Pitfalls and challenges when assessing the depth of hypnosis during general anaesthesia by clinical signs and electronic indices

Relationships between the qNOX, qCON, burst suppression ratio, and muscle activity index of the CONOX monitor during total intravenous anesthesia: a pilot study

BACKGROUND

Processed electroencephalographic (EEG) indices can help to navigate general anesthesia. The CONOX (Fresenius Kabi) calculates two indices, the qCON (hypnotic level) and the qNOX (nociception). The CONOX also calculates indices for electromyographic (EMG) activity and EEG burst suppression (BSR). Because all EEG parameters seem to influence each other, our goal was a detailed description of parameter relationships.

METHODS

We used qCON, qNOX, EMG, and BSR information from 14 patients receiving propofol anesthesia. We described index relationships with linear models, heat maps, and box plot representations. We also evaluated associations between qCON/qNOX and propofol/remifentanil effect site concentrations (ceP/ceR).

RESULTS

qNOX and qCON (qCON = 0.79*qNOX + 5.8; p < 0.001; R2 = 0.84) had a strong linear association. We further confirmed the strong relationship between qCON/qNOX and BSR for qCON/qNOX < 25: qCON=-0.19*BSR + 25.6 (p < 0.001; R2 = 0.72); qNOX=-0.20*BSR + 26.2 (p < 0.001; R2 = 0.72). The relationship between qCON and EMG was strong at higher indices: qCON = 0.55*EMG + 33.0 (p < 0.001; R2 = 0.68). There was no qCON > 80 without EMG > 0. The relationship between ceP and qCON was qCON=-3.8*ceP + 70.6 (p < 0.001; R2 = 0.11). The heat maps also suggest that the qCON and qNOX can at least partially separate the hypnotic and analgetic components of anesthesia.

CONCLUSION

We could describe relationships between qCON, qNOX, EMG, BSR, ceP, and ceR, which may help the anaesthesiologist better interpret the information provided. One major finding is the dependence of qCON > 80 on EMG activity. This may limit the possibility of detecting wakefulness in the absence of EMG. Further, qNOX seems generally higher than qCON, but high opioid doses may lead to higher qCON than qNOX indices.

Hypnosis measured with monitors of anesthetic depth - EEG changes during the test for Harvard Group Scale of Hypnotic Susceptibility

Introduction

Hypnotic trance can be defined as a non-ordinary state of consciousness that is accompanied by a number of neurophysiological changes, including brain electrophysiology. In addition to subjective measures, corresponding objective parameters are needed in experimental and clinical hypnosis research but are complex, impractical, or unspecific. A similar challenge exists for the measurement and monitoring of drug-induced hypnosis, namely general anesthesia. The observation of changes in EEG induced by narcotics has led to the development of monitors for the depth of anesthesia based on EEG parameters. We investigated whether two such monitors react to the induction and maintenance of hypnosis during a highly standardized procedure.

Methods

A total of 56 volunteers were monitored for the bispectral index (BIS) and cerebral state index (CSI) (range 0-100, >95 considered "awake") during the Harvard Group Scale of Hypnotic Susceptibility test. For this test, trance is induced by a taped text and followed by 12 tasks performed under hypnosis. In contrast to random forms of hypnosis, this represents a standardized, worldwide-established condition. According to the resulting score, participants were classified into suggestibility groups in order to evaluate whether the electrophysiological measurements of BIS and CIS indices differ between high and low suggestible persons. Furthermore, participants were asked to rate their hypnotic depth (HD, 1-10) at every task of the test.

Results

Scores dropped significantly from a mean of 97.7 to 86.4 for BIS and from 94.6 to 77.7 for CSI with the induction of hypnosis to stay throughout hypnosis at levels of approximately 88.6 or 82.9, respectively. Results did not differ between high- and low-suggestible participants. The means of the subjective score of hypnotic depth and of the electrophysiological measurements showed a similar course. However, no correlation was found between BIS or CSI values and scores of hypnotic depths.

Conclusion

Monitors for depth of anesthesia respond to changes in consciousness, including trance states of hypnosis. However, specificity is unclear. Practically, in hypnosis research with the exclusion of drug effects or sleep, these monitors might be helpful to test and compare the efficacy of induction texts and to detect disturbances of trance state.

Comparative analysis of the effect of electromyogram to bispectral index and 95% spectral edge frequency under remimazolam and propofol anesthesia: a prospective, randomized, controlled clinical trial

Background

Bispectral index (BIS), an index used to monitor the depth of anesthesia, can be interfered with by the electromyogram (EMG) signal. The 95% spectral edge frequency (SEF95) also can reflect the sedation depth. Remimazolam in monitored anesthesia care results in higher BIS values than propofol, though in the same sedation level assessed by Modified Observers Assessment of Alertness and Sedation (MOAA/S). Our study aims to illustrate whether EMG is involved in remimazolam causing higher BIS value than propofol preliminarily and to explore the correlations among BIS, EMG, and SEF95 under propofol and remimazolam anesthesia.

