The Auditory Evoked Response as an Indicator of Awareness

Abstract

The latency of the early cortical wave Nb of the auditory evoked response (AER) was compared with responses to Tunstall's isolated forearm test, while the concentration of nitrous oxide was progressively reduced during light anaesthesia in seven patients. A threshold Nb latency of 44.5 ms was chosen to discriminate between an early cortical AER containing three waves and that with two waves of longer latency. When Nb latency decreased below this threshold, four of the patients has positive responses, indicating awareness. The addition of a volatile anaesthetic abolished any response, and increased Nb latency to more than 44.5 ms. The three wave AER pattern, therefore, is associated with a depth of anaesthesia at which awareness occurs.

Effects of benzodiazepines on mid-latency auditory evoked potentials

“…Nevertheless, there is experimental evidence from several studies indicating a close relationship between MLAEP and motor signs of wakefulness, intraoperative awareness and explicit and implicit memory functions during general anaesthesia. Thornton et at 15 and Newton et at 16 studied the relation between MLAEP and response to verbal commands, conscious awareness and recall and in surgical patients and anaesthetized volunteers during different sub-MAC concentrations of N20 and isoflurane. When MLAEP peak latencies were significantly increased compared with the awake state no response to verbal commands, conscious awareness or recall could be observed.…”

Motor signs of wakefulness during general anaesthesia with propofol, isoflurane and flunitrazepam/fentanyl and midlatency auditory evoked potentials

“…Nevertheless, there is experimental evidence from several studies indicating a relationship between MLAEP and wakefulness during general anaesthesia. Thornton et al [15] and Newton et al [17] studied the relationship between MLAEP and response to verbal commands, conscious awareness and recall in surgical patients and anaesthetised volunteers during different sub-MAC concentrations of N,O and isoflurane. When MLAEP peak latencies were significantly increased compared with the awake state no response to verbal commands, conscious awareness or recall could be observed.…”

Effect of ephedrine on auditory‐evoked potentials during light general anaesthesia

“…Auditory-evoked potentials (AEP) have been shown to change in response to inhalation and intravenous administration of anaesthetics [4±7] and to surgical stimulation [8,9], and to be related to the level of consciousness [10], suggesting that they may indicate depth of anaesthesia. The purpose of this study was to evaluate the CNS effect of ephedrine during light general anaesthesia by measuring AEP; its effect was compared with that of etilefrine, a sympathomimetic drug without a CNS effect.…”