Knowledge of actual energy expenditure is important for optimal nutrition management

Providing optimal nutrition for the critically ill patient is an important part of ICU therapy, reducing both morbidity and mortality1,2. Knowledge of actual energy expenditure is important for optimal nutrition management3. Expired gas analysis indirect calorimetry, i.e. determining energy expenditure from oxygen (O2) consumption and carbon dioxide (CO2) production, is considered the gold standard for assessing energy expenditure; however, the accuracy and reliability of different indirect calorimeters may differ.


Quantifying energy expenditure from indirect calorimeters

Such systems measure respiratory air flow ((expired (VE) or inspired (VI) ventilation), and fractions of oxygen and carbon dioxide in expired air (FECO2 and FEO2, respectively). From these measurements, computations are performed for oxygen consumption (VO2), carbon dioxide production (VCO2), and the respiratory exchange ratio (RER; ratio of VO2 and VCO2). Therefore the performance of an indirect calorimeter depends on its ability to measure the actual respiratory air flow and gas fractions and its reproducibility, or repeatability of measuring them.


Nutrition management

For measurements of energy expenditure to be optimally useful for nutrition management, researchers and healthcare professionals need to have confidence in the accuracy of the measurements and “true” variability in repeated measures. A recent article by Kaviani et al4 suggested that an ideal indirect calorimeter should have a coefficient of variation (CV; expressed as a ratio of standard deviation to the mean) for resting energy expenditure (REE) of less than 3%. This is in agreement with an earlier article by Blond et al.5 which recommended that an ideal respiratory gas analyzer for nutritional studies should be accurate within several percent and have a within-person reliability of 3% or better. This is important in order not to inflate physiological variation and is clinically quite acceptable.


Actual REE= 1500 Kcal/day

Measured REE (with 3% CVIC) = 1500 ± 45 Kcal/day

Measured REE (with 6% CVtotal) = 1500 ± 90 Kcal/day = 1410 – 1590 Kcal/day

For a healthy subject with a REE of 1500 Kcal/day, a coefficient of variation (CVIC) of 3% represents a standard deviation of ±45 Kcal/day or a total difference of less than 100 Kcal/day due to instrumental variation. In consideration that a 5% day-to-day subject variability is generally observed6, the total coefficient of variation, CVTotal = (52 + 32)1/2 = 6%, total standard deviation of ±90 Kcal/day could be expected.

An over or under estimation of nutritional needs over a period of a few days can lead to unfavorable outcomes for a patient. Based on observational data derived from a study of supplemental parenteral nutrition, a mean total energy deficit of 2300 kcal over 8 days is associated to an increased rate of nosocomial infections7.

Considering the clinical importance of providing patients with optimal nutritional therapy, it is not only important to use an instrument which accurately measures respiratory gases and REE but also one that reliably measures actual physiological variation. In a previous article, we discussed the difference in performance of various indirect calorimeters and had a look at their validity and reliability. Consequently, to have confidence in measurements, it is essential to choose an instrument with exceptional accuracy and reliability such as the Omnical. Maastricht instruments supplies the Omnical system that allows researchers to easily conduct various studies. Contact us today for more information.


  1. Strack van Schijndel, R.J., et al., Optimal nutrition during the period of mechanical ventilation decreases mortality in critically ill, long-term acute female patients: a prospective observational cohort study. Crit Care, 2009. 13(4): p. R132.
  2. Danish Ahmad, K.J.a.C.H., Nutrition and Indirect Calorimetry, in Oncology Critical Care, D.J. Hoag, Editor. 2016.
  3. Singer, P., et al., The tight calorie control study (TICACOS): a prospective, randomized, controlled pilot study of nutritional support in critically ill patients. Intensive Care Med, 2011. 37(4): p. 601-9.
  4. Kaviani, S., et al., Determining the Accuracy and Reliability of Indirect Calorimeters Utilizing the Methanol Combustion Technique. Faseb Journal, 2017. 31.
  5. Blond, E., et al., A new indirect calorimeter is accurate and reliable for measuring basal energy expenditure, thermic effect of food and substrate oxidation in obese and healthy subjects. European e-Journal of Clinical Nutrition and Metabolism. 6(1): p. e7-e15.
  6. St-Onge, M.P., et al., A new hand-held indirect calorimeter to measure postprandial energy expenditure. Obes Res, 2004. 12(4): p. 704-9.
  7. Heidegger, C.P., et al., Optimisation of energy provision with supplemental parenteral nutrition in critically ill patients: a randomised controlled clinical trial. Lancet, 2013. 381(9864): p. 385-93.