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BACKGROUND: Several measures have been developed to quantify the change in EEG from wakefulness to deep anaesthesia. Measures of signal complexity or entropy have been popular and even applied in commercial monitors. These measures quantify different features of the signal, however, and may therefore behave in an incomparable way when calculated for standardized EEG patterns. METHODS: Two measures widely studied for anaesthesia EEG analysis were considered: spectral entropy and approximate entropy. First, we generated surrogate signals which had the same spectral entropy as a prototype signal, the sawtooth wave. Secondly, EEG samples where rhythmic pattern caused a peak in the power spectrum in the α-frequency band were modified by enhancing or suppressing the corresponding rhythm. RESULTS: We found that the value of spectral entropy does not, in general, correlate with the visual impression of signal regularity. Also, the two entropy measures interpret a standardized artificially modified EEG signal in opposite directions: spectral peak of increasing amplitude in the α-frequency band causes spectral entropy to increase but decreases approximate entropy when low frequencies are present in the signal. CONCLUSIONS: Spectral entropy and approximate entropy of EEG are two totally different measures. They change similarly in deepening anaesthesia due to an increase in slow activity. In some cases, however, they may change in opposite directions when the EEG signal properties change during anaesthesia. Failure to understand the behaviour of these measures can lead to misinterpretation of the monitor readings or study results if no reference to the raw EEG signal is taken.

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BACKGROUND: Epileptiform patterns, spikes, polyspikes and periodic epileptiform discharges (PED) have been reported in electroencephalograms (EEGs) during anaesthesia induction with sevoflurane in healthy adults and children. Published recordings have been performed with a limited number of channels, and therefore the topographic distributions of these patterns are not known. METHODS: Twenty ASA I children aged 4-10 years undergoing routine operations were anaesthetized with 8% sevoflurane in 50%/50% oxygen and nitrous oxide using mask induction with controlled normoventilation. An EEG was recorded with a full 10-20 electrode system including orbitofrontal and ear electrodes, and a recording band of 0.016-70 Hz. Beat-to-beat heart rate (HR) was calculated off-line. RESULTS: Nineteen out of 20 children developed multifocal spikes and polyspikes with a maximum over the frontal lobes. Four patients developed suppression, which was almost continuous and lasted several minutes, and thereafter a continuous EEG resumed, a few spikes were seen and then a nonepileptiform pattern. In three children a couple of PED waves were seen at the onset of a continuous EEG. HR increased maximally before the onset of spikes. No motor phenomena were seen. CONCLUSION: These recordings confirm the epileptogenic property of sevoflurane in mask induction. The spikes and polyspikes had frontal multifocal maxima and may be missed in recordings from frontopolar electrodes used by depth-of-anaesthesia monitors. PED and burst suppression were synchronous over the whole cortex. Epileptiform activity was indiscernible from epileptiform waveforms without anaesthesia, such as the patterns seen in status epilepticus.

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BACKGROUND: Measurement of slow EEG activity and burst suppression are the main tasks in monitoring the effects of anaesthestics with EEG, which is often done with commercial univariate indexes such as BIS. The aim of this study was to describe the characteristics of burst suppression EEG during propofol anaesthesia using scalp electrodes and depth electrodes in the subthalamic nucleus. Specifically, we describe the electrical fields of the three EEG patterns we have previously described: the sharp wave, the burst and the spindle. METHODS: We recorded the EEG of three Parkinson patients during propofol anaesthesia from the scalp electrodes and the depth electrode implanted in the subthalamic nucleus for treating parkinsonism. RESULTS: (1) The slow waves of bursts recorded from all surface electrodes on scalp or neck with depth electrode reference are positive and have the highest amplitude in frontal electrodes, suggesting synchronous generation in the whole cerebral cortex. (2) The sharp wave and spindles have the highest amplitude at vertex. They are opposite in polarity in vertex and depth electrodes when referred to the neck electrode, suggesting generation in the sensorimotor cortex. CONCLUSIONS: Recording simultaneously EEG from the depth and scalp electrodes shows that bursts and their slow wave oscillations are synchronous in the whole cortex while spindles and sharp waves are produced by the sensorimotor cortex. The amplitude of slow waves recorded with surface electrodes is equal to the difference of the wave at two electrodes and therefore only a small part of that generated by the cortex.

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Cortical evoked responses to median nerve stimulation were recorded from 21 subjects during sevoflurane anaesthesia at the level of burst suppression in EEG. The N20/P22 wave had the typical form of a negative wave postcentrally, and positive precentrally. The amplitude exceeded 4 microV in all patients, making it easily visible without averaging on the low-amplitude suppression. These results show that two kinds of somatosensory evoked potential can be studied without averaging during EEG suppression in deep anaesthesia. One is the localised N20/P22 wave, which is seen regularly during suppression after stimuli with intervals exceeding 1 s. The other is the burst, involving the whole cortex, which is not evoked by every stimulus. We suggest that somatosensory evoked potentials can be monitored during sevoflurane-induced EEG suppression, and often can be evaluated reliably from a couple of single sweeps with stimulation interval exceeding 1 s. The enhancement of early cortical components of SEP, their adaptation to repeated stimuli, and the disappearance of later polysynaptic components during EEG suppression, give new possibilities to study the generators of SEP and the different effects of anaesthetics.

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