Validation of carbon dioxide production (VCO2) as a tool to calculate resting energy expenditure (REE) in mechanically ventilated critically ill patients: a retrospective observational study

Background: Indirect calorimetry (IC) measurement is considered the gold standard for the assessment of resting energy expenditure (REE). It is based on the measurement of oxygen and carbon dioxide consumption (VO2 and VCO2, respectively). However, its use is limited by cost and technical issues. It has been proposed that, in critically ill patients, the analysis of VCO2 obtained from the ventilator alone may be used as an accurate method to assess REE in ventilated patients. This retrospective study aimed to assess the accuracy of VCO2 measurement alone in the determination of REE. Methods: This was a retrospective study conducted at the general intensive care unit of a single university-affiliated tertiary medical center. Patients included were invasively ventilated and their REE was measured by using IC. The respiratory quotients (RQs) were set at 0.8, 0.85, and 0.89. Data were collected from computerized patient files. REE obtained from the ventilator by using VCO2 (REE-VCO2) alone was compared with REE obtained from IC (REE-IC). Results: Measurements were obtained for 80 patients, and 497 REE-IC measurements were compared with REEVCO2 obtained at the same time. The mean REE-IC was 2059.5 +/- 491.7 kcal/d. The mean REE-RQs corresponding to RQs of 0.80, 0.85, and 0.89 were 1936.8 +/- 680.0, 2017.8 +/- 708.8, and 2122.1 +/- 745.4 kcal/d, respectively. REE-VCO2 derived from an RQ of 0.85 had the lowest mean difference from REE-IC. Whereas accuracy was higher using an RQ of 0.85, agreement (between 85% and 115%) was highest using an RQ of 0.89. Conclusions: The level of agreement of REE obtained from VCO2 readings with REE obtained from IC was generally low. IC continues to be the recommended method for REE assessment.