Propofol-induced alpha rhythm.

The electroencephalographic effects of two intravenous sedative/hypnotic drugs, propofol and thiopental, were studied at three stable blood concentrations in 52 normal healthy volunteers. The higher concentration resulted in unresponsiveness (lack of response to auditory/tactile stimuli) in all subjects. This report describes the strong frontal-central rhythms apparent in this state using a quantitative description of oscillatory systems underlying the rhythm. These rhythms occur when sedative drug concentrations are greater than those producing the well-described increase in broadband beta-power associated with many sedative drugs. Propofol induces rhythms in the alpha-range, while thiopental produces rhythms in the beta-range. Quasistationary for a period of about 1 h, these rhythms exceed the baseline alpha-rhythm in power. By their resonant nature, these propofol-induced rhythms are analogous to 'the classic alpha-rhythm', but quantitative characteristics of the underlying oscillatory systems are different. Baseline properties of the oscillatory system underlying the initial resting alpha-rhythm recover completely as drug concentration decays to negligible values.