ALTERATIONS IN PAIN THRESHOLD AND PSYCHOMOTOR RESPONSE ASSOCIATED WITH SUBANAESTHETIC CONCENTRATIONS OF INHALATION ANESTHETICS IN HUMANS

We studied the effects of six inhalation anaesthetics at subanaestheti c concentrations of 0.2 MAC on pain threshold and psychomotor function in six healthy volunteers. When compared with 100% oxygen inhalation, nitrous oxide and methyoxy-flurane significantly increased pain thres hold as measured by a radiant heat algometer, and prolonged the respon se time to auditory stimuli. In contrast, halothane, enflurane, isoflu rane and servo-flurane produced prolongation of the response time to a uditory stimuli but did not influence pain perception. The pain thresh old with nitrous oxide remained significantly increased 30 min after i ts discontinuation, while the response time returned to the preinhalat ion value. We conclude that nitrous oxide and methoxyflurane possess b oth analgesic and hypnotic actions but halothane, enflurane, isofluran e and sevoflurane do not have an analgesic action at subanaesthetic co ncentrations, and the analgesic action of nitrous oxide persists after its elimination.