Patients and methods

Twenty-eight patients were randomly divided into propofol (P) and remimazolam (RM) groups. Patients in the two groups received alfentanil 10 μg/kg, followed by propofol 2 mg/kg and remimazolam 0.15 mg/kg. Blood pressure (BP), heart rate (HR), and oxygen saturation (SpO2) were routinely monitored. The BIS, EMG, and SEF95 were obtained through BIS VISTATM. The primary outcomes were BIS, EMG, and the correlation between BIS and EMG in both groups. Other outcomes were SEF95, the correlation between BIS and SEF95, and the correlation between EMG and SEF95. And all the statistical and comparative analysis between these signals was conducted with SPSS 26.0 and GraphPad Prism 8.

Results

BIS values, EMG, and SEF95 were significantly higher in the RM group than in the P group (all p < 0.001). There was a strong positive correlation between BIS and EMG in the RM group (r = 0.416). Nevertheless, the BIS in the P group showed a weak negative correlation with EMG (r = -0.219). Both P (r = 0.787) and RM group (r = 0.559) had a reasonably significant correlation coefficient between BIS and SEF95. SEF95 almost did not correlate with EMG in the RM group (r = 0.101).

Conclusion

Bispectral index can be interfered with high EMG intensity under remimazolam anesthesia. However, EMG can hardly affect the accuracy of BIS under propofol anesthesia due to low EMG intensity and a weak negative correlation between EMG and BIS. Moreover, SEF95 may have a great application prospect in predicting the sedation condition of remimazolam.

Changes in bispectral index and patient state index during sugammadex reversal of neuromuscular blockade under steady-state sevoflurane anesthesia

Few studies have investigated the changes in patient state index (PSI) and bispectral index (BIS) in response to abrupt increase in electromyographic (EMG) activity. These were performed using intravenous anesthetics or reversal agents for neuromuscular blockade (NMB) other than sugammadex. We compared the changes in BIS and PSI values caused by the sugammadex reversal of NMB during steady-state sevoflurane anesthesia. We enrolled 50 patients with American Society of Anesthesiologists physical status 1 and 2. At the end of the surgery, we administered 2 mg kg-1 sugammadex while maintaining sevoflurane for a 10-min study period. The changes in BIS and PSI from baseline (T0) to train of four ratio of 90% were not significantly different (median difference 0; 95% CI - 3 to 2; P = 0.83), neither were the changes in BIS and PSI values from T0 to their maximum values (median difference 1; 95% CI - 1 to 4; P = 0.53). Maximum BIS and PSI were significantly higher than their baseline values (median difference 6; 95% CI 4-9; P < 0.001 and median difference 5; 95% CI 3-6; P < 0.001, respectively). We found weak positive correlations between BIS and BIS-EMG (r = 0.12, P = 0.01), as well as PSI and PSI-EMG (r = 0.25, P < 0.001). Both PSI and BIS were affected to some extent by EMG artifacts after sugammadex administration.

Usefulness and Reliability of the Bispectral Index during Balanced Anesthesia for Neurovascular Surgery in New Zealand White Rabbits

Few data about the electroencephalogram and its calculated indices, such as the bispectral index (BIS), have been reported in rabbits. We aimed to evaluate whether a clinically stable anesthesia was mirrored by consistent and stable BIS values and to investigate the effects of modified cerebral blood supply, due to bilateral carotid clamping and re-opening, on BIS values. We also investigated the effects of fentanyl, as an antinociceptive drug, on the BIS. Sixty-eight rabbits undergoing general anesthesia for surgical creation of carotid bifurcation aneurysms were enrolled. The BIS values were recorded at nine selected time points (TPs) during each procedure and before and after fentanyl administration. The BIS values over time were compared with two-way repeated-measures analysis of variance followed by Tukey test, while the Wilcoxon signed rank test was performed to compare values at clamping and re-opening of the carotids as well as before and after fentanyl administration. The BIS values were significantly lower during anesthesia than at the end of anesthesia and at tracheal extubation; no significant differences were found among other TPs. Adequate depth of anesthesia was mirrored by consistent BIS values among rabbits, and alteration of cerebral blood supply did not modify BIS values, except once. Following fentanyl, BIS values did not change in a clinically relevant way.

