Effect of low carrier gas flows (50% oxygen/50% nitrous oxide) on inspired oxygen tension in anesthetized dogs.

Low carrier gas flows of 50% oxygen/50% nitrous oxide were evaluated with regard to their effect on inspired oxygen (PIO2) and carbon dioxide (PICO2) tension during maintenance inhalational anesthesia in dogs, using a circle-system anesthetic machine. A total flow rate of 15 ml/min/kg was associated with a PIO2 of 230 +/- 25 mm of Hg and a PICO2 of 0.4 +/- 0.4 mm of Hg after 4 hours of methoxyflurane anesthesia. End-tidal carbon dioxide, minute volume ventilation, and ventilation frequency did not change during the 4 hours. Lower total flow rates were associated with collapse of the reservoir bag, or a PIO2 less than 200 mm of Hg, or both.