THE AUDITORY STEADY STATE RESPONSE DURING SUFENTANIL ANAESTHESIA

The auditory steady state response (ASSR) is a sinusoidal evokedpotential elicited by rapidly repeated auditory stimuli. TheASSR was recorded in eight patients during high-dose sufentanilanaesthesia for cardiac surgery in order to assess its usefulnessas a measure of the level of consciousness. The electroencephalogram(EEG) was recorded for comparison. The ASSR was present beforeinduction in all patients. It was attenuated severely or possiblyabolished with loss of consciousness, and reappeared at lowamplitude 5–10 min later and remained attenuated untilthe end of surgery. The amplitude increased with early signsof awakening in the Intensive Care Unit. With few exceptions,changes in the simultaneously recorded EEG were similar to thoseof the ASSR. The ASSR deserves further evaluation as a toolfor monitoring level of consciousness during high-dose opioidanaesthesia. ^*Present address: Department of Anaesthesia, Toronto GeneralHospital, 200 Elizabeth Street, Toronto (Ontario), Canada M5G1L7.     

Ketamine increases the amplitude of the 40-Hz auditory steady-state response in humans

The auditory middle latency response (AMLR) and the 40-Hz auditory steady-state response (40-Hz ASSR) are evoked potentials which possibly arise from the same generators in the primary auditory cortex. Both responses are attenuated by most general anaesthetics. Ketamine, however, has been reported to have no effect on the AMLR. Our aim was to evaluate the effects of ketamine on the 40-Hz ASSR. Spectral analysis of the electroencephalogram (EEG) was also conducted to independently examine the effects of ketamine. Ketamine 1.5 mg kg-1 was given to 12 patients for induction of general anaesthesia. Recordings of the 40-Hz ASSR and EEG were obtained every minute from 3 min before administration of ketamine to 5 min after injection, when the study was terminated. Similar recordings were obtained in three control subjects under identical conditions except that no medication was administered. Consciousness, defined as responsiveness to verbal commands, was assessed before each recording. Ketamine caused an increase in the amplitude of the 40-Hz ASSR (P < 0.01). Using published AMLR data, we conducted a simulation experiment that suggested that the effect of ketamine on the AMLR can explain its effects on the amplitude of the 40- Hz ASSR. There was a pronounced increase in relative theta (3.9-7.9 Hz) EEG power and a decrease in relative alpha (8.0-12.8 Hz) power (P < 0.001). These changes were not observed in the control group. Ketamine produced unconsciousness until the end of the study in five patients and transient unconsciousness in five patients. Two patients did not lose consciousness after administration of ketamine. The 40-Hz ASSR and EEG revealed no consistent differences between conscious and unconscious patients. No relationship could be demonstrated between the increase in amplitude of the 40-Hz ASSR or of relative theta power (the hallmark of ketamine effect) and loss of responsiveness to commands. We conclude that ketamine, unlike other anaesthetics, increases the amplitude of the 40-Hz ASSR.      

Effect of Isoflurane on the Auditory Steady-state Response and on Consciousness in Human Volunteers

Background: The auditory steady state response (ASSR) is a sustained electrical response of the brain to auditory stimuli delivered at fast rates (30 - 50 responses/s). The aim of this study was to evaluate the effect of 0.26-0.50% isoflurane on the ASSR and on consciousness, defined as responsiveness to verbal commands.

Methods: Ten volunteers (21 - 31 yr) participated. Isoflurane was administered at three stable, end-tidal concentrations: 0.26%, 0.38%, and 0.50%. The ASSR in response to 18,000 stimuli (500-Hz tonebursts, 10 ms, 82-dB, the right ear, 35 - 45 bursts/s) was recorded from the vertex with reference to the right mastoid. Recordings were made during baseline, at each isoflurane concentration, and during recovery.

Results: The mean (SD) ASSR amplitudes were 0.32 (0.23) [micro sign]V during baseline, 0.24 (0.17) [micro sign]V during 0.26% isoflurane, 0.09 (0.05) [micro sign]V during 0.38% isoflurane, 0.04 (0.03) [micro sign]V during 0.50% isoflurane, and 0.29 (0.33) [micro sign]V during recovery. The amplitude during baseline and recovery was larger than during 0.38% and 0.50% isoflurane (P < 0.001). The amplitude at 0.26% was larger than at the other concentrations (P < 0.025). The logarithm of the ASSR amplitude was related linearly to the concentration of isoflurane (r = 0.85; P < 0.0001). The prediction probability (Pk) for loss of consciousness was 0.95 for both ASSR and measured isoflurane concentration. An ASSR amplitude < 0.07 [micro sign]V was always associated with unconsciousness.     

