COMPARISON OF THE EFFECTS OF HALOTHANE, ISOFLURANE AND METHOXYFLURANEON THE ELECTROENCEPHALOGRAM OF THE HORSE

We have investigated in eight ponies the effects of three different en d-tidal concentrations of halothane, isoflurane and methoxyflurane on median (F50) and 95% spectral edge (F95) frequencies of the EEG and th e second differential (DD) of the middle latency auditory evoked poten tial (MLAEP). The three concentrations of each agent were chosen to re present approximately the minimum alveolar concentration (MAC), 1.25 M AC and 1.5 MAC for each agent. During halothane anaesthesia, F95 decre ased progressively as halothane concentration increased, from mean 13. 9 (SD 2.6) at 0.8% to 11.9 (1.1) at 1.2%. DD was lower during anaesthe sia with the highest concentration (21 (6.5)) compared with the lowest (27.6 (11.4)). There were no significant changes in F50. During isofl urane anaesthesia, there was a small, but significant increase in F95 between the intermediate and highest concentrations (10.2 (1.5) to 10. 8 (1.6)). There were no changes in F50 and DD. Values of F95, F50 and DD at all isoflurane concentrations were similar to those of halothane at the highest concentration. During methoxyflurane anaesthesia, F95 and F50 decreased progressively as methoxyflurane concentration was in creased, from 21.3 (0.7) and 6.5 (1), respectively, at 0.26%, to 20.1 (0.6) and 5.6 (0.8), respectively, at 0.39%. DD was lower during anaes thesia with the highest concentration of methoxyflurane (25.7 (7.8)) c ompared with the lowest (39.7 (20.6)). Values of F95, F50 and DD at al l methoxyflurane concentrations were higher than those seen with halot hane at the lowest concentration. The different relative positions of the dose-response curves for EEG and MLAEP changes compared with antin ociception (MAC) changes suggest differences in the mechanisms of acti on of these three agents. These differences may explain the incomplete adherence to the Meyer-Overton rule.