Depth of anaesthesia, awareness and EEG

Inapparent adverse intraoperative wakefulness is still a relevant problem i n modern anaesthetic routine. It can be associated with serious negative ef fects on the postoperative recovery of the patients. Several different proc edures have been developed to monitor and therefore avoid intraoperative si tuations of wakefulness during general anaesthesia. The most promising meth ods are the PRST-score, calculated from changes in the blood pressure, hear t rate, sweating and tear production, the so-called isolated forearm techni que, spontaneous EEG and its derived parameters such as spectral edge frequ encies or BIS and finally mid-latency auditory evoked potentials. The obser vation of clinical autonomic signs, even including the calculation of the P RST-score does not seem to be valid enough to indicate or predict intraoper ative wakefulness. The isolated forearm technique can be regarded as the mo st reliable tool to detect intraoperative wakefulness, but it can only be a pplied for a very limited period of time. The processed EEG with the median frequency, spectral edge frequency or bispectral index are important scien tific tools to quantify central anaesthetic effects especially to develop p harmacodynamic-pharmacokinetic models of anaesthetic action. But they seem to be less suitable to indicate situations of intraoperative wakefulness or awareness. The mid-latency auditory evoked potentials are depressed dose-d ependently by a series of anaesthetic agents, which correlate with the occu rrence of situations of intraoperative wakefulness and awareness. There is a hierarchical correlation between certain values of the MLAEP and intraope rative wakefulness defined by purposeful movements, amnesic awareness with only implicit recall and conscious awareness with explicit recall. For some of the most commonly used anaesthetics reasonable threshold values of the MLAEP for the different states of consciousness have already been determine d. Future studies in broad patient populations with all of the different ro utinely used anaesthetics and procedures will have to finally identify the importance of the recording of midlatency auditory evoked potentials as a r outine method to assess the depth of anaesthesis.