Midlatency auditory evoked potentials do not allow the prediction of recovery from general anesthesia with isoflurane

PURPOSE: To investigate midlatency auditory evoked potentials (MLAEP) waveforms during recovery from anesthesia. The hypothesis was that MLAEP are sensitive variables to discriminate between states of consciousness and unconsciousness during emergence from anesthesia. METHODS: MLAEP were recorded in the awake state and during the wake-up phase from isoflurane anesthesia in 22 female patients undergoing ophthalmologic surgery. During emergence from anesthesia the changes in latency and amplitude of MLAEP components Na, Pa and Nb were compared with the awake level. The next day the patients were asked for explicit memory for the recovery period. RESULTS: In 72% of the patients the MLAEP waveforms were completely suppressed during isoflurane anesthesia. When the patients responded and opened their eyes spontaneously 38 +/- 12 min after anesthesia, the latencies of Na (18.3 +/- 1.2 vs 17.6 +/- 1.3; P = 0.013) and Nb (47.4 vs 7.1 vs 44.7 +/- 7.8; P = 0.048) remained prolonged compared with awake values. In contrast, the amplitudes NaPa and PaNb had regained baseline level. Nine patients had explicit memory for the immediate recovery period. However, there was no difference for any MLAEP component between patients with and without memory at any time. CONCLUSIONS: The persistent changes of MLAEP latency components Na and Nb indicated impaired auditory signal processing 38 min after isoflurane anesthesia. There was a marked intra- and inter-individual variability during reversal of the anesthetic induced MLAEP changes. This limits the prediction of recovery of consciousness in the individual patient during emergence from anesthesia.     

Electroencephalographic change during induction of high-dose fentanyl anesthesia--evaluation by LIFESCAN EEG monitor]

The relation between the patient's consciousness level and the change in electroencephalographic (EEG) activity during general anesthesia has not been well understood. In the present study, we evaluated such relationship quantitatively during the induction of high-dose fentanyl anesthesia in adult open-heart cases. There are significant increase in delta activity and significant decrease in alpha and beta activity in response to the induction of anesthesia. The loss of consciousness coincided with the sharp reduction in delta ratio and edge frequency (80%). Such reduction was more prominent in delta ratio than in edge frequency. These findings suggest that delta ratio could indicate the consciousness level during the induction of high-dose fentanyl anesthesia. Further study is needed to clarify the EEG response during inhalation and intravenous anesthesia in relation to EEG activity.     

Auditory steady-state response and bispectral index for assessing level of consciousness during propofol sedation and hypnosis

We assessed the effect of propofol on the auditory steady-state response (ASSR), bispectral (BIS) index, and level of consciousness in two experiments. In Experiment 1, propofol was infused in 11 subjects to obtain effect-site concentrations of 1, 2, 3, and 4 microg/mL. The ASSR and BIS index were recorded during baseline and at each concentration. The ASSR was evoked by monaural stimuli. Propofol caused a concentration-dependent decrease of the ASSR and BIS index values (r(2) = 0.76 and 0.93, respectively; P<0.0001). The prediction probability for loss of consciousness was 0.89, 0.96, and 0.94 for ASSR, BIS, and arterial blood concentration of propofol, respectively. In Experiment 2, we compared the effects of binaural versus monaural stimulus delivery on the ASSR in six subjects during awake baseline and propofol-induced unconsciousness. During baseline, the ASSR amplitude with binaural stimulation (0.47+/-0.13 microV, mean +/- SD) was significantly (P<0.002) larger than with monaural stimulation (0.35+/-0.11 microV). During unconsciousness, the amplitude was 0.09+/-0.09 microV with monaural and 0.06+/-0.04 microV with binaural stimulation (NS). The prediction probability for loss of consciousness was 0.97 (0.04 SE) for monaural and 1.00 (0.00 SE) for binaural delivery. We conclude that the ASSR and BIS index are attenuated in a concentration-dependent manner by propofol and provide a useful measure of its sedative and hypnotic effect. BIS was easier to use and slightly more sensitive. The ASSR should be recorded with binaural stimulation. The ASSR and BIS index are both useful for assessing the level of consciousness during sedation and hypnosis with propofol. However, the BIS index was simpler to use and provided a more sensitive measure of sedation. Implications: We have compared two methods for predicting whether the amount of propofol given to a human subject is sufficient to cause unconsciousness, defined as failure to respond to a simple verbal command. The two methods studied are the auditory steady-state response, which measures the electrical response of the brain to sound, and the bispectral index, which is a number derived from the electroencephalogram. The results showed that both methods are very good predictors of the level of consciousness; however, bispectral was easier to use.     

