Awareness and the EEG power spectrum: analysis of frequencies

BACKGROUND

Power spectral analysis is a well-established method for the analysis of EEG signals. Spectral parameters can be used to quantify pharmacological effects of anaesthetics on the brain and the level of sedation. This method, in numerous variations, has been applied to depth of anaesthesia monitoring and has been incorporated into several commercially available EEG monitors. Because of the importance of EEG spectral analysis, we evaluated the performance of each frequency in the power spectrum regarding detection of awareness.

METHODS

Ninety artefact-free EEG segments of length 8 s were obtained from a database that contains perioperatively recorded EEG data. For the present analysis, EEG data were selected from 39 patients with propofol-remifentanil or sevoflurane-remifentanil anaesthesia with a period of awareness. Half of the EEG segments were recorded during periods of awareness as defined by an adequate response to the command 'squeeze my hand'. The other half were from unresponsive patients. The power spectral density was calculated for each segment. The performance of each frequency bin of the power spectrum as a detector of awareness was assessed with a remapped prediction probability rPK, i.e. the prediction probability PK mapped to a range of 0.5-1.

RESULTS

The remapped prediction probability was high (rPK>0.8) for low frequencies (<15 Hz) and for high frequencies (>26 Hz), with a minimum (rPK<0.55) at 21 Hz. Indentations in the 'performance spectrum' occur at the power-line frequency (50 Hz) and its harmonics and at 78 Hz, probably caused by the continuous impedance measurement of another device used in parallel. With the exception of the indentations, the remapped prediction probability of the high frequencies (>35 Hz) was >0.95.

CONCLUSIONS

The best performance for the detection of awareness was achieved by EEG power spectral frequencies from >35 Hz up to 127 Hz. This frequency band may be dominated by muscle activity. The frequency band between 15 and 26 Hz may be of limited value, as reflected by lower rPK values.

A BIS-guided study of sevoflurane requirements for adequate depth of anaesthesia in Caesarean section

Caesarean section carries a high risk of awareness, especially in the period prior to neonatal delivery. We investigated the concentration of sevoflurane required to maintain bispectral index (BIS) < 60 until delivery occurred. We enrolled 23 parturients into an up-down sequential allocation study. The median effective end-tidal concentration (EC(50)) of sevoflurane was defined as that which maintained BIS < 60 between skin incision and delivery in 50% of patients. This was calculated using Dixon and Massey's method. Receiver operating characteristic curve analysis was used to establish BIS response probability thresholds. The EC(50) for sevoflurane was 1.22% (1.08-1.33, 95% CI). The probability of maintaining BIS < 60 was < 55% at a concentration of < 1.1%; this increased to 80% at concentrations of 1.2-1.3%. We conclude that sevoflurane concentrations of at least 1.2-1.3% should be administered in uncomplicated Caesarean section, so as to minimise the risk of awareness and recall.

Narcotrend® Does Not Adequately Detect the Transition between Awareness and Unconsciousness in Surgical Patients

Background

The Narcotrend index (MonitorTechnik, Bad Bramstedt, Germany) is a dimensionless number between 0 and 100 that is calculated from the electroencephalogram and inversely correlates with depth of hypnosis. The current study evaluates the capability of the Narcotrend to separate awareness from unconsciousness at the transition between these levels.

Methods

Electroencephalographic recordings of 40 unpremedicated patients undergoing elective surgery were analyzed. Patients were randomly assigned to receive (1) sevoflurane-remifentanil (&lt;/= 0.1 microg . kg . min), (2) sevoflurane-remifentanil (&gt;/= 0.2 microg . kg . min), (3) propofol-remifentanil (&lt;/= 0.1 microg . kg . min), or (4) propofol-remifentanil (&gt;/= 0.2 microg . kg . min). Remifentanil and sevoflurane or propofol were given until loss of consciousness. After tracheal intubation, propofol or sevoflurane was stopped until return of consciousness and then restarted to induce loss of consciousness. After surgery, drugs were discontinued. Narcotrend values at loss and return of consciousness were compared with each other, and anesthetic groups were compared. Prediction probability was calculated from values at the last command before and at loss and return of consciousness.

