Pain during continuous nitrous oxide administration.

The continuous administration of nitrous oxide, in subjects who remain conscious, caused only a relatively small increase in the threshold of appreciation of pain, induced experimentally by tibial pressure and a hot wire applied to the thenar eminence. The threshold reached a maximum at 10 min. A concentration of 50% nitrous oxide caused only a marginally greater effect than 33%. In some subjects pain thresholds returned to control values during administration of the gas. In contrast, when a concentration of 50% nitrous oxide was approached in step changes of 7-10% over a period of 45 min, the effect on the pain threshold was more than doubled. It was concluded that, with respect to analgesia, adaptation of the nervous system to a constant concentration of nitrous oxide can occur in some subjects.