Auditory steady-state response and bispectral index for assessing level ofconsciousness during propofol sedation and hypnosis

We assessed the effect of propofol on the auditory steady-state response (A SSR), bispectral (BIS) index, and level of consciousness in two experiments . In Experiment 1, propofol was infused in 11 subjects to obtain effect-sit e concentrations of 1, 2, 3, and 4 mug/mL. The ASSR and BIS index were reco rded during baseline and at each concentration. The ASSR was evoked by mona ural stimuli. Propofol caused a concentration-dependent decrease of the ASS R and BIS index values (r(2) = 0.76 and 0.93, respectively; P < 0.0001). Th e 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 stimu lus 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 <mu>V, mean +/- SD) was significantly (P < 0.00 2) larger than with monaural stimulation (0.35 +/- 0.11 <mu>V). During unco nsciousness, the amplitude was 0.09 +/- 0.09 muV with monaural and 0.06 +/- 0.04 muV with binaural stimulation (NS). The prediction probability for lo ss 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 i n a concentration-dependent manner by propofol and provide a useful measure of its sedative and hypnotic effect. BIS was easier to use and slightly mo re sensitive. The ASSR should be recorded with binaural stimulation. The AS SR and BIS index are both useful for assessing the level of consciousness d uring sedation and hypnosis with propofol. However, the BIS index was simpl er to use and provided a more sensitive measure of sedation.