Monitoring intraoperativer Wachzustände Vegetative Zeichen, isolierte Unterarmtechnik, Elektroenzephalogramm und akustisch evozierte Potentiale

Zusammenfassung

Es sind verschiedene Methoden entwickelt worden, um Wachzustände während Allgemeinanästhesie zu erfassen. Die wichtigsten sind der PRST-Score, der aus Veränderungen von Blutdruck, Herzfrequenz, Schweißsekretion und Tränenfluß errechnet wird, die isolierte Unterarmtechnik, das verarbeitete EEG sowie die akustisch evozierten Potentiale mittlerer Latenz. Der PRST-Score erscheint nicht sehr verläßlich, um intraoperative Wachzustände anzuzeigen. Die isolierte Unterarmtechnik kann nur kurzzeitig angewendet werden. Das verarbeitete EEG, vor allem die abgeleiteten Parameter Medianfrequenz und spektrale Eckfrequenz sind wichtige wissenschaftliche Instrumente, um zentrale Anästhetikaeffekte zu quantifizieren. Mit ihrer Hilfe sind pharmakodynamische-pharmakokinetische Modelle entwickelt worden. Sie erscheinen jedoch weniger geeignet, um intraoperative Wachzustände anzuzeigen. Die akustisch evozierten Potentiale mittlerer Latenz sind unter einer Vielzahl von Anästhetika dosisabhängig unterdrückt. Sie korrelieren mit Wachzuständen während Narkose, expliziten und impliziten Gedächtnisfunktionen nach Anästhesie. Sie erscheinen als vielversprechende Methode, um intraoperative Wachzustände zu erfassen. Zukünftige Studien werden Schwellenwerte der verschiedenen Parameter der akustisch evozierten Potentiale für intraoperative Wachzustände, explizite und implizite Erinnerungen an intraoperativ präsentierte Information für verschiedene Anästhetika zu bestimmen haben. Erst dann wird man definitiv die Brauchbarkeit der Methode im klinischen Alltag bewerten können.

Abstract

Several methods have been developed to quantify central anaesthetic effects and monitor awareness during general anaesthesia. The most important of these are the PRST score, calculated from changes in blood pressure, heart rate, sweating, and tear production, the isolated forearm technique, where the patient is allowed to move during surgery, the processed electroencephalogram (EEG) and the derived parameters median frequency (MF) and spectral-edge frequency (SEF), and midlatency auditory evoked potentials (MLAEP). In clinical practice, the application of individual doses of anaesthetics is generally guided by autonomic vegetative clinical signs such as changes in blood pressure, heart rate, sweating, and tear production, quantified as the PRST score. Unfortunately, these parameters are not very reliable with regard to predicting the suppression of consciousness and awareness, especially when high-dose opioids are used in patients with cardiovascular medications and a variety of accompanying diseases. The PRST score probably indicates mainly the autonomic responses to painful stimuli, and seems to be useful in guiding the individual use of analgesics. The isolated forearm technique is a useful test of the patient's responsiveness during general anaesthesia, and thus an instrument for investigating the incidence of awareness during different anaesthetic regimens and when muscle relaxants are imployed. A disadvantage is that it can only be used for 20 to 30 min because of pressure-induced nerve blocks or lesions. It can not be employed when long-term relaxation is necessary and consciousness and awareness are to be monitored continuously. The processed EEG and the derived parameters MF and SEF are important scientific tools to quantify central effects of many anaesthetics and opioid analgesics that allow the development of pharmacodynamic-pharmacokinetic models of anaesthetic action. MF has proven to be useful in monitoring closed-loop feedback of intravenous drug administration. Unfortunately, until now there have been no clinical studies that document the usefulness of MF or SEF with regard to predicting intraoperative arousal or awareness. To the contrary, some experimental data failed to predict imminent arousal and response to surgical incision or verbal commands by MF or SEF. Therefore, the EEG seems to be of limited value for monitoring awareness, consciousness, or memory formation during anaesthesia. MLAEP are suppressed in a dose-dependent fashion by many general anaesthetics and correlate with wakefulness, awareness, and explicit and implicit memory during anaesthesia and seem to be a promising method of monitoring awareness during anaesthesia. Nevertheless, future studies will have to determine threshold values for the different MLAEP parameters for intraoperative awareness and explicit and implicit recall of intraoperatively presented information for the different commonly used anaesthetics. Only then will it be possible to determine the usefulness of the method in clinical practice.