SELECTION OF CHANNELS FOR EEG MONITORING IN ANESTHESIOLOGY

The conventional multichannel electroencephalogram is quite inconvenie nt for long-term monitoring in the operating theatre or intensive care unit. Recording of the EEG would be easier if a small number of chann els was sufficient. Aiming at reduction of channels, leads from differ ent regions of the scalp were analysed visually and with regard to the ir spectral content. Methods. Electrode placements corresponded to the International 10/20 System (Fig.1). EEG recordings were made with a c onventional device (ES 12000), a personal computer, and a spectral ana lyser. Two-channel recordings. Retrospective analysis was performed on data from 392 patients (age 14-90 years) whose anaesthesia was induce d with various anaesthetics/narcotics, for instance thiopental, ketami ne, etomidate, halothane, and enflurane. The EEG was recorded using C- 3-P-3 and C-z-A(1). For each patient the changes of spectral parameter s during the course of the induction were plotted and visually analyse d. For statistical analyses a 30-s epoch of each patient was randomly selected from the first few minutes after the beginning of induction. Ten-channel recordings. In ten gynaecological patients (age 26-55 year s) EEG recordings were performed during induction of anaesthesia with thiopental in combination with fentanyl, N2O and O-2. The set of chann els consisted of F-z-Cb-1, F-3-Cb-1, C-z-Cb-1, C-3-Cb-1, P-3-Cb-1, O-z -Cb-1, F-z-F-3, F-3-C-3, C-3-P-3, and P3Oz. The electrodes F-z. and F- 3. were positioned on the forehead near to F-z and F-3, respectively. These sites were chosen because they allow easy application of electro des. The relationship between channels was calculated with Bravais-Pea rson's coefficient of correlation for the power and the absolute power in the frequency bands delta (0.5-3.5 Hz), theta (3.5-7.5 Hz), alpha (7.5-12.5 Hz), and beta (>12.5 Hz). Results. In visual and statistical analyses of the two- and ten-channel recordings under the influence o f anaesthetics/narcotics, similar changes of EEG activity could be obs erved in all channels. Although differences in the absolute power of t he frequency bands were present, there was high conformity in the comp osition of the spectral content of the different channels. Classificat ion of the EEG into stages of anaesthesia by means of a single channel led to consistent results for all channels. Alpha activity as leading feature of the awake state predominated occipitally. In channels incl uding the region around the ears, contamination with ERG artifacts was observed. Conclusions. EEG patterns under the influence of different anaesthetics/narcotics are adequately represented by a reduced number of channels. For the choice of an appropriate set of channels the foll owing aspects should be considered. Contamination with artifacts shoul d be as low as possible, electrode sites should easily be accessible, and special features of the awake state should be identifiable. Experi ence with routinely conducted EEG recordings in the operating theatre and the intensive care unit showed that the channels C-3-P-3 or C-4-P- 4 provide a sufficient basis for automatic staging of the depth of ana esthesia, which is implemented in the EEG monitor Narkograph.