Second gas effect of N2O on oxygen uptake

Purpose: The concept of the second gas effect is well known, however, there have been no studies that showed the relationship between alveolar oxygen concentration and arterial oxygen tension (PaO2) after the inhalation of ni trous oxide (N2O) in humans. The purpose of this study was to examine the c hanges in both end-tidal oxygen fraction (FETO2) and PaO2 after N2O inhalat ion in patients under general anesthesia, Methods: Fifteen patients scheduled for elective orthopedic surgery were en rolled in this study. Anesthesia was maintained with the continuous infusio n of propofol and with nitrogen (N-2) and oxygen (O-2) (6 L.min(-1), F1O2, 0.33). In all patients, the lungs were ventilated with a Servo 900C ventila tor equipped with a gas mixer for O-2, N2O, and N-2. After obtaining baseli ne data, N-2 was replaced with N2O maintaining F1O2 constant at 0.33. The c hanges in fractional concentration of O-2, N2O, and N-2 were continuously m easured using mass spectrometer in a breath-by-breath basis. PaO2 and hemod ynamic data were obtained at 1, 5, 10, 30 and 60 min after the start of N2O inhalation, Results: Five minutes after N2O inhalation, FETO2 increased from 0.27 +/- 0 .01 to 0.3 +/- 0.02 (P < 0.01) and PaO2 increased from 172.0 +/- 22.5 mmHg to 201.0 +/- 0.3 mmHg (P < 0.01). These effects produced by N2O were observ ed for 30 min, Conclusions: These results confirm the concept of second gas effect of N2O on oxygen uptake in humans and provide evidence that the PaO2 increase corr elated with the increase in FETO2 after N2O inhalation.