Zur Ableitungsauswahl für ein EEG-Monitoring in der Anästhesie

Zusammenfassung

EEG-Registrierungen mit einer größeren Anzahl von Ableitungen sind technisch aufwendig. Als Routinemonitoringverfahren z.B. zur Beurteilung der Effekte von Anästhetika/Narkotika bei der Steuerung der Narkose sind sie daher wenig geeignet. Die EEG-Ableitung wäre einfacher, wenn sie mit wenigen Kanälen erfolgen könnte. Daher sollten in dieser Arbeit EEG-Ableitungen von unterschiedlichen Regionen des Kopfs mit dem Ziel einer Kanalreduktion visuell und im Hinblick auf den spektralen Gehalt verglichen werden. Dazu wurden 392 EEG-Kurven mit zwei Kanälen aus Narkoseeinleitungen mit unterschiedlichen Anästhetika/Narkotika und zehn EEG-Aufzeichnungen mit jeweils zehn Kanälen bei Narkoseeinleitungen mit Fentanyl und Thiopental untersucht. Die visuellen und statistischen Auswertungen zeigen, daß unter dem Einfluß von Anästhetika/Narkotika in unterschiedlichen Ableitungen gleichartige EEG-Bilder auftreten und anhand einer einzelnen dieser Ableitungen eine Stadienklassifikation möglich ist. Bei eigenen Routinemessungen haben sich die Ableitungen C ₃ −P ₃ oder C ₄ −P ₄ als Grundlage für ein EEG-Monitoring in der Anästhesie als geeignet erwiesen. Diese Ableitungen bieten neben leichter Zugänglichkeit der Ableiteelektroden gute Voraussetzungen für die Erkennung des Wach-EEG und sind im Vergleich mit anderen Ableitungen artefaktarm.

Abstract

The conventional multichannel electroencephalogram is quite inconvenient for long-term monitoring in the operating theatre or intensive care unit. Recording of the EEG would be easier if a small number of channels was sufficient. Aiming at reduction of channels, leads from different regions of the scalp were analysed visually and with regard to their spectral content.

Methods. Electrode placements corresponded to the International 10/20 System (Fig. 1). EEG recordings were made with a conventional device (ES 12000), a personal computer, and a spectral analyser.

Two-channel recordings. Retrospective analysis was performed on data from 392 patients (age 14–90 years) whose anaesthesia was induced with various anaesthetics/narcotics, for instance thiopental, ketamine, etomidate, halothane, and enflurane. The EEG was recorded using C₃–P₃ and C_z–A₁. For each patient the changes of spectral parameters during the course of the induction were plotted and visually analysed. 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 years) EEG recordings were performed during induction of anaesthesia with thiopental in combination with fentanyl, N₂O and O₂. The set of channels consisted of F_z′–Cb₁, F_3′–Cb₁, C_z–Cb₁, C₃–Cb₁, P₃–Cb₁, O_z–Cb₁, F_z′–F_3′, F_3′–C₃, C₃–P₃, and P₃–O_z. The electrodes F_z′ and F_3′ were positioned on the forehead near to F_z and F₃, respectively. These sites were chosen because they allow easy application of electrodes. The relationship between channels was calculated with Bravais-Pearson'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 of anaesthetics/narcotics, similar changes of EEG activity could be observed in all channels. Although differences in the absolute power of the frequency bands were present, there was high conformity in the composition of the spectral content of the different channels. Classification 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 including the region around the ears, contamination with EKG 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 following aspects should be considered. Contamination with artifacts should be as low as possible, electrode sites should easily be accessible, and special features of the awake state should be identifiable. Experience with routinely conducted EEG recordings in the operating theatre and the intensive care unit showed that the channels C₃–P₃ or C₄–P₄ provide a sufficient basis for automatic staging of the depth of anaesthesia, which is implemented in the EEG monitor Narkograph.