Importance of temperature and humidity in the measurement of pulmonary oxygen uptake per breath during anesthesia.

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 degrees C) caused percent errors in VO2,br of 5.6% during FIO2 = 0.2% and 28.8% during FIO2 of unity. Per(-57.6 x FIO2-0.115) VO2,br was given by (change in RH) (R2 > 0.999). Errors in inspired T (during constant inspired RH of 0.5) had similar effects on percent error in VO2,br( =-8.75 x FIO2-0.093) x (change in T) (R2 = 0.999). Because inspired VO2 is larger at higher FIO2 and because VO2,br is the difference between inspired and expired VO2, VO2,br is most affected by the inspired V error at the largest FIO2 . 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.