INTRAOPERATIVE AWARENESS AND AUDITORY-EVO KED POTENTIALS

Midlatency auditory evoked potentials (MLAEP) are suppressed dose-depe ndently during anaesthesia with a variety of general anaesthetics. The refore, MLAEP have been proposed to measure depth of anaesthesia and t o indicate intraoperative awareness. Several studies give evidence of a close relationship between MLAEP and motor signs of wakefulness, int raoperative awareness, and explicit and implicit memory functions duri ng general anaesthesia. Summarising these data, one may conclude that there is a close hierarchical relation between cognitive function, mem ory and wakefulness during anaesthesia, and MLAEP latencies. A short N b latency below 45 ms is consistent with conscious awareness and unimp aired memory function with explicit recall and adequate response to co mmands. When Nb latency increases to 45-50 ms, it may be associated wi th conscious awareness. Patients still respond to commands, but memory formation is impaired and explicit recall is lost. A further increase of Nb latencies seems to be consistent with unconscious awareness, ch aracterised by implicit memory of intraoperative events; 60 ms seems t o be the threshold value for motor signs of wakefulness during anaesth esia. With a further increase of MLAEP latency during anaesthesia, con scious awareness and memory formation, explicit and implicit recall, r esponse to commands, and spontaneous purposeful movements during anaes thesia are blocked. The new volatile anaesthetic sevoflurane leads to a dose-dependent increase in MLAEP peak latencies and a decrease in ML AEP amplitudes. At about 1.5 vol.% end-expiratory sevoflurane concentr ation, MLAEP are significantly suppressed and Nb latency is in the ran ge of 68-80 ms. Therefore, from the present data and those from the li terature, one may expect that sevoflurane at concentrations greater th an 1,5 vol.% for general anaesthesia would be able to suppress awarene ss phenomena such as purposeful movements, auditory perception, intrao perative wakefulness and awareness, memory formation, and explicit and implicit recall of intraoperative events.