Nitrous oxide analgesia and altitude.

The analgesic effectiveness of 50% nitrous oxide and oxygen or oxygen-enriched air, measured by variations in pain threshold, was studied at an altitude of 1460 m and simulated altitudes of sea level and 3300 m. At sea level pressures, 50% nitrous oxide exerted a similar analgesic effect to that found for morphine, raising the pain thresholds by a mean of 71.5%. At 1460 m, the increase in pain threshold produced was 40% and at 3300 m the increase in pain threshold was only 19%. The difference between the analgesic effects of nitrous oxide at each altitude was statistically significant. It is concluded that moderate altitudes significantly reduce the effectiveness of nitrous oxide in a manner directly related to the partial pressure of nitrous oxide at each altitude.