Background: Indirect calorimetry allows the determination of energy expenditure in critically ill patients by measuring oxygen consumption (VO2) and carbon dioxide production (VCO2). Recent studies have demonstrated variable performance of "breath-by-breath" instruments compared to mixing chamber technology. The aim of this study was to validate two modern devices (E-sCOVX and Quark RMR) against a reference method (Deltatrac II).
Method: Measurements of VO2/VCO2 with the test and reference devices were performed simultaneously over a 20-min period in mechanically ventilated adult intensive care unit patients. Accuracy and precision of instruments were analyzed using Bland-Altman plots.
Results: Forty-eight measurements in 22 patients were included for analysis. Both E-sCOVX and Quark RMR overestimated VO2 and VCO2 compared to Deltatrac II, corresponding to a 10% higher mean resting energy expenditure. Limits of agreement of resting energy expenditure within ± 2 standard deviations were ± 461 kcal/24 h (± 21% expressed as percentage error) for ΔE-sCOVX-Deltatrac II and ± 465 kcal/24 h (± 22%) for ΔQuark RMR-Deltatrac II.
Conclusion: Both test devices overestimate VO2 and VCO2 compared to Deltatrac II. The observed limits of agreement are comparable to those commonly accepted in evaluations of circulatory monitoring, and significantly less than results from predictive equations. We hypothesize that the discrepancy between methods is due to patient/ventilator-related factors that affect the synchronization of gas and spirometry waveforms.
Trial registration: Australian New Zealand Clinical Trials Registry, Trial ID ACTRN12615000205538. Date registered 3 March 2015.