Slow potentials of EEG burst suppression pattern during anaesthesia

Substance-Specific Differences in Human Electroencephalographic Burst Suppression Patterns

Different anesthetic agents induce burst suppression in the electroencephalogram (EEG) at very deep levels of general anesthesia. EEG burst suppression has been identified to be a risk factor for postoperative delirium (POD). EEG based automated detection algorithms are used to detect burst suppression patterns during general anesthesia and a burst suppression ratio (BSR) is calculated. Unfortunately, applied algorithms do not give information as precisely as suggested, often resulting in an underestimation of the patients’ burst suppression level. Additional knowledge of substance-specific burst suppression patterns could be of great importance to improve the ability of EEG based monitors to detect burst suppression. In a re-analysis of EEG recordings obtained from a previous study, we analyzed EEG data of 45 patients undergoing elective surgery under general anesthesia. The patients were anesthetized with sevoflurane, isoflurane or propofol (n = 15, for each group). After skin incision, the used agent was titrated to a level when burst suppression occurred. In a visual analysis of the EEG, blinded to the used anesthetic agent, we included the first distinct burst in our analysis. To avoid bias through changing EEG dynamics throughout the burst, we only focused on the first 2 s of the burst. These episodes were analyzed using the power spectral density (PSD) and normalized PSD, the absolute burst amplitude and absolute burst slope, as well as permutation entropy (PeEn). Our results show significant substance-specific differences in the architecture of the burst. Volatile-induced bursts showed higher burst amplitudes and higher burst power. Propofol-induced bursts had significantly higher relative power in the EEG alpha-range. Further, isoflurane-induced bursts had the steepest burst slopes. We can present the first systematic comparison of substance-specific burst characteristics during anesthesia. Previous observations, mostly derived from animal studies, pointing out the substance-specific differences in bursting behavior, concur with our findings. Our findings of substance-specific EEG characteristics can provide information to help improve automated burst suppression detection in monitoring devices. More specific detection of burst suppression may be helpful to reduce excessive EEG effects of anesthesia and therefore the incidence of adverse outcomes such as POD.

Electroencephalogram signatures of loss and recovery of consciousness from propofol

Unconsciousness is a fundamental component of general anesthesia (GA), but anesthesiologists have no reliable ways to be certain that a patient is unconscious. To develop EEG signatures that track loss and recovery of consciousness under GA, we recorded high-density EEGs in humans during gradual induction of and emergence from unconsciousness with propofol. The subjects executed an auditory task at 4-s intervals consisting of interleaved verbal and click stimuli to identify loss and recovery of consciousness. During induction, subjects lost responsiveness to the less salient clicks before losing responsiveness to the more salient verbal stimuli; during emergence they recovered responsiveness to the verbal stimuli before recovering responsiveness to the clicks. The median frequency and bandwidth of the frontal EEG power tracked the probability of response to the verbal stimuli during the transitions in consciousness. Loss of consciousness was marked simultaneously by an increase in low-frequency EEG power (<1 Hz), the loss of spatially coherent occipital alpha oscillations (8-12 Hz), and the appearance of spatially coherent frontal alpha oscillations. These dynamics reversed with recovery of consciousness. The low-frequency phase modulated alpha amplitude in two distinct patterns. During profound unconsciousness, alpha amplitudes were maximal at low-frequency peaks, whereas during the transition into and out of unconsciousness, alpha amplitudes were maximal at low-frequency nadirs. This latter phase-amplitude relationship predicted recovery of consciousness. Our results provide insights into the mechanisms of propofol-induced unconsciousness, establish EEG signatures of this brain state that track transitions in consciousness precisely, and suggest strategies for monitoring the brain activity of patients receiving GA.

EEG sources of noise in intraoperative somatosensory evoked potential monitoring during propofol anesthesia

OBJECTIVE

It was hypothesized that somato- sensory evoked potentials can be achieved faster by selective averaging during periods of low spontaneous electroen- cephalographic (EEG) activity. We analyzed the components of EEG that decrease the signal-to-noise ratio of somatosensory evoked potential (SEP) recordings during propofol anesthesia.

METHODS

Patient EEGs were recorded with a high sampling frequency during deep anesthesia, when EEGs were in burst suppression. EEGs were segmented visually into bursts, spindles, suppressions, and artifacts. Tibial somatosensory evoked potentials (tSEPs) were averaged offline separately for burst, suppression, and spindle segments using a signal bandwidth of 30-200 Hz. Averages achieved with 2, 4, 8, 16, 64, 128, and 256 responses were compared both visually, and by calculating the signal-to-noise ratios.

RESULTS

During bursts and spindles, the noise levels were similar and significantly higher than during suppressions. Four to eight times more responses had to be averaged during bursts and spindles than during suppressions in order to achieve a similar response quality. Averaging selectively during suppressions can therefore yield reliable tSEPs in approximately one-fifth of the time required during bursts.

