British Journal of Anaesthesia, Vol 77, Issue 6 720-726, Copyright © 1996 by The Board of Management and Trustees of the British Journal of Anaesthesia
M. A. Tooley, G. L. Greenslade and C. Prys-Roberts
We have studied the effects of propofol, as the sole agent, at blood
concentrations of 1-10 micrograms ml-1, on the first 100 ms of the auditory
evoked response (AER) in 41 women before gynaecological surgery. AER were
recorded with the patients awake and then after 30 min of one of seven
stepped infusion regimens. Each patient was studied at only one blood
concentration. The recordings were edited and processed off-line by
coherent signal averaging, to obtain reliable estimates of each AER. We
measured standard features, such as amplitudes and latencies of brainstem
wave V and the mid-latency waves Na, Pa and Nb. In addition, we studied
several composite indices, intended to give a more global characterization
of the AER. We derived relationships between the doses and blood
concentrations of propofol, features of the AER and response to eyelash
stimulus and venepuncture. Nb latency was better than either concentration
or dose rate of propofol in providing a confident explanation of the
likelihood of eyelash response (which parallels the response to command). A
cut-off value of 53 ms had a sensitivity of 100%, a specificity of 96% and
an overall correctness of 98% as a discriminator of eyelash response vs no
response. Several alternative AER-derived indices provided more than 90%
correctness in discrimination, as did a dose rate of propofol of 6.3-7.8 mg
kg-1 h-1 or a blood concentration of 2.9 micrograms ml-1. We conclude that
the concentration and dose of propofol were good discriminators of response
to venepuncture, while the latency of the Na wave was the most successful
of the AER features.
CLINICAL INVESTIGATIONS
Concentration-related effects of propofol on the auditory evoked response
University of Bristol, Sir Humphry Davy Department of Anaesthesia, Bristol Royal Infirmary, Upper Maudlin Street, Bristol BS2 8HW
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