Anaesthetic depth and auditory evoked potentials

We have investigated the auditory evoked response (AER) in a series of experimental and clinical studies to assess its usefulness as a measure of depth of anaesthesia. Three questions were posed. Does the AER show: –l) graded changes with increasing concentrations of general anaesthetics which are similar for different anaesthetic drugs 2) appropriate changes with surgical stimulation 3) changes with loss of consciousness? The AER was found to satisfy these criteria in that six general anaesthetics, belonging to different chemical groups, showed similar graded reductions in the amplitudes and increases in the latencies of the AER waves originating from the primary cortical section of the auditory nerve pathway. These changes were reversed by surgical stimulation indicating that the AER does not simply act as a concentration monitor but reflects true depth of anaesthesia. In addition there were changes in these early cortical AER waves which indicate potential awareness. The clinical investigations also suggested that the AER is a valid measure of depth of anaesthesia. A numerical index derived from the AER decreased when the anaesthetic blood concentration increased and increased when surgical stimulation increased in severity. The numerical index gave the same value at the same depth of anaesthesia in different patients. This means that a monitoring device based on the AER would not have to be calibrated separately for each patient and further that it could be attached to the patient when they are already anaesthetised. Further work is in progress on the display of information for the anaesthetist and in reducing the time taken to produce a reliable response.