The Effects of Subanaesthetic Doses of Isoflurane and Enflurane on the Auditory Evoked Response and Two Tests of Psychometric Performance

We investigated the effects of subanaesthetic doses of isoflurane and enflurane on the auditory evoked response, and two tests of psychoraotor performance, the choice reaction time and tracking task. The use of subanaesthetic doses of isoflurane has been used for dental sedation. Thirty fasting volunteers (mean age 24y) had scalp electrodes placed at vertex and both mastoids. Fifteen were randomly allocated to receive doses of isoflurane of inspired concentrations (0, 0.31, 0.5, 0.75%), placebo or enflurane (0, 0.17, 0.42, 0.6%) and 15 received isoflurane (0, 0.2, 0.31, 0.4%) or placebo. At each step change in volatile agent concentration 20min were allowed for equilibration. The reclining volunteers received the gases through a Hudson mask connected to a Bain circuit (flow > 10lmin-1), delivering 30% oxygen in air. Basic physiological variables, heart rate, arterial pressure, axillary temperature, and oxygen saturation were monitored throughout each 2.5h experiment. End-tidal concentrations of carbon dioxide could not be practically measured in these conscious volunteers. Control baseline brainstem, and long latency recordings were made before introducing the volatile agents or placebo. A probability of p s 0.05 was considered significant. The basic physiological variables remained within normal ranges throughout each experiment. The brainstem auditory evoked responses (BSAER) latencies and amplitudes did not change significantly for either drug compared with the placebo, except for the wave V latency which increased significantly at 0.5% isoflurane and 0.42% enflurane. The N100 latency of the long-latency auditory evoked responses (LLAER) increased significantly from the placebo at 0.3, 0.4, 0.5% isoflurane and enflurane 0.42%. The N100 amplitude differed significantly from the placebo at 0.2, 0.3, 0.5% isoflurane and 0.6% enflurane. The N100 latency seems to produce a graded response to isoflurane which might be useful in automatic control of anaesthesia, providing the waveforms can be reliably identified automatically. The two tests of psychometric function were affected by the drugs. Mean reaction times and tracking times increased. The variability of the above two measures increased as measured by the coefficients of variation