The analgesic effect in humans of subanaesthetic isoflurane concentrations evaluated by experimentally induced pain

Abstract

The analgesic effect at subanaesthetic concentrations of ether, trichloroethylene, methoxyflurane, and halothane has been investigated with either clinical assessment or pain threshold measurements [1,2], but with partly contradicting results. The purpose of the present study was to evaluate the analgesic effect of isoflurane on different pain modalities, using experimental pain models. Twelve healthy volunteers were studied at three randomly chosen subanaesthetic isoflurane concentrations from a low (0.10-0.14 vol%), a middle (0.16-0.20 vol%), and a high (0.22-0.26 vol%) concentration group. Thermal pain detection and the pain tolerance threshold for argon laser stimulation, mechanical pressure pain detection and pain tolerance thresholds, a 2 min immersion of the hand in ice water with continuous recording of the perceived pain, the threshold of the nociceptive reflex to single electrical stimulation, and the threshold for the summation of the nociceptive flexor reflex to repeated electrical stimulation (temporal summation) were used as experimental models to assess analgesia [3]. Statistical analysis was performed with the Friedman repeated measures ANOVA on ranks. No significant changes could be observed in the response to heat, cold, or mechanical pressure, at any of the subanaesthetic concentrations of isoflurane used. Compared with baseline values, a significant increase in all three isoflurane concentration groups was found for the nociceptive reflex threshold to single stimulation, but the difference between the different isoflurane concentrations was not statistically significant. The threshold for the temporal summation of the nociceptive reflex was unchanged. Reaction time did not change in the low concentration group, but was significantly changed in the middle and high concentration groups, presumably as a result of sedation. Subanaesthetic isoflurane concentrations only changed the reaction of the nociceptive reflex to single stimulations. This could be interpreted as an analgesic effect, but in a comparative study with propofol and alfentanil, a far greater increase in the reflex threshold to single stimulations was found with propofol than with alfentanil (unpublished results), indicating that sedation can also increase the threshold. Furthermore, isoflurane even in high concentrations does not attenuate the blood pressure increase to standard painful stimuli [4]. In experimental pain models, subanaesthetic isoflurane concentrations have little or no analgesic potency.