ANALYSIS OF THE FREQUENCY-SPECTRUM OF THE EQUINE ELECTROENCEPHALOGRAMDURING HALOTHANE ANESTHESIA

The electroencephalogram (EEG) has been used in human clinical anaesth esia as an indicator of cortical activity and as an indicator of the d epth of anaesthesia. It would be useful if it provided a reliable indi cation of the depth of anaesthesia of horses. In this study anaesthesi a was induced with thiopentone and maintained with halothane in nine p onies. The end tidal halothane concentration (P-E-Hal) was monitored a nd 20 seconds of EEG were recorded at 0.8 per cent, 1.0 per cent and 1 .2 per cent halothane, equivalent to the minimum alveolar concentratio n (MAC), 1.25 MAC and 1.5 MAC. Each 20 second block of data was divide d into one second segments and averaged to give one second of averaged EEG from which a frequency spectrum was obtained by using a fast Four ier transformation. The power of the waveform at low frequency (1 to 3 Hz) was compared with that at higher frequency (9 to 11 Hz). The medi an frequency and 95th percentile (spectral edge) were also calculated. The spectral edge frequency had the best correlation with P-E-Hal.