The levels of nitrous oxide analgesia.

The literature on the effects of various levels of nitrous oxide is reviewed. The rapid uptake and excretion of the gas facilitates the recognition of four levels or zones depending upon the concentration used. In the lightest level of nitrous oxide, Zone 1, moderate analgesia is present with few side effects. In Zone 2 psychological side effects of dissociation are present in addition to more potent analgesia. With the third level of nitrous oxide. Zone 3, marked somnolence is present in addition to almost total analgesia and amnesia, but patient contact is still preserved. This zone is known as "analgesic anaesthesia". Patient contact is lost in the deepest level of nitrous oxide, that of light anaesthesia (Zone 4). In this level there is a risk of involuntary movements. In conclusion it is suggested that, of the four zones, the first, of moderate analgesia, is the one of greatest clinical use for analgesia though "analgesic anaesthesia" may prove to have special indications. The fourth zone is of use in modem anaesthetic techniques.