Abstract
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 60s 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 4.6s (SD=16.6) after clinical amnesia onset and a late peak at a mean 58s (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 60s 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.