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A Comparison of Carbon Dioxide Elimination Measurements Between a Portable Indirect Calorimeter and Volumetric Capnography Monitor: An In Vitro Simulation

BACKGROUND: Gas exchange measurements for carbon dioxide elimination (V̇CO2) and oxygen consumption (V̇O2) have been used to derive resting energy expenditure and guide energy prescription. Volumetric capnography is used in intensive care units and provides V̇CO2 measurements that could be used for titrating respiratory and nutritional support. We have recently suggested that measuring V̇CO2 may be sufficient to obtain a reasonable estimate of energy expenditure. However, data describing the accuracy of gas exchange measurement devices are limited. METHODS: We used an in vitro simulation model to test the accuracy of gas exchange measurements by 2 devices: the CCM Express indirect calorimeter and the NM3, a volumetric capnography monitor. A Huszczuk gas injection system combined with a high-fidelity lung simulator was used to simulate V̇O2 and V̇CO2 values in the pediatric and adult range. Bland-Altman analysis was used to examine the agreement between the measured and simulated values across a range of tidal volumes and gas exchange values. Additionally, agreement between the 2 devices was examined. RESULTS: During the adult simulation with the CCM Express, the mean bias (95% CI) for V̇CO2 values was −12.6% (−16.4 to −8.8%) and −17.5% (−19.9 to −15.1%) for V̇O2 values. For the pediatric simulation with the CCM Express, mean bias for V̇O2 was −14.7% (−16.4 to −13.0%) and V̇CO2 was −10.9% (−13.5 to −8.3%). For the adult and pediatric simulations with the NM3, the bias for V̇CO2 was −8.2% (−15.7 to −0.7%) and −8.3% (−19.4 to −2.8%), respectively. Between the 2 devices, the mean bias was −4.4% (−10.2 to 1.3%) and −2.3% (−11.4 to 6.8%) for the adult and pediatric V̇CO2 simulations, respectively. CONCLUSIONS: Currently available portable gas exchange monitors demonstrated acceptable agreement with reference V̇O2 and V̇CO2 values in an in vitro simulation. The devices demonstrated good agreement with each other.