DESFLURANE AND NITROUS-OXIDE, BUT NOT NONIMMOBILIZERS, AFFECT NOCICEPTIVE RESPONSES

Nonimmobilizers (previously called nonanesthetics) do not prevent move ment in response to a noxious stimulus, even at doses predicted to pro duce anesthesia. We hypothesized they would also lack antinociceptive effects. We tested this prediction using the tail-flick latency (TFL) test. As predicted, the two nonimmobilizers tested (1,2-dichlorohexafl uorocyclobutane and perfluoropentane) did not alter TFL, whereas desfl urane and nitrous oxide both lengthened TFL (nitrous oxide at a lower minimum alveolar anesthetic concentration [MAC]-multiple than desflura ne). In addition, we found that 0.1 MAC desflurane had a hyperalgesic effect (shortened TFL). Implications: We studied the response of anima ls inhaling anesthetics or nonimmobilizers (compounds predicted to be anesthetics from the Meyer-Overton relation) to painful stimuli. Nonim mobilizers had no effect on these responses; at a low partial pressure , desflurane was hyperalgesic; nitrous oxide and, at higher partial pr essures, desflurane were antinociceptive.