Annals of biomedical engineering
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Traditionally, measurement of pulmonary O2 uptake uses mass balance of N2 to correct for differences between inspired and expired volume (V) due to temperature (T) and relative humidity (RH). Often during anesthesia, N2 balance cannot be invoked due to high inspired O2 fraction (FIO2) or nonsteady state conditions. Then, O2 uptake per breath (VO2,br) must use assumed or measured T and RH differences between inspirate and expirate. ⋯ When tissue O2 consumption decreases relative to minute ventilation, T and RH errors have a greater effect on VO2 br because the error in inspired V affects a smaller VO2,br. At lower barometric pressure, RH errors affect VO2,br more because water vapor V occupies a larger fraction of inspired V. In summary, because inspired RH and T can vary significantly during anesthesia, a fast-response humidity and T sensor, combined with flow and FO2 measurements, are needed to allow accurate determination of VO2,br x VO2,br should become an important measure of metabolism and patient wellness during anesthesia.