Alterations in pain threshold and psychomotor response associated with subanaesthetic concentrations of inhalation anaesthetics in humans

Abstract

Summary: We studied the effects of six inhalation anaesthetics at subanaesthetic 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 threshold as measured by a radiant heat algometer, and prolonged the response time to auditory stimuli. In contrast, halothane. enflurane, isoflurane and sevo-flourane produced prolongation of the response time to auditory stimuli but did not influence pain perception. The pain threshold with nitrous oxide remained significantly increased 30 min after its discontinuation, while the response time returned to the preinhalation value. We conclude that nitrous oxide and methoxyflurane possess both analgesic and hypnotic actions but halothane, enflurane, isoflurane andsevoflurane do not have an analgesic action at subanaesthetic concentrations, and the analgesic action of nitrous oxide persists after its elimination. (Br. J. Anaesth. 1993; 70: 684-686). © 1993 British Journal of Anaesthesia.