Results

At 105 of 316 analyzed time points, the Narcotrend did not calculate an index, and the closest calculated value was analyzed. No significant differences between loss and return of consciousness were found. In group 1, Narcotrend values were significantly higher than in group 3. Prediction probability was 0.501.

Conclusions

In these challenging data, the Narcotrend did not differentiate between awareness and unconsciousness. In addition, Narcotrend values were not independent from the anesthetic regimen.

Spectral entropy measurement of patient responsiveness during propofol and remifentanil. A comparison with the bispectral index †

BACKGROUND

We compared two spectral entropies, state entropy (SE) and response entropy (RE), based on the irregularity of the EEG, to measure loss of response to verbal command (LOR(verbal)) and noxious stimulus (LOR(noxious)) with the bispectral index (BIS) during propofol infusion with and without remifentanil.

METHODS

Three groups of 20 patients received an effect-site controlled propofol infusion (Ce(PROP)) starting at 1 microg ml(-1) and increased in steps of 0.5 microg ml(-1) at 4 min intervals. In addition, a remifentanil infusion was maintained at a group-dependent, fixed effect-site target concentration (Ce(REMI)) (0, 2 or 4 ng ml(-1)). The ability of BIS, SE or RE to predict LOR(verbal) and LOR(noxious) were compared with the changes in BIS, SE and RE using logistic regression, prediction probability (P(K)), and sensitivity/specificity.

RESULTS

In all groups, BIS, SE and RE decreased with increasing Ce(PROP). However, BIS decreased more smoothly than SE and RE at deeper levels of sedation. At LOR(verbal), BIS(50), SE(50) and RE(50) increased with increasing Ce(REMI). BIS, SE and RE all detected LOR(verbal) accurately but BIS performed better at 100% sensitivity. Sensitivity/specificity for detection of LOR(verbal) decreased for all methods with increasing Ce(REMI). LOR(noxious) was poorly described by all measures.

CONCLUSION

LOR(verbal) was detected accurately by BIS, SE and RE except for 100% sensitivity, where BIS performed better. Though BIS, SE and RE were influenced by remifentanil during propofol administration, their ability to detect LOR(verbal) remained accurate. None of the measures predicted LOR(noxious).

[Unwanted wakefulness during general anesthesia]

Intraoperative wakefulness ("awareness") is still a relevant problem. Different stages of wakefulness exist: conscious awareness with explicit recall of pain in 0.03% and with nonpainful explicit recall in 0.1-0.2% of all anesthesias; amnesic awareness or implicit recall may occur with unknown, even higher incidences. Sufficient analgesia minimizes possible painful perceptions. Opioids, benzodiazepines, and N(2)O alone or combined lead to the highest incidences of nonpainful intraoperative wakefulness. Volatile anesthetics, etomidate, barbiturates, and propofol in sufficient doses effectively block any sensory processing and therefore abolish intraoperative wakefulness. Intraoperative awareness with recall may lead to sustained impairment of the patients, in severe cases even to a post-traumatic stress disorder (PTSD). The observation of clinical signs does not reliably detect intraoperative wakefulness in all cases; monitoring of end-tidal gas concentrations, EEG, or evoked potentials may help in prevention. Active information is recommended only for patients at higher risk. Complaints about recall of intraoperative events should be taken seriously; in cases of sustained symptoms psychological help may be necessary.

Melatonin als Alternative zur Sedierung bei Durchführung einer Hirnstammaudiometrie bei Kindern

BACKGROUND

Though one of the most important investigations in paediatric audiology, brainstem evoked response audiometry (BERA) often necessitates sedation or general anaesthetics, especially in newborn and infants. In paediatric neurology, melatonin has been successfully used for some years to induce sleep prior to EEG investigations. Melatonin as a hormone regulating the circadian rhythm induces natural sleep without the risks of sedation. Side effects are not known.