The long-latency auditory evoked potential as a measure of the level of consciousness during sufentanil anesthesia

The long-latency auditory evoked potential was recorded during opioid anesthesia to evaluate its usefulness for assessing the level of consciousness. Eight patients undergoing coronary artery surgery under high-dose sufentanil anesthesia following a lorazepam premedication were tested. Four waves were studied: the N1, P2, P3, and the slow wave (SW). The amplitude of N1 and that of the SW were reduced by sufentanil, but the differences did not reach significance. The amplitude and latency of P2 and the latency of the SW were significantly reduced. The P3, which is possibly a specific indicator for consciousness, was present before induction and absent during anesthesia. There was no conscious recall for intraoperative events. Preservation of the N1, P2, and SW, which are abolished by nonopioid general anesthetics, suggests that auditory processing proceeds further during anesthesia with sufentanil. However, no electrophysiologic evidence of consciousness was found during sufentanil anesthesia.     

Power spectrum correlates of changes in consciousness during anesthetic induction with enflurane

In order to correlate electroencephalogram (EEG) changes during anesthetic induction with level of consciousness, four-channel parasagittal EEG recordings were made during anesthetic induction with enflurane and enflurane-nitrous oxide in oxygen. The EEG was quantitated using power spectrum analysis. Significant EEG changes were identified during all anesthetic inductions; however, the frequency of occurrence of change was significant only during development of amnesia (15 of 20 subjects, P = 0.04). The nature of the EEG changes at this time was agent-specific (P less than 0.05 by chi-square), with high-frequency changes evident in the enflurane group and shifts in amplitude in the 8-12 Hz activity predominating in the nitrous oxide-enflurane group. Anterior dominance could not be documented as a correlate of amnesia or unresponsiveness. The identification of such EEG changes may be valuable in assessing anesthetic depth, but other effects, such as the response of the EEG to surgical stimulation, must be determined before the results are clinically applicable.     

Attenuation of the 40-Hertz Auditory Steady State Response by Propofol Involves the Cortical and Subcortical Generators

Background: The 40-Hz auditory steady state response (40-Hz ASSR) provides a reliable marker of anesthetic-induced unconsciousness. Brain electric source analysis indicates that the 40-Hz ASSR arises from cortical and subcortical generators. The authors used source analysis to assess the effect of propofol anesthesia on the cerebral generators of the 40-Hz ASSR. They also examined the effect of propofol on two auditory evoked potentials of cortical origin: the N1 and the sustained potential.

Methods: Eleven healthy human volunteers were anesthetized with propofol given in target-concentration mode at the minimal concentration causing unconsciousness. The 40-Hz ASSR was recorded before, during, and after anesthesia. The source model consisted of five concurrently active generator dipoles: two in the contralateral auditory cortex (one tangentially oriented, one radially oriented), two in the ipsilateral auditory cortex (same orientations), and one in the midline brainstem.

Results: During anesthesia, the strength of the cortical and brainstem dipoles was reduced to the same extent (to 54% of baseline for the four cortical dipoles pooled vs. 53% for the brainstem dipole). Dipole strength during anesthesia was significantly less (P < 0.01) than during baseline and recovery for both cortical and brainstem dipoles. The N1 and sustained potential were no longer recordable during anesthesia.     

