title: Energy expenditure in critically ill patients estimated by population-based equations, indirect calorimetry and CO2-based indirect calorimetry.
abstract: BACKGROUND
  Indirect calorimetry (IC) is the reference method for measurement of energy expenditure (EE) in mechanically ventilated critically ill patients. When IC is unavailable, EE can be calculated by predictive equations or by VCO2-based calorimetry. This study compares the bias, quality and accuracy of these methods.
  
  
  METHODS
  EE was determined by IC over a 30-min period in patients from a mixed medical/postsurgical intensive care unit and compared to seven predictive equations and to VCO2-based calorimetry. The bias was described by the mean difference between predicted EE and IC, the quality by the root mean square error (RMSE) of the difference and the accuracy by the number of patients with estimates within 10 % of IC. Errors of VCO2-based calorimetry due to choice of respiratory quotient (RQ) were determined by a sensitivity analysis, and errors due to fluctuations in ventilation were explored by a qualitative analysis.
  
  
  RESULTS
  In 18 patients (mean age 61 ± 17 years, five women), EE averaged 2347 kcal/day. All predictive equations were accurate in less than 50 % of the patients with an RMSE ≥ 15 %. VCO2-based calorimetry was accurate in 89 % of patients, significantly better than all predictive equations, and remained better for any choice of RQ within published range (0.76-0.89). Errors due to fluctuations in ventilation are about equal in IC and VCO2-based calorimetry, and filtering reduced these errors.
  
  
  CONCLUSIONS
  This study confirmed the inaccuracy of predictive equations and established VCO2-based calorimetry as a more accurate alternative. Both IC and VCO2-based calorimetry are sensitive to fluctuations in respiration.
authors.affiliation: Center for Model-based Medical Decision Support (MMDS), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7E, 9220, Aalborg East, Denmark. mlr@hst.aau.dk.
authors.forename: Mark Lillelund
authors.initials: ML
authors.lastname: Rousing
authors.affiliation: Center for Model-based Medical Decision Support (MMDS), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7E, 9220, Aalborg East, Denmark.
authors.forename: Mie Hviid
authors.initials: MH
authors.lastname: Hahn-Pedersen
authors.affiliation: Center for Model-based Medical Decision Support (MMDS), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7E, 9220, Aalborg East, Denmark.
authors.forename: Steen
authors.initials: S
authors.lastname: Andreassen
authors.affiliation: Center for Model-based Medical Decision Support (MMDS), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7E, 9220, Aalborg East, Denmark.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Pielmeier
authors.affiliation: Department of Intensive Care, Erasme University Hospital, Université Libre de Bruxelles, 808 Route de Lennik, 1070, Brussels, Belgium.
authors.forename: Jean-Charles
authors.initials: JC
authors.lastname: Preiser
keywords: Caloric intake; Critically ill; Energy expenditure; Indirect calorimetry; Metabolic rate; Nutritional support; Respiratory quotient; VCO2