Bispectral index monitoring during electroconvulsive therapy under propofol anaesthesia

Background. The accuracy of the bispectral index (BIS) as a monitor of consciousness has not been well studied in patients who have abnormal electroencephalograms (EEG). Methods. We studied the changes in BIS, its subparameters, and spectral entropy of the EEG during 18 electroconvulsive treatments under propofol and succinylcholine anaesthesia. A single bifrontal EEG, and second subocular channel (for eye movement estimation) was recorded. Results. The median (interquartile range) BIS value at re-awakening was only 57 (47-78)-thus more than a quarter of the patients woke at BIS values of less than 50. The changes in spectral entropy values were similar: 0.84 (0.68-0.99) at the start, 0.65 (0.42-0.88) at the point of loss-of-consciousness, 0.63 (0.47-0.79) during the seizures, and 0.58 (0.31-0.85) at awakening. Conclusions. Post-ictal slow-wave activity in the EEG (acting via the SynchFastSlow subparameter) may cause low BIS values that do not correspond to the patient's clinical level of consciousness. This may be important in the interpretation of the BIS in other groups of patients who have increased delta-band power in their EEG.