Electroencephalographic indices related to hypnosis and amnesia during propofol anaesthesia for cardioversion

Most previous studies which have assessed the depth of anaesthesia using the Bispectral Index (BIS) have used multi-agent anaesthetics with relatively slow induction and recovery times. Elevation of electroencephalographic (EEG) Beta/Alpha Ratio has been linked to onset of midazolam amnesia. Propofol anaesthesia for cardioversion in 19 patients enabled us to profile the changes in BIS and Beta/Alpha Ratio during a short, single-agent anaesthetic with a relatively rapid induction and recovery period. BIS values did not alter significantly through onset of amnesia and hypnosis as compared with awake values. Cardioversion occurred over a wide range of BIS values (38 to 94), with recovery at a median BIS value of 79. We observed the BIS to lag about 60 s behind the clinical situation (termed BIS60). The BIS60 more closely tracked the clinical level of consciousness, showing statistically significant differences from the uncorrected BIS. (amnesia (median BIS60 = 82, P = 0.02), hypnosis (median BIS60 = 67, P = 0.01), eye opening (median BIS60 = 90, P = 0.001)). An early EEG Beta/Alpha Ratio peak occurred at a mean of 4.6 s (SD = 16.6) after clinical amnesia onset and a late peak at a mean 58 s (SD = 144) after eye opening. There was no significant EEG response to cardioversion. We conclude that: (1) in rapidly changing conditions, the value of the BIS most accurately reflects the level of consciousness of the patient about 60 s in the past, and (2) the onset and offset of propofol-induced amnesia commonly corresponds to a peak in Beta/Alpha Ratio of the EEG.