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. This numerical analysis study examines how errors in inspired RH and T can affect VO2,br. Equations were developed to simulate a baseline metabolic and ventilatory condition. A unit error in inspired RH of 0.5 (during constant inspired T of 22°C) caused percent errors in VO2,br of 5.6% during FIO2 = 0.2% and 28.8% during FIO2 of unity. Percent error in VO2,br was given by (-57.6 FIO2 -0.115) ⋅(change in RH) \documentclass[12pt]{minimal}
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$$ \left( {R^2 >0.999} \right) $$
\end{document}. Errors in inspired T (during constant inspired RH of 0.5) had similar effects on percent error in \documentclass[12pt]{minimal}
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$$ V_{O_2 ,br} \left( { = - 8.75F_{I_{O_2 } } - 0.093} \right) $$
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$$ V_{O_2 ,br} \left( { = - 8.75F_{I_{O_2 } } - 0.093} \right) $$
\end{document} Because inspired \documentclass[12pt]{minimal}
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$$ V_{O_2 } $$
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$$ F_{I_{O_2 } } $$
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$$ V_{O_2 } $$
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$$ V_{O_2 ,br} $$
\end{document} is most affected by the inspired V error at the largest \documentclass[12pt]{minimal}
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$$F_{I_{O_2 } } $$
\end{document} When tissue O2 consumption decreases relative to minute ventilation, T and RH errors have a greater effect on \documentclass[12pt]{minimal}
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$$ V_{_{O_2 } ,br} $$
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$$ V_{_{O_2 } ,br} $$
\end{document} At lower barometric pressure, RH errors affect \documentclass[12pt]{minimal}
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$$ V_{_{O_2 } ,br} $$
\end{document} 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 \documentclass[12pt]{minimal}
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$$ F_{O_2 } $$
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$$ V_{O_2 ,br} $$
\end{document} should become an important measure of metabolism and patient wellness during anesthesia. © 2000 Biomedical Engineering Society.