METHODS

Click-induced BERA was first performed in 10 adults with normal hearing with and without previous melatonin administration, and click thresholds and latencies of evoked potentials were compared. 50 children then underwent BERA in melatonin-induced sleep.

RESULTS

Click thresholds in adults were mostly identical (r = 0,88), while the mean latencies of evoked potentials seemed to be minimally prolonged (r from 0,82 to 0,95). Click-induced BERA was successful in 45 of the 50 children, and notched-noise BERA in at least 2 frequencies in 38 of 43 children.

CONCLUSIONS

Offering a high success rate with no side effects, melatonin-induced sleep seems to be a good alternative to sedation. This method is widely accepted by parents and permits earlier diagnosis of hearing impairment in the routine clinical setting.

Sedation after spinal anesthesia in elderly patients: a preliminary observational study with the PSA-4000

PURPOSE

Neuraxial blockade is known to have a sedative effect, decreasing the need for inhalational and iv anesthetic agents. The purpose of the present study was to quantify the sedative effect of spinal anesthesia and to determine the time of maximum sedation.

METHODS

This is an observational study in which 20 unsedated patients were scheduled to undergo urologic and orthopedic surgeries under spinal anesthesia. Patients with pre-existing neurological conditions or receiving psychotropic medications were excluded from the study. All received 1.5 mL (11.25 mg) of hyperbaric bupivacaine 0.75% intrathecally. No sedative or narcotic was administered intravenously or intrathecally. The Patient State Analyzer, (PSA-4000) was used to monitor sedation along with Observer's Assessment of Alertness and Sedation (OAA/S) scores every five minutes. Differences in patient state index (PSI) and OAA/S scores are expressed as median and range and were evaluated by Wilcoxon's signed rank test for non-parametric data; P < 0.05 was considered significant. PSI, OAA/S and time at lowest score are expressed as median(range).

RESULTS

PSI scores decreased from baseline 99 (96-99) to 78 (56-87) at 35(14.5-54) min into the spinal anesthetic (P < 0.05). OAA/S scores decreased from baseline 5 to 4 (range 3-5) at the time of the lowest PSI scores (P < 0.05).

CONCLUSIONS

In this elderly patient population, spinal anesthesia induced changes in the processed electroencephalogram with reduction in PSI and OAA/S scores. The reduction in afferent input to the reticular activating system could possibly explain the sedation that has been observed and the reduction in the PSA scores.

The Patient State Index as an indicator of the level of hypnosis under general anaesthesia

BACKGROUND

This retrospective study describes the performance of the Patient State Index (PSI), under standard clinical practice conditions. The PSI is comprised of quantitative features of the EEG (QEEG) that display clear differences between hypnotic states, but consistency across anaesthetic agents within the state.

METHODS

The PSI was constructed from a systematic investigation of a database containing QEEG extracted from the analyses of continuous 19 channel EEG recordings obtained in 176 surgical patients. Induction was accomplished with etomidate, propofol, or thiopental. Anaesthesia was maintained by isoflurane, desflurane, or sevoflurane, total i.v. anaesthesia using propofol, or nitrous oxide/narcotics. It was hypothesized that a multivariate algorithm based on such measures of brain state, would vary significantly with changes in hypnotic state.

RESULTS

Highly significant differences were found between mean PSI values obtained during the different anaesthetic states selected for study. The relationship between level of awareness and PSI value at different stages of anaesthetic delivery was also evaluated. Regression analysis for prediction of arousal level using PSI was found to be highly significant for the combination of all anaesthetics, and for the individual anaesthetics.

CONCLUSIONS

The PSI, based upon derived features of brain electrical activity in the anterior/posterior dimension, significantly co-varies with changes in state under general anaesthesia and can significantly predict the level of arousal in varying stages of anaesthetic delivery.

