Auditory Steady-State Response and Bispectral Index for Assessing Level of Consciousness During Propofol Sedation and Hypnosis

We assessed the effect of propofol on the auditory steady-state response (ASSR), bispectral (BIS) index, and level of consciousness in two experiments. In Experiment 1, propofol was infused in 11 subjects to obtain effect-site concentrations of 1, 2, 3, and 4 &mgr;g/mL. The ASSR and BIS index were recorded during baseline and at each concentration. The ASSR was evoked by monaural stimuli. Propofol caused a concentration-dependent decrease of the ASSR and BIS index values (r2 = 0.76 and 0.93, respectively;P < 0.0001). The prediction probability for loss of consciousness was 0.89, 0.96, and 0.94 for ASSR, BIS, and arterial blood concentration of propofol, respectively. In Experiment 2, we compared the effects of binaural versus monaural stimulus delivery on the ASSR in six subjects during awake baseline and propofol-induced unconsciousness. During baseline, the ASSR amplitude with binaural stimulation (0.47 ± 0.13 &mgr;V, mean ± sd) was significantly (P < 0.002) larger than with monaural stimulation (0.35 ± 0.11 &mgr;V). During unconsciousness, the amplitude was 0.09 ± 0.09 &mgr;V with monaural and 0.06 ± 0.04 &mgr;V with binaural stimulation (NS). The prediction probability for loss of consciousness was 0.97 (0.04 se) for monaural and 1.00 (0.00 se) for binaural delivery. We conclude that the ASSR and BIS index are attenuated in a concentration-dependent manner by propofol and provide a useful measure of its sedative and hypnotic effect. BIS was easier to use and slightly more sensitive. The ASSR should be recorded with binaural stimulation. The ASSR and BIS index are both useful for assessing the level of consciousness during sedation and hypnosis with propofol. However, the BIS index was simpler to use and provided a more sensitive measure of sedation. Implications We have compared two methods for predicting whether the amount of propofol given to a human subject is sufficient to cause unconsciousness, defined as failure to respond to a simple verbal command. The two methods studied are the auditory steady-state response, which measures the electrical response of the brain to sound, and the bispectral index, which is a number derived from the electroencephalogram. The results showed that both methods are very good predictors of the level of consciousness; however, bispectral was easier to use.

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