Analgesic effect in humans of subanaesthetic isoflurane concentrations evaluated by evoked potentials

The aim of this study was to see if an analgesic effect of subanaesthetic concentrations of isoflurane could be detected with evoked potentials elicited by nociceptive stimuli. We studied 10 healthy volunteers breathing three steady-state subanaesthetic concentrations of isoflurane (0.08, 0.16 and 0.24 vol% end-tidal). Reaction time, subjective pain intensities and evoked vertex potentials to laser (LEP) and electrical (SEP) stimuli were recorded and compared with auditory evoked potentials (AEP). Compared with baseline, the subanaesthetic concentrations of isoflurane did not change the latencies of the evoked potentials, but caused a significant reduction in the amplitudes of the LEP and SEP at 0.16 and 0.24 vol% and of the AEP at all three concentrations. There were no changes in perceived pain intensity, and isoflurane produced similar reductions in evoked potentials elicited by both nociceptive and non-nociceptive stimuli. The reaction time was increased significantly at 0.24 vol% isoflurane. The results demonstrated that subanaesthetic isoflurane concentrations caused similar changes in evoked potentials with both painful and non-painful stimuli, with no effect on perceived pain intensity.