Complexity of Brain Dynamics as a Correlate of Consciousness in Anaesthetized Monkeys

The use of anaesthesia is a fundamental tool in the investigation of consciousness. Anesthesia procedures allow to investigate different states of consciousness from sedation to deep anesthesia within controlled scenarios. In this study we use information quantifiers to measure the complexity of electrocorticogram recordings in monkeys. We apply these metrics to compare different stages of general anesthesia for evaluating consciousness in several anesthesia protocols. We find that the complexity of brain activity can be used as a correlate of consciousness. For two of the anaesthetics used, propofol and medetomidine, we find that the anaesthetised state is accompanied by a reduction in the complexity of brain activity. On the other hand we observe that use of ketamine produces an increase in complexity measurements. We relate this observation with increase activity within certain brain regions associated with the ketamine used doses. Our measurements indicate that complexity of brain activity is a good indicator for a general evaluation of different levels of consciousness awareness, both in anesthetized and non anesthetizes states.

Different perspectives for monitoring nociception during general anesthesia

Safe anesthesia is achieved using objective methods that estimate the patient’s state during different phases of surgery. A patient’s state under anesthesia is characterized by three major aspects, which are linked to the main effects produced by each of the families of anesthetic agents administered: hypnosis, analgesia, and muscular relaxation.

While quantification techniques designed to assess muscular relaxation under neuromuscular blocking agents have a relatively long history with a high degree of standardization and understanding (e.g., the train-of-four), the knowledge and techniques used to the depth of hypnosis assessment suffer from a lesser degree in both standardization and interpretation due to brain complexity.

The problem of standardization and interpretation in the analgesia and nociception assessment increases since it involves more systems, the central nervous system, and the autonomic nervous system.

This helps to explain why there are multiple a priori valid approaches to develop nociception monitoring from different interpretations and physiological bases of noxious stimuli processing. Thus, in this review, the current monitoring technologies clinically available for estimating a patient’s nociception under general anesthesia are described.

Improving Stereotaxic Neurosurgery Techniques and Procedures Greatly Reduces the Number of Rats Used per Experimental Group-A Practice Report

Simple Summary

Stereotaxic surgery techniques are commonly used today in research laboratories by a range of students, technicians, and researchers. Over the past twenty years, technical and scientific progress has been made in neurosurgery to meet the evolving requirements imposed by international legislation, and to promote the implementation of 3R rules. These improvements were motivated by a greater awareness of animal welfare and the necessary effort in the reduction of the number of animals used in experiments. The data presented in the present study show that technical and methodological improvements brought to our surgical procedures from 1992 resulted in reproducible stereotaxic neurosurgeries and in a significant reduction in experimental errors and animal morbidity. The effects of these improvements include a decrease in the final number of animals used in our experiments as well as better management of pain during and after surgery and the use of appropriate aseptic techniques. Correct stereotaxic surgical approaches are precisely described throughout the text.

Abstract

Techniques of stereotaxic surgery are commonly used in research laboratories by a range of students, technicians, and researchers. To meet the evolving requirements imposed by international legislation, and to promote the implementation of 3R rules (replacement, reduction, and refinement) by reducing experimental error, animal morbidity, and mortality, it is essential that standard operating procedures and proper conduct following such complex surgeries be precisely described and respected. The present report shows how refinements of our own neurosurgical techniques over decades, have significantly reduced the number of animals (rats) used in experiments and improved the animals’ well-being during the post-surgical recovery period. The current pre-, per-, and post-surgical procedures used in our laboratory are detailed. We describe the practical aspects of stereotaxic neurosurgery that have been refined in our laboratory since 1992 and that cover various areas including appropriate anesthesia and pain management during and after surgery, methods to determine the stereotaxic coordinates, and the best approach to the target brain structure. The application of these optimal surgical methods that combine reliable and reproducible results with an acute awareness of ethics and animal welfare leads to a significant reduction in the number of animals included in experimental research in accordance with ethical and regulatory rules as required by the European Directive on laboratory animal welfare.

Propofol-Bispectral Index (BIS) Electroencephalography (EEG) Pharmacokinetic-Pharmacodynamic Model in Patients With Post-Cerebral Hemorrhage Hydrocephalus

