Assessment of graded changes in the central nervous system, during general anaesthesia and surgery in man, using the auditory evoked response
The thesis examines the use of the auditory evoked response (AER) to measure 'depth of anaesthesia'. The historical background to general anaesthesia is reviewed. Developments in recording the evoked responses with particular reference to the auditory evoked response and the factors which influence this are described. The effects of increasing concentrations of six general anaesthetic agents (halothane, enflurane, isoflurane, etomidate, Althesin and propofol) on the brainstem and early cortical auditory evoked responses and the modification of these effects by surgical stimulation were investigated. The AER as an indicator of 'awareness' was also assessed. These studies showed that all six general anaesthetics produced qualitatively similar changes in the early cortical section of the AER. These were increases in latency and reductions in amplitude of the waves Pa and Nb with increasing concentrations of anaesthetics. The amplitude changes were partially reversed during surgery. Only the inhalation anaesthetics (halothane, enflurane and isoflurane) produced changes in the brainstem response. The latencies of waves III and V and their interpeak intervals increased with increasing anaesthetic concentrations. These changes were not reversed during surgery. The latency of the early cortical wave Nb, emerged as a possible indicator of 'awareness' in that, in a group of patients anaesthetised with nitrous oxide and oxygen prior to general surgery, latencies below 44.5 ms were associated with a positive response using the isolated forearm technique. In a second study,during Caesarian section surgery, latencies below 44.5 ms occurred more frequently following delivery in patients in whom anaesthesia was maintained with nitrous oxide-opiate anaesthesia only, compared to those to whom an enflurane supplement was given. These findings and their theoretical implications are examined in the light of the literature. The practical application of the technique as a clinical monitor of anaesthetic depth is discussed.