Bispectral index, serum drug concentrations and emergence associated with individually adjusted target-controlled infusions of remifentanil and propofol for laparoscopic surgery

BACKGROUND

Target-controlled infusions (TCI) are used to simplify administration and increase precision of i.v. drugs during general anaesthesia. However, there is a limited relationship between preset targets and measured concentrations of drugs and between measured concentrations and measures of brain function, such as the bispectral index (BIS).

METHODS

We set out to evaluate the performance of TCI devices for propofol (Diprifusor) and remifentanil (Remifusor, prototype), during laparoscopic cholecystectomy in 21 patients. We also checked if there was any correlation between serum concentrations of propofol and BIS during individually adjusted anaesthesia.

RESULTS

The Diprifusor and Remifusor had a median absolute performance error of 60% and 25% respectively. Propofol concentrations were underpredicted by a median of 60%, and remifentanil concentrations were slightly overpredicted by a median of 7%. When anaesthesia was adjusted to keep BIS values between 45 and 60, no correlation existed between measured concentrations of propofol and the corresponding BIS values, although both BIS and serum propofol concentration discriminated well between the awake and asleep states. Emergence was rapid and uneventful in all patients. Female patients had a more rapid emergence than male patients (6.6 and 11.6 min respectively).

CONCLUSIONS

TCI devices for remifentanil and propofol result in large variation in measured serum concentrations. The lack of correlation between BIS and serum concentrations of propofol adds to the debate about whether BIS measures hypnosis as a graded state during surgery. This study confirms that women wake up faster than men, but provides no explanation for this repeatedly shown difference.

Reduction in the incidence of awareness using BIS monitoring

BACKGROUND

Explicit recall (ER) is evident in approximately 0.2% of patients given general anaesthesia including muscle relaxants. This prospective study was performed to evaluate if cerebral monitoring using BIS to guide the conduction of anaesthesia could reduce this incidence significantly.

PATIENTS AND METHODS

A prospective cohort of 4945 consecutive surgical patients requiring muscle relaxants and/or intubation were monitored with BIS and subsequently interviewed for ER on three occasions. BIS values between 40 and 60 were recommended. The results from the BIS-monitored group of patients was compared with a historical group of 7826 similar cases in a previous study when no cerebral monitoring was used.

RESULTS

Two patients in the BIS-monitored group, 0.04%, had ER as compared with 0.18% in the control group (P < 0.038). Both BIS-monitored patients with ER were aware during intubation when they had high BIS values (> 60) for 4 min and more than 10 min, respectively. However, periods with high BIS = 4 min were also evident in other patients with no ER. Episodes with high BIS, 4 min or more, were found in 19% of the monitored patients during induction, and in 8% of cases during maintenance.

CONCLUSIONS

The use of BIS monitoring during general anaesthesia requiring endotracheal intubation and/or muscle relaxants was associated with a significantly reduced incidence of awareness as compared with a historical control population.

Awareness under general anesthesia

General anesthesia aims to eliminate patients' awareness of excruciating pain during surgery. Nevertheless, rare occurrences of patient awareness continue because the problem is not yet completely preventable. One study puts the incidence of awareness at 0.18% for patients receiving muscle relaxants and at 0.10% for patients not given relaxant drugs. Awareness experiences frighten patients and impact their implicit and explicit memories in ways that can leave a lifetime of residual emotional and psychological problems ranging from sleep disturbances, nightmares, and daytime anxiety that may subside with time to development of post-traumatic stress disorder. Most anesthetists monitor depth of anesthesia by assessing intraoperative hemodynamic responses to surgical stimuli--an approach questioned by some authors. Several depth-of-anesthesia monitors are available, but there is no ideal monitor that is 100% reliable. This review provides an overview of literature that reports findings associated with the monitoring and occurrence of intraoperative awareness. These studies indicate assessment methods that can be trusted when we provide general anesthesia and what measures can be taken to prevent recall by patients under general anesthesia.