Background. Titrating hypnotic agents for patients who suffer from a cerebral insult is a challenging task. To date there is no real gold standard to precisely quantify electroencephalography (EEG) response in a fashion that could be utilized for patients with post-cerebral hemorrhage hydrocephaly. While we must administer "as per usual" analgesics for noxious stimuli, we have to administer the hypnotic agents more "sparingly" due to lack of objective monitoring. Methods. We compared 15 adult post-cerebral hemorrhage hydrocephalus patients undergoing ventriculo-peritoneal shunt placement with 15 controls matched for gender and approximate age. We set propofol target controlled infusion estimated plasma concentrations (C_p) to gradually reach 4 µg/mL over 4 minutes. To precisely quantify post-cerebral hemorrhage mental dysfunction, we used electronically retrieved bispectral index (BIS) and propofol C_p data points to create individual inhibitory monophasic mathematical model for each patient that incorporates an independent hysteresis "lag" function. Results. In post-cerebral hemorrhage patients C_p-BIS curve, C₅₀ (propofol concentration associated with inhibitory 50% BIS response) cutoff point was significantly shifted to the left by 39%. Whereas before infusion and at stable propofol 4 µg/mL aneurismal surgical sides ipsilateral (75 ± 13, 25 ± 9) and contralateral (73 ± 15, 27 ± 9) mean ± SD BIS values were significantly lower than ipsilateral (95 ± 3, 46 ± 12) and contralateral (94 ± 3, 46 ± 12) matched controls. Conclusions. Using BIS as surrogate marker of propofol hypnotic effect, BIS monitoring in patients with post-cerebral hemorrhage hydrocephaly showed a pattern of change and trend that was similar albeit 39% significantly lower than subjects without.

Effects of neuromuscular blockade reversal on bispectral index and frontal electromyogram during steady-state desflurane anesthesia: a randomized trial

The degree of neuromuscular blockade reversal may affect bispectral index (BIS) value. One possible reason is that the reverse of neuromuscular blockade affects electromyographic (EMG) signals of fascial muscle. Another reason is, the afferentation theory, the reverse of neuromuscular blockade relieves block signals generated in muscle stretch receptors from accessing the brain through afferent nerve pathways and induces arousal. Inaccurate BIS value may lead to overdose of drugs or the risk of intraoperative awareness. We compared changes in BIS and EMG values according to neuromuscular blockade reversal agents under steady-state desflurane anesthesia. A total of 65 patients were randomly allocated to receive either neostigmine 0.05 mg/kg, sugammadex 4 mg/kg, or pyridostigmine 0.25 mg/kg for neuromuscular blockade reversal under stable desflurane anesthesia, and 57 patients completed the study. The primary outcome was change in BIS and EMG values before and after administration of neuromuscular blockade reversal agents (between train-of-four [TOF] count 1-2 and TOF ratio 0.9). The change in BIS and EMG values before and after administration of neuromuscular blockade reversal agents were statistically different in each group (BIS: Neostigmine group, P < 0.001; Sugammadex group, P < 0.001; Pyridostigmine group, P = 0.001; EMG: Neostigmine group, P = 0.001; Sugammadex group, P < 0.001; Pyridostigmine group, P = 0.001; respectively). The BIS and EMG values had a positive correlation (P < 0.001). Our results demonstrate that the EMG and BIS values have increased after neuromuscular blockade reversal under desflurane anesthesia regardless of the type of neuromuscular blockade reversal agent. BIS should be applied carefully to measure of depth of anesthesia after neuromuscular blockade reversal.

Assessing pain objectively: the use of physiological markers

Pain diagnosis and management would benefit from the development of objective markers of nociception and pain. Current research addressing this issue has focused on five main strategies, each with its own advantages and disadvantages. These encompass: (i) monitoring changes in the autonomic nervous system; (ii) biopotentials; (iii) neuroimaging; (iv) biological (bio-) markers; and (v) composite algorithms. Although each strategy has shown areas of promise, there are currently no validated objective markers of nociception or pain that can be recommended for clinical use. This article introduces the most important developments in the field and highlights shortcomings, with the aim of allowing the reader to make informed decisions about what trends to watch in the future.

To clarify features of photoplethysmography in monitoring balanced anesthesia, compared with Cerebral State Index

Background

Although photoplethysmography and cerebral state index (CSI) have been used as indices in monitoring vital signs perioperatively, there are only a few reports comparing the performance of photoplethysmography with CSI in monitoring anaesthesia depth. The aim of the present study was to clarify features of photoplethysmography in monitoring balanced general anesthesia compared with CSI.

Material/Methods

Forty-five patients undergoing elective operation under general anaesthesia were enrolled in this study. Anaesthesia was induced with target-controlled infusion propofol. The photoplethysmogram, CSI, Modified Observer’s Assessment of Alertness/Sedation Scale (MOAAS), and mean arterial pressure (MAP) were continuously monitored and recorded. Finger photoplethysmogram amplitude (PPGA) and pulse beat interval (PBI) were calculated off-line.

Results

For the period of time from pre-induction to pre-intubation, the coefficient of correlation between MOAAS and CSI was higher than those between MOAAS and PPGA, PBI, and MAP. CSI showed higher prediction probabilities (P_k) to differentiate the levels of MOAAS than did PPGA, PBI