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Second gas effect of N₂O on oxygen uptake

Koh-ichi Nishikawa, MD^*, Fumio Kunimoto, MD, Yukitaka Isa, MD, Sotaro Miyoshi, MD^*, Ken-ichiro Takahashi, MD^*, Toshihiro Morita, MD^*, Hidehiro Arii, MD and Fumio Goto, MD^*

^* From the Departments of Anesthesiology and
Intensive Care Unit,
Gunma University School of Medicine, Maebashi, Japan and the Department of Anesthesia, Ashikaga Red Cross Hospital, Ashikaga, Japan

Address correspondence to: Koh-ichi Nishikawa MD PhD, Molecular Neuropharmacology Lab., Department of Anesthesiology, A-1050, Weill Medical College of Cornell University, 1300 York Ave., New York, NY 10021, USA. Phone: 212-746-1150; Fax: 212-746-4879; E-mail: nishikaw{at}news.sb.gunma-u.ac.jp

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 nitrous oxide (N₂O) in humans. The purpose of this study was to examine the changes in both end-tidal oxygen fraction (F_ETO₂) and PaO₂ after N₂O inhalation in patients under general anesthesia.

Methods: Fifteen patients scheduled for elective orthopedic surgery were enrolled in this study. Anesthesia was maintained with the continuous infusion of propofol and with nitrogen (N₂) and oxygen (O₂) (6 L•min^–1, F_IO₂, 0.33). In all patients, the lungs were ventilated with a Servo 900C ventilator equipped with a gas mixer for O₂, N₂O, and N₂. After obtaining baseline data, N₂ was replaced with N₂O maintaining F_IO₂ constant at 0.33. The changes in fractional concentration of O₂, N₂O, and N₂ were continuously measured using mass spectrometer in a breath-by-breath basis. PaO₂ and hemodynamic data were obtained at 1, 5, 10, 30 and 60 min after the start of N₂O inhalation.

Results: Five minutes after N₂O inhalation, F_ETO₂ increased from 0.27 ± 0.01 to 0.31 ± 0.02 (P < 0.01) and PaO₂ increased from 172.0 ± 22.5 mmHg to 201.0 ± 10.3 mmHg (P < 0.01). These effects produced by N₂O were observed 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 correlated with the increase in F_ETO₂ after N₂O inhalation.

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