Can calculation of energy expenditure based on CO(2) measurements replace indirect calorimetry?

BACKGROUND: Methods to calculate energy expenditure (EE) based on CO(2) measurements (EEVCO(2)) have been proposed as a surrogate to indirect calorimetry. This study aimed at evaluating whether EEVCO(2) could be considered as an alternative to EE measured by indirect calorimetry. METHODS: Indirect calorimetry measurements conducted for clinical purposes on 278 mechanically ventilated ICU patients were retrospectively analyzed. EEVCO(2) was calculated by a converted Weir’s equation using CO(2) consumption (VCO(2)) measured by indirect calorimetry and assumed respiratory quotients (RQ): 0.85 (EEVCO(2)_0.85) and food quotient (FQ; EEVCO(2)_FQ). Mean calculated EEVCO(2) and measured EE were compared by paired t test. Accuracy of EEVCO(2) was evaluated according to the clinically relevant standard of 5% accuracy rate to the measured EE, and the more general standard of 10% accuracy rate. The effects of the timing of measurement (before or after the 7th ICU day) and energy provision rates (<90 or ≥90% of EE) on 5% accuracy rates were also analyzed (chi-square tests). RESULTS: Mean biases for EEVCO(2)_0.85 and EEVCO(2)_FQ were -21 and -48 kcal/d (p = 0.04 and 0.00, respectively), and 10% accuracy rates were 77.7 and 77.3%, respectively. However, 5% accuracy rates were 46.0 and 46.4%, respectively. Accuracy rates were not affected by the timing of the measurement, or the energy provision rates at the time of measurements. CONCLUSIONS: Calculated EE based on CO(2) measurement was not sufficiently accurate to consider the results as an alternative to measured EE by indirect calorimetry. Therefore, EE measured by indirect calorimetry remains as the gold standard to guide nutrition therapy.