MONITORING INTRAOPERATIVE AWARENESS - VEG ETATIVE SIGNS, THE ISOLATEDFOREARM TECHNIQUE, THE ELECTROENCEPHALOGRAM, AND AUDITORY-EVOKED POTENTIALS

Several methods have been developed to quantify central anaesthetic ef fects and monitor awareness during general anaesthesia. The most impor tant of these are the PRST score, calculated from changes in blood pre ssure, heart rate, sweating, and tear production, the isolated forearm technique, where the patient is allowed to move during surgery, the p rocessed electroencephalogram (EEG) and the derived parameters median frequency (MF) and spectral-edge frequency (SEF), and midlatency audit ory evoked potentials (MLAEP). In clinical practice, the application o f individual doses of anaesthetics is generally guided by autonomic ve getative clinical signs such as changes in blood pressure, heart rate, sweating, and tear production, quantified as the PRST score. Unfortun ately, these parameters are not very reliable with regard to predictin g 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 us eful in guiding the individual use of analgesics. The isolated forearm technique is a useful test of the patient's responsiveness during gen eral anaesthesia, and thus an instrument for investigating the inciden ce 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 conscio usness and awareness are to be monitored continuously. The processed E EG and the derived parameters MF and SEF are important scientific tool s to quantify central effects of many anaesthetics and opioid analgesi cs that allow the development of pharmacodynamic-pharmacokinetic model s of anaesthetic action. MF has proven to be useful in monitoring clos ed-loop feedback of intravenous drug administration. Unfortunately, un til now there have been no clinical studies that document the usefulne ss of MF or SEF with regard to predicting intraoperative arousal-or aw areness. To the contrary, some experimental data failed to predict imm inent arousal and response to surgical incision or verbal commands by MF or SEF. Therefore, the EEG seems to be of limited value for monitor ing awareness, consciousness, or memory formation during anaesthesia. MLAEP are suppressed in a dose-dependent fashion by many general anaes thetics and correlate with wakefulness, awareness, and explicit and im plicit 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 parame ters 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 usefulne ss of the method in clinical practice.