The electroencephalograph during anesthesia and emergence in infants and children

BACKGROUND: The characteristics of the electroencephalogram (EEG) during anesthesia in children are poorly described. An understanding of the EEG during anesthesia may help explain and predict the performance of EEG-derived depth of anesthesia monitors in children. This study aims to describe the association between age and some basic characteristics of the EEG during nonstandardized anesthesia in children and infants. METHODS: Sixty-four children aged 9 days to 12 years were enrolled in this observational physiological study. Anesthesia was given at the discretion of the anesthetist. EEG was recorded from after induction to emergence using a BRM2 brain monitor 'BrainZ Instruments, Auckland, New Zealand' in parietal and frontal montages. For comparison, 90% spectral edge frequency (SEF-90) and power were determined at three points (equilibrium during anesthesia, when the volatile agent was discontinued and at emergence), and children were divided into three age groups (0-6 months, 6-24 months and 2-12 years). RESULTS: Fifty-seven children had artifact-free EEG data. From equilibrium to emergence the mean forehead power decreased substantially in children aged 2-12 years (3171-153 microV(2), P < 0.001) and 6-24 months (756-140 microV(2), P < 0.001) but no difference was found in infants aged 0-6 months (93-63 microV(2), P = 0.4). From equilibrium to emergence mean forehead spectral edge frequency increased in children aged 2-12 years (10.8-13.7 Hz, P = 0.01) but no change was found in children aged 6-24 months (12.7-12.7 Hz, P = 0.9) or 0-6 months (9.45-7.26 Hz, P = 0.08). Throughout emergence, infants demonstrated a discontinuous EEG pattern with intermittent bursts separated with low amplitude. CONCLUSIONS: During anesthesia, the EEG in infants is fundamentally different from the EEG in older children. This study supports the need for specific infant-derived algorithms if EEG-derived anesthesia depth monitors are to be used in infants.     

A comparison of patient state index and bispectral index values during the perioperative period

The patient state index (PSI), a quantitative electroencephalographic index, has been recently introduced into clinical practice as a monitor for assessing consciousness during sedation and general anesthesia. We designed this observational study to compare the sensitivity and specificity of the PSI with that of the bispectral index (BIS) with respect to their ability to predict the loss of consciousness and emergence from anesthesia, as well as to assess changes in IV (propofol) and inhaled (desflurane) anesthetics during the maintenance period. Twenty consenting patients scheduled for elective laparoscopic surgical procedures were enrolled in this prospective clinical study. Anesthesia was induced with propofol 2 mg/kg IV and fentanyl 1 micro g/kg IV, and tracheal intubation was facilitated with cisatracurium 0.3 mg/kg IV. Desflurane 4% in combination with nitrous oxide 60% in oxygen was administered for the maintenance of anesthesia. Comparative PSI and BIS values were obtained at specific time intervals during the induction, maintenance, and emergence periods. The changes in these indices were recorded after the administration of propofol (20 mg IV) or with 2% increases or decreases in the inspired concentration of desflurane during the maintenance period. With logistic regression models, both the BIS and PSI were found to be effective as predictors of unconsciousness (i.e., failed to respond to verbal stimuli) (P < 0.01). The PSI also correlated with the BIS during both the induction of (r = 0.78) and emergence from (r = 0.73) general anesthesia. However, the area under the receiver operating characteristic curve for detection of consciousness indicated a better performance with the PSI (0.95 +/- 0.04) than the BIS (0.79 +/- 0.04). During the maintenance period, the PSI values were comparable to the BIS in response to changes in propofol and desflurane but displayed greater interpatient variability. Finally, the PSI (versus BIS) values were less interfered with by the electrocautery unit during surgery (16% versus 65%, respectively). In conclusion, the PSI may prove to be a viable alternative to the BIS for evaluating consciousness during the induction of and emergence from general anesthesia, as well as for titrating the administration of propofol and desflurane during the maintenance period. However, further studies with the PSA device are needed to determine its role in anesthesia. IMPLICATIONS: The patient state index could be a useful alternative to the bispectral index for assessing level of consciousness during the induction of and emergence from anesthesia, as well as for titrating IV and volatile anesthetics during surgery.     

