding. The different cases reflect the use of a new generation device as a metabolic monitor that should be combined with other clinical observations and laboratory biomarkers. The tutorial also points to some shortcomings of the method, proposing alternatives.

Several IC devices exist which have a variable precision as shown by studies comparing different devices. The above text focuses on one single device (COSMED Q-NRG®) which has been shown to be extremely precise, and is used by the authors of this text. Despite some differences existing between devices, the principal data provided (VO2, VCO2, RQ and EE) are the same across devices, enabling the analysis of the measurement results: therefore, the above case analysis extends to other available IC devices. IC is a tool that provides metabolic information far beyond the only value of EE. With the RQ, it tells exactly how the patient is handling the received amount of energy: low values <0.80 reflect insufficient feeding and values over 1.0 indicate too much. IC is a metabolic monitor, not only a guide of energy target and has a wide range of applications in clinical practice. Training is required as numbers should not be applied blindly.