Department of Anaesthesia, Waikato Hospital, Hamilton, New Zealand*Corresponding author
Accepted for publication: September 18, 2001
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Abstract |
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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 (4778)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 (0.680.99) at the start, 0.65 (0.420.88) at the point of loss-of-consciousness, 0.63 (0.470.79) during the seizures, and 0.58 (0.310.85) 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 patients 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.
Br J Anaesth 2002; 88: 1847
Keywords: monitoring, bispectral index; anaesthesia, depth; brain, electroconvulsive therapy; monitoring, electroencephalography; brain, entropy
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Introduction |
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Methods |
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Recording was commenced using an Aspect A-1000 EEG monitor (software version 3.31, Aspect Medical Systems), and data downloaded to a portable computer for storage and analysis. The sampling rate was 128/s, and the filters set at 0.5 and 70 Hz. An anaesthetist uninvolved with the study gave their usual anaesthetic consisting of: induction with propofol (70170 mg i.v.) and neuromuscular block with succinylcholine (4560 mg i.v.). Times were recorded for the following events: (1) start of induction (T1); (2) loss of response to verbal command (T2); (3) 10 s after the end of the electroconvulsive shock (T3); (4) first eye opening to verbal command (T4); and (5) 1 min after first eye opening (T5).
The data were analysed using Matlab (Matlab 5.3, The Mathworks Inc., Natick, MA, USA). The subparameters of the BIS (the BetaRatio and the SynchFastSlow) were calculated using published algorithms.1 Burst suppression was not observed during convulsions or in the post-ictal phase. Because there is some evidence that the spectral entropy of the EEG signal is a promising measure of depth of anaesthesia, this parameter was also calculated as described by Inoue.5 We used the frequency-band 240 Hz, and a resolution of 1 Hz.
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Results |
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The median (2575th percentiles) values of the spectral entropy at the start were 0.84 (0.680.99), dropping to 0.65 (0.420.88) at the point of loss-of-consciousness, and further decreasing to 0.63 (0.470.79) during the seizures. Spectral entropy did not increase at the point of awakening (0.58 (0.310.85)). The value at awakening was significantly different from the start value (P=0.001, paired t-test), but not different from the value during the seizures (P=0.44, paired t-test). If the spectral entropy was calculated using only the higher frequencies of the EEG signal (2040 Hz), the values at awakening (0.96 (0.860.99)) were significantly higher (P<0.001, t-test) than those calculated using the whole range of frequencies (240 Hz).
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Discussion |
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Which subcomponent of the BIS was responsible for the falsely depressed values? The BetaRatio recovered towards pre-induction levels at the time of first eye-opening to verbal command and at 60 s after, but the SynchFastSlow remained low. This may be explained by the presence of low-frequency (delta) EEG activity in the post-ictal state. Because of the denominator used in the respective calculations, activity below 10 Hz will affect the calculation of the SynchFastSlow, but not the BetaRatio.1 The regression model was derived from only 18 data points, and therefore must be interpreted with caution, but supports the predominance of the SynchFastSlow in lowering the BIS during recovery.
The spectral entropy calculated at each stage showed changes that were similar to those shown by the BIS. Neither parameter increased reliably at the point of awakening. However, the behaviour of the spectral entropy is somewhat dependent on technicalities of the range of frequencies over which it is calculated, and the amount of pre-processing and filtering. If the spectral entropy was calculated over higher frequencies (e.g. 2040 Hz) it is sensitive to increases in higher frequency activity that are characteristic of the return of consciousness.11 However it is difficult to be sure how much of this activity is frontal EMG, and how much is true return of high-frequency EEG gamma rhythms at the point of awakening. Conversely, if the lower frequencies are excluded, then the spectral entropy will be less sensitive to the influence of the strong delta rhythms that indicate deep anaesthesia. One may speculate that in order to cover fully the range of anaesthetic concentrations and effects it would be logical to have a meta-function that varied and combined the influence of different frequency-bands in the spectral entropy calculations. This is analogous to the construction of the BIS. If the value of the spectral entropy (2040 Hz) was high (e.g. >0.9) it would predominate. As the high-frequency spectral entropy (2040 Hz) fell, then the proportional influence of spectral entropies calculated using progressively lower frequencies (e.g. 220 Hz) would increase.
The interpretation of the EYE parameter is problematic. The diagonal placement of the eye electrodes should have been able to detect both horizontal and vertical eye movements. The EYE parameter decreased only minimally during the pre-ECT period when the patients were anaesthetized with propofol and paralysed with suxamethoniumas evidenced by minimal movements during the grand mal seizure. Because EYE still had activity when the patient was supposedly paralysed, this clearly indicates that the EYE parameter was not a pure estimate of eye-movements, but included an unknown and significant quantity of EEG signal and, possibly, frontalis EMG activity. Indeed, the BIS derived from the second (eye) channel correlated well with the true channel-one BIS (derived from the frontal montage) when the patient was anaesthetizedPearsons correlation coefficient (r): r(T2)=0.99, r(T3)=0.97, r(T4)=0.95, r(T5)=0.57.
In conclusion, our study suggests that a low BIS value does not correspond to a deep hypnotic level in patients after ECT. By inference, this may also be applicable to patients with other pre-existing EEG abnormalities. In our patient group, the recovery of spectral power in the higher frequencies (as estimated by the BetaRatio, or spectral entropy (2040 Hz)) was more predictive than the BIS, and the broadband spectral entropy (240 Hz), of the recovery of consciousness.
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References |
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