The coherent frequency in the electroencephalogram as an objective measure of cognitive function during propofol sedation.

Ten volunteers were studied during six stages of propofol sedation, namely awake (no propofol), light sedation (small dose of propofol), deep sedation (large dose), deep sedation with stimulation of the ulnar nerve, then light sedation again (small dose), and awake (recovery). Light and deep sedation were defined in terms of performance on a test of cognitive function: the within-list recognition (WLR) test. At each stage, the steady-state auditory evoked potential was measured at different stimulating frequencies to derive the frequency needed to achieve the maximal coherence index. This frequency is called the "maximum coherent frequency". WLR performance correlated with infusion dose (r = -0.71), plasma propofol concentration (r = -0.75), and maximum coherent frequency (r = 0.75). When the correlations were examined for the propofol sedation stages only, there remained a strong correlation between WLR performance and maximum coherent frequency (r = 0.47, P < 0.005), but no significant correlations between infusion dose and WLR (r = -0.11) or infusion dose and plasma concentration (r = 0.13). These data suggest that maximum coherent frequency provides a better measure of depth of sedation than does the dose of propofol alone.