The auditory steady-state response is not a suitable monitor of anesthesia

Previous studies show that the human 40-Hz auditory steady-state response (ASSR) disappears on induction of general anesthesia, suggesting that it may be a good candidate for a monitor of anesthesia. In this study, we aimed to learn whether all normal alert adults display ASSRs with adequate signal-to-noise ratio. Clicks were presented at a series of frequencies between 35 and 70 Hz and electroencephalographic records taken at the vertex were Fourier transformed. ASSRs were observable as sharp peaks in the electroencephalograph spectrum at the frequency of the clicks. Initial results showed that a discernible ASSR could not be obtained from about half the subjects studied at any click frequency used. Further investigation revealed that in subjects whose ASSR was undetectable in the alert state, induction of a drowsy mental state resulted in appearance of an observable ASSR. This was attributable to an increase in signal in the drowsy state, not to a decrease in noise. We conclude that, because a significant proportion of subjects do not display easily recordable ASSRs when alert, it is not practical to use disappearance of the ASSR as a routine test for adequacy of anesthesia. IMPLICATIONS: Auditory steady-state responses (ASSRs) are brain waves evoked by auditory stimuli. Because they reportedly disappear under general anesthesia, they have been suggested as potential indicators of anesthetic depth. However, in this study, we show that about half our normal adult subjects did not produce measurable ASSRs when awake. This suggests that ASSRs are not good candidates for use in monitoring anesthetic depth during surgery.     

不同镇静程度下健康志愿者事件相关电位与意识和记忆的关系

目的探讨不同镇静程度下事件相关电位(ERPs)与意识和记忆的关系。方法 10名志愿者加入本研究。采用异丙酚静脉靶控输注,记录不同镇静清醒(OAA/S)评分、ERPs各波(N1、P3) 波幅及潜伏期、无创血压、心电图、脉搏血氧饱和度。于OAA/S评分3分、2分时给志愿者听学习表内容。志愿者清醒后4 h进行记忆测试。采用加工分离程序(PDP)分离内隐记忆和外显记忆。计算学习表、干扰表包含测验和排除测验的测试成绩,计算外显记忆、内隐记忆成绩。结果与基础值比较, OAA/S评分3分时,N1潜伏期延长,波幅降低;P3波幅降低,潜伏期延长(P<0．05或0．01);OAA/S评分为2分时,N1潜伏期延长,波幅降低,P3消失;OAA/S评分3分时存在外显记忆和内隐记忆;评分2 分时,意识和外显记忆一同消失,内隐记忆存在。与OAA/S评分3分比较,2分时内隐记忆成绩下降 (P<0．01)。P3波幅与外显记忆成绩呈正相关(n=10,P<0．05或0．01)。结论 P3可作为判断镇静状态下意识和外显记忆存在或消失的指标,但不能反映内隐记忆的存在与否。     

Middle latency auditory evoked responses and electroencephalographic derived variables do not predict movement to noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia.

The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. However, a reliable monitor of the adequacy of anesthesia has not yet been defined. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict subsequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N2O. We also investigated whether the absolute values of any of these variables before skin incision was able to predict subsequent movement. After the induction of anesthesia with propofol and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N2O 50% without an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair. MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the changes in EEG and MLAER were recorded. EEG and MLAER values were evaluated before and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which patients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin incision for any of the variables. MLAER amplitude increased after skin incision. We conclude that it is unlikely that linear EEG measures or MLAER variables can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation. IMPLICATIONS: Reliable estimation of anesthetic adequacy remains a challenge. Changes in spontaneous or auditory evoked brain activity after a brief electrical stimulus at the wrist could not be used to predict whether anesthetized patients would subsequently move at the time of surgical incision.     

Depth of sedation using Cerebral State Index in infants undergoing spinal anesthesia

Background: Infants are noted to frequently sleep during spinal anesthesia, with a concomitant fall in Bispectral Index. However, there are suggestions that EEG derived anesthesia depth monitors have inferior performance in infants. The aim of this study was to quantify the degree of sedation during spinal anesthesia in infants using another EEG derived measure of anesthesia effect – the Cerebral State Index (CSI). Methods: Twelve infants, <52 weeks postconceptual menstrual age, scheduled for bilateral inguinal hernia repair under spinal anesthesia were enrolled. Patients received a standard anesthetic protocol with a subarachnoid dose of 1 mg·kg^?1 of levobupivacaine 0.5%. No premedication, sedatives, opioids or anticholinergics were administrated during the perioperative period and patients were left undisturbed during the surgical time, without tactile stimulation or loud auditory stimuli. CSI score (0–100) and bust suppression (BS) (0–100%) were continuously recorded during the surgical time and then statistically re‐evaluated. Results: In all patients the CSI fell during the procedure and there were significant levels of BS recorded by the CSI monitor. The BS occurred between 12 and 34 min after spinal anesthesia with the peak being at 30 min and mean onset time being 15 (2.6) min after spinal block. A statistical significant difference was found between the lowest mean CSI as well as the highest BS if compared with their baseline values. A negative correlation was found between CSI and BS. Conclusions: The degree of burst suppression detected by the CSI in our study supports the hypothesis that infants may have discontinuous patterns of EEG during spinal anesthesia similar to those seen during emergence from general anesthesia. Moreover, the limitations in the application of the adult algorithms to infant EEG may lead to an overestimation of the degree of sedation.     