CONCLUSION

The major source of EEG noise in tSEP recordings is the mixed frequency activity of the slow waves of bursts that occur during propofol anesthesia. Spindles also have frequency components that increase noise levels, but these are less important, as the number of spindles is fewer. The fastest way to obtain reliable tSEPs is by averaging selectively during suppressions.

Burst suppression during propofol anaesthesia recorded from scalp and subthalamic electrodes: report of three cases

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.

Evoked EEG patterns during burst suppression with propofol

BACKGROUND

During EEG suppression with isoflurane or sevoflurane anaesthesia, median nerve stimulation causes cortical responses of two kinds: an N20 wave with a latency of 20 ms and an EEG burst with a latency of 200 ms. We tested the possibility that median nerve stimulation during EEG suppression with propofol would cause an EEG response that was consistent enough to be of use for neuromonitoring.

METHODS

Eight patients were anaesthetized with propofol to allow burst suppression. Electrical stimulation of the median nerve was applied during general anaesthesia and the EEG was measured.

RESULTS

The EEG response to a painful stimulus had four successive components: (i) N20 and P22 potentials, reflecting activation of fast somatosensory pathways; (ii) a high-amplitude negative wave, possibly reflecting activation of the somatosensory cortex SII bilaterally; (iii) a burst (i.e. a negative slow wave with superimposed 10 Hz activity, probably reflecting an arousal mechanism); and (iv) a 13-15 Hz spindle, probably originating from the thalamus, similar to sleep spindles. These could be seen separately and in different combinations. Bursts and spindles during burst suppression were also seen without stimulation. In deepening propofol anaesthesia, spindles were seen in the continuous EEG before burst suppression was achieved. In deep anaesthesia, spindles were seen when bursts had ceased, and painful stimuli evoked sharp waves without subsequent bursts.

CONCLUSION

In addition to SSEP (somatosensory evoked potentials), three different evoked responses are noted that could be useful for clinical monitoring.

Single-sweep cortical somatosensory evoked potentials: N20 and evoked bursts in sevoflurane anaesthesia

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.

Suppression of F-VEP during isoflurane-induced EEG suppression

We recorded visual evoked potentials (VEPs) to flash stimuli in moderately deep anaesthesia when EEG showed burst suppression pattern. Flash VEPs could consistently be recorded in all 8 test subjects during bursts but not during suppressions. We conclude that during isoflurane-induced EEG suppression VEPs to flash stimuli are also suppressed. This effect should be taken into account in evoked potential testing during anaesthesia.

MEG burst suppression in an anaesthetized dog

Recording of the magnetic fields of the brain, magnetoencephalography (MEG), has proved to be a valuable method in neurophysiological research. In order to study the feasibility of MEG recording during anaesthesia we recorded magnetoecephalographic burst suppression in a dog during enflurane and propofol anaesthesia. The observed signal distribution implies a complex current distribution underlying the burst activity. This experiment also proves that an essentially artefact-free MEG recording can be obtained during respirator-assisted anaesthesia.

Somatosensory evoked potentials during isoflurane anaesthesia

Short latency somatosensory evoked potentials (SEPs) to median nerve stimulation during isoflurane anaesthesia were recorded in 12 elective-surgery patients. The effect of isoflurane on the shape, amplitude and latency of SEPs was evaluated. SEPs were recorded at awake, 1 MAC, 1.5 MAC, at electroencephalogram (EEG) burst suppression and at continuous suppression levels. Finally, SEPs were recorded when anaesthesia was lightened back to 1 MAC. The peak latency and amplitude of the first cortical N20 wave were measured. The latencies increased with increasing isoflurane concentrations. At high concentrations only an almost monophasic N20 wave was recorded, reduced in shape and amplitude. No specific changes could be correlated with the burst suppression or suppression patterns. This suggests that EEG and SEP generators are differently affected with increasing isoflurane concentration. The results indicate that SEPs can also be recorded in drug-induced EEG suppression.

Vibration stimulus induced EEG bursts in isoflurane anaesthesia

The EEG and heart rate reactions to vibration stimulus were studied in 14 patients during moderately deep surgical isoflurane anaesthesia, at a level when EEG showed a burst suppression pattern. Vibration applied to the palm of the hand induced bursts in EEG in 12 patients, usually with a latency of about 0.5 sec from the onset, or from the end of the 3 sec stimulus. Increases in heart rate were seen at bursts related to both vibration onset and offset, as well as at spontaneous bursts. With spontaneous bursts, an initial positive wave was frequently seen. In 6 patients the vibration induced bursts were different in shape from the spontaneous bursts; no initial positive wave was seen before the negative DC shift in Cz-Fz recording. We conclude that EEG bursts can be evoked by a non-noxious stimulus such as vibration in patients during isoflurane anaesthesia.