Comparison of resting energy expenditure measured with metabolic cart and calculated with predictive formulas in critically ill patients on mechanical ventilation

Introduction: The purpose was to compare the resting energy expenditure (REE) measured with the Q-NRGTM+ metabolic-cart (MREE) with REE predicted by equations (the Harris-Benedict formula and an equation developed in ward, REE-HB and REE-W, respectively). We also aimed to assess the agreement of the measurements of oxygen consumption (V̇O2) and carbon dioxide production (V̇CO2) at different inspired fractions of oxygen (FiO2). Methods: 27 mechanically ventilated ICU patients were enrolled. V̇O2 and V̇CO2 were measured by Q-NRGTM+ during breathing 40% and 60% FiO2. MREE was compared with REE-W and REE-HB normalized for body weight. Results: V̇O2 was 233.0 (95.2) ml/min and 217.5 (89.8) ml/min at FiO2 40% and 60%, respectively (NS). V̇CO2 was 199.0 (91.7) ml/min at FiO2 40%, and 197.5 (85.5) ml/min at FiO2 60% (NS). The REE estimated from the equations was significantly different from the MREE. The best agreement was found for the Harris-Benedict equation without correction for stress-factors. Harris-Benedict equation corrected overestimates REE. Conclusions: This new metabolic cart Q-NRGTM+ provides a concordance of values for V̇O2 and V̇CO2 when measured at different FiO2, and is a reliable tool for estimating energy expenditure and assessing the nutritional needs of the patient. This study demonstrates that the estimation of REE using predictive formulas does not allow accurate calculation of metabolic demands in ventilated intensive care patient. However, predictive equations allow for a rapid assessment of REE and calculation of the amount of energy derived from different substrates.