Concentration-related effects of propofol on the auditory evoked response.

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.