Bennett C et al. Practical use of the raw electroencephalogram waveform during general anesthesia: the art and science. Anesth Analg. 2009 Aug;109(2):539-50.This review article attempts to educate clinicians so that they are better equipped to problem solve when faced with an unexpectedly high or low quantitative EEG index (qEEGI) number from either a BIS or spectral entropy monitor. The authors emphasize that clinical judgement is crucial when using qEEG, as is a clear mental picture of the expected raw EEG and knowledge of common EEG artifacts. Beta-band activity (13-30 Hz) predominates in the EEG of an awake subject (fuzzy flat line). As the sedative/hypnotic drug concentration increases, the beta activity slows to spindle-like/alpha waves (short bursts of activity with frequency of around 7-14 Hz) and then slows further to the theta range (3.5-7 Hz). Delta waves (1-4 Hz) are the hallmark of slow wave sleep and deeper anaesthesia, where the whole EEG slowly wanders up then down around the base-line. Burst suppression and isoelectricity are features of progressively deeper anaesthesia. The presence of spindle-like waves and background slow delta waves, with no fast waves, are probably the most important EEG signs of anaesthesia. Common artifacts include EMG signal (frontalis, masseter or extraocular muscles), cerebral pathology and certain drugs i.e ketamine. Of note, cerebral ischaemia is accurately detected by BIS with a decreasing of the qEEGI due to cortical inactivation. Given this background, the authors conclude that it is not possible to adequately monitor a patient by blind obedience to the processed number.

Take home message: Optimal use of qEEG monitors, such as BIS and M-entropy, requires an understanding of expected raw EEG changes during anaesthesia as well as potential sources of non-EEG artifact.

For related reading this month: -
Nishiyama T. Cerebral state index vs. bispectral index during sevoflurane-nitrous oxide anaesthesia. Eur J Anaesthesiol. 2009 Aug;26(8):638-4

Cirodde A et al. Is your patient sleeping? Eur J Anaesthesiol. 2009 Aug;26(8):704-6.