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