Subjective and objective sensory responses to inhalation of nitrous oxide and methoxyflurane.

An investigation, in volunteers, into the sensory effects of nitrous oxide and methoxyflurane showed that the subjective changes of reduction in visual awareness and heightened auditory awareness were not supported by objective measurement. Vision remained normal until the subjects became unresponsive, whilst hearing was in fact diminished. The analgesia produced by nitrous oxide was late in appearance relative to the other sensory changes and it disappeared very rapidly upon recovery. In contrast, analgesia appeared early with methoxyflurane and persisted well into the recovery period. The principal changes found were alterations in orientation in space, of intellectual attitudes, of mood and of emotional attitudes. The emotional attitudes were easily disturbed by extraneous noise or conversation and the need for a quiet approach not only to the induction of anaesthesia but also to the recovery from anaesthesia is stressed.