Quantitative electroencephalographic analysis of the biphasic concentration-effect relationship of propofol in surgical patients during extradural analgesia

We studied effects on the EEG of propofol infused at a rate of 0.5 mg kg-1 min-1 for 10 min in 10 healthy male surgical patients under extradural analgesia. The EEG amplitude in six frequency bands was related to arterial blood propofol concentrations and responsiveness to verbal commands. The EEG amplitude showed a characteristic biphasic response to increasing blood propofol concentrations in all frequency bands. During the infusion, patients lost responsiveness when EEG amplitudes in the high frequency bands were decreasing after having reached a maximum. EEG changes were different during infusion and emergence. Pharmacodynamic modelling, using two effect compartments with dissimilar equilibration constants, resulted in satisfactory fits. We conclude that propofol exerts a biphasic effect on the EEG amplitude in all frequency bands. The dissimilarity of EEG changes during infusion and during emergence suggests that two effect compartments with different equilibration constants exert opposing effects on the EEG.      

Correlation of bispectral index and Guedel's stages of ether anesthesia

Bispectral index (BIS) analysis is a method of electroencephalograph (EEG) analysis based on the interfrequency phase relationships of the EEG, designed to quantify anesthetic hypnosis. The BIS was created after concurrent collection of EEG and clinical data from a large number of patients anesthetized with various drugs over a prolonged period and then performing a Fourier analysis followed by a bispectral calculation. The clinical stages of anesthetic depth are very well demonstrated in etherized patients. In this study, we studied the BIS changes during various stages of ether anesthesia and quantified the hypnotic depth during the surgical stage of ether anesthesia. The values for BIS under various stages and planes of ether anesthesia were recorded in 21 patients listed for short surgical procedures. During diethyl ether anesthesia, BIS initially increased and subsequently decreased. During surgical anesthesia, a BIS value of 30 was observed. IMPLICATIONS: For the first time, bispectral index (BIS) has been studied in patients being anesthetized solely with ether. Ether both causes an increase and decrease in BIS during induction and emergence. The index observed during the surgical stage of ether anesthesia is probably the correct value for the depth of hypnosis because no other volatile anesthetic can produce the true anesthetic state when used alone. This value could be taken as the value to be attained when balanced anesthesia is being practiced.     

The potentialities of EEG analysis using the Lifescan in anesthesiology]

In 87 patients a new computerized EEG monitor was used in clinical anesthesia. Following aperiodic analysis, the Lifescan shows the computerized EEG in a striking color visualization allowing recognition of changes in the course of the EEG and of interhemispheric differences. The computerized EEG can be read after a brief training period. The changes caused by different clinical situations are visualized in detail. Different changes were observed with the anesthetics used. During induction with thiopentone a sudden increase in activity was obvious. During a lighter level of anesthesia with thiopentone beta activity was easily recognizable. During induction with high-dose fentanyl frequency was slowed over a long period and activity was increased. The monitor shows the changes in the lower frequency range in detail. A bimodal pattern occurred with a higher dosage of enflurane. During lightening of anesthesia with enflurane a typical pattern indicated return to consciousness. When nitrous oxide was administered during anesthesia with halothane, and to a lesser degree during anesthesia with enflurane or isoflurane, remarkable slowing and decrease in activity occurred. The monitor allowed detailed recognition of the different patterns obtained with the various anesthetics. In some cases it was possible to say what dosage of the anesthetic used had been given or what depth of anesthesia had been achieved. The effect of combined anesthesia, however, was difficult to judge. In carotid artery surgery a quick unilateral decrease in activity and slowing of frequency indicated cerebral ischemia and quickly disappeared after insertion of a shunt. This change was particularly obvious with the new monitor.(ABSTRACT TRUNCATED AT 250 WORDS)