Awareness: Monitoring versus remembering what happened

BACKGROUND

Awareness during anesthesia is foremost assessed with postoperative interviews, which may underestimate its incidence. On-line monitors such as the Bispectral Index and patient response to verbal command are not necessarily commonly used. This study investigated response to command during deep sedation (Bispectral Index 60-70) and the ability of prevailing monitoring techniques to indicate awareness and predict recall.

METHODS

The authors systematically assessed the response to command using the isolated forearm technique while monitoring electroencephalographic and hemodynamic variables. Fifty-six elective surgical patients were repeatedly given verbal instructions to squeeze the observer's hand during target-controlled infusion with propofol and alfentanil. After recovery, conscious recall was assessed with a short structured interview.

RESULTS

Overall, 1,082 commands were given. No response was observed to 887 (82%) commands, an equivocal response was observed to 56 (5%) commands, and an unequivocal response was observed to 139 (13%) commands. Of the 37 patients (66%) with an unequivocal response to command ("awareness"), nine (25%) reported conscious recall after recovery. Their reports provided valuable insights as to how awareness may be adequately addressed. Hemodynamic variables poorly predicted awareness, whereas parameters derived from the encephalogram, especially the Bispectral Index, were highly significant predictors (P < 0.0001). Electroencephalographic parameters did not discriminate between patients with or without conscious recall, whereas heart rate and responsiveness to command did.

CONCLUSIONS

The incidence of awareness is underestimated when conscious recall is taken as evidence. Awareness can be monitored on-line with behavioral and modern neurophysiologic measures. Providing feedback during intra-anesthetic awareness helps patients to cope with a potentially stressful situation.

Detection of awareness in surgical patients with EEG-based indices--bispectral index and patient state index

BACKGROUND

Patient state index (PSI) and bispectral index (BIS) are values derived from the EEG, which can measure the hypnotic component of anaesthesia. We measured the ability of PSI and BIS to distinguish consciousness from unconsciousness during induction and emergence from anaesthesia and a period of awareness in surgical patients.

METHODS

Forty unpremedicated patients were randomized to receive: (1) sevoflurane/remifentanil (< or =0.1 microg kg(-1) min(-1)), (2) sevoflurane/remifentanil (> or = 0.2 microg kg(-1) min(-1)), (3) propofol/remifentanil (< or =0.1 microg kg(-1) min(-1)), (4) propofol/remifentanil (> or = 0.2 microg kg(-1) min(-1)). Every 30 s after the start of the remifentanil, patients were asked to squeeze the investigator's hand. Sevoflurane or propofol were given until loss of consciousness (LOC1). Tunstall's isolated forearm technique was used during neuromuscular block with succinylcholine. After tracheal intubation, propofol or sevoflurane were stopped until return of consciousness (ROC1). Propofol or sevoflurane were re-started to induce LOC2. After surgery, drugs were discontinued and recovery (ROC2) was observed. PSI and BIS at LOC (LOC1 and LOC2) were compared with those at ROC (ROC1 and ROC2) (t-test). Prediction probability (P(k)) was calculated from values at the last command before and at LOC and ROC. Values are mean (SD).

RESULTS

At non-responsiveness, BIS (66 (17)) and PSI (55 (23)) were significantly less than at responsiveness (BIS, 79 (14); PSI, 77 (18); P<0.05). The wide variation with both BIS and PSI measurements of the 80 'awareness' values led to an erroneous classification as unconscious in some cases (BIS, six patients; PSI, nine patients). P(k) was 0.68 (0.03) (BIS) and 0.69 (0.03) (PSI).

CONCLUSIONS

Despite significant differences between mean values at responsiveness and non-responsiveness for BIS and PSI, neither measure may be sufficient to detect awareness in an individual patient, reflected by a P(k) less than below 70%.

Propofol and sufentanil titration with the bispectral index to provide anesthesia for coronary artery surgery

BACKGROUND

To provide anesthesia for cardiac surgery, hypnotics and opioids are frequently titrated on variables such as mean arterial pressure and heart rate. In this study conducted in patients scheduled to undergo coronary artery bypass grafting, propofol and sufentanil, both administered by computer-controlled infusion, were titrated on the Bispectral Index (BIS) values using a predefined algorithm.

METHODS

After written informed consent, 110 patients, 95 men and 15 women aged 61 (9) yr [mean (SD)], were randomly allocated to receive predicted sufentanil effect site concentrations (Ce) of 0.5, 0.75, 1, 1.25, and 1.5 ng/ml, decreased by a third after sternotomy (groups 1-5). Target induction propofol concentration was 1.5 microg/ml and subsequently adjusted on BIS values. The following parameters were recorded: BIS values, predicted propofol Ce, the number of changes of propofol target, mean arterial pressure, heart rate, the number of bolus injection and doses of vasoconstrictor and vasodilator drugs, time to tracheal extubation, postoperative awareness and satisfaction scores, and cumulative morphine doses for the first postoperative day.

RESULTS

One patient randomized to group 1 required 0.75 ng/ml sufentanil Ce instead of 0.5 ng/ml for increased BIS values on tracheal intubation. BIS values were similar in the five groups. The predicted propofol Ce values were different (P < 0.05; analysis of variance) among the five groups: 1.59 (0.47) to 1.23 (0.25) microg/ml in group 1 and group 4, respectively. Significantly fewer changes of propofol target were required in group 4 as compared to group 1. There were no differences among the five groups for mean arterial pressure, heart rate, time to tracheal extubation, awareness, satisfaction scores, and morphine requirements.

CONCLUSION

These results suggest the BIS, as part of an algorithm that uses both the absolute BIS value and its increase following tracheal intubation, can be used to effectively titrate both propofol and sufentanil. A predicted sufentanil Ce of 1.25 ng/ml before and 0.8 ng/ml after sternotomy was associated with the lowest predicted propofol Ce and fewer changes of propofol target. Lower sufentanil concentrations required higher propofol concentrations and more frequent changes of the target propofol concentration and were associated with similar hemodynamic tolerance.

Absence of explicit and implicit memory in unconscious patients using a TCI of propofol

BACKGROUND

Episodes of implicit memory have been described during propofol anaesthesia. It remains unclear whether implicit memory is caused by short periods of awareness or occurs in an unconscious subject.

METHODS

Sixty patients were randomized in an experimental group (EG), a control group (CG) and a reference group (RG). Loss of consciousness (LOC) was obtained by progressive stepwise increases of propofol using a target-controlled infusion device (Diprifusor, Alaris Medical Systems, San Diego, CA). A tape containing 20 words was played to the patients in the CG before the start of anaesthesia and to the patients in the EG at a constant calculated concentration of propofol associated with LOC. The tape was not played to the patients in the RG. Three memory tests were performed postoperatively.

RESULTS

Explicit and implicit memories were evidenced in the CG but not in the EG.

CONCLUSION

In our group of young ASA I/II patients, in the absence of any noxious stimulus, no implicit or explicit memory was found when the calculated concentration of propofol using a Diprifusor was maintained at the level associated with LOC.

The AAI index, the BIS index and end-tidal concentration during wash in and wash out of sevoflurane

The bispectral index (BIS), auditory evoked potential index (AAI) and the end-tidal sevoflurane concentration were studied during induction and emergence in 10 ASA I-II patients. Both during 'wash-in' and 'wash-out' of sevoflurane, the AAI and BIS indices show huge variability and an overlap of indices between awake and not responding to command. This was the most pronounced during induction and the range of values was larger for the AAI index as compared with the BIS index. Mean (range) BIS was 85 (73-98) and 48 (10-83) awake and unconscious, respectively, and mean AAI index was 71 (43-99) and 21 (4-85), respectively. This study demonstrates the difficulties of using processed EEG variables in real time in a clinical situation of non-steady state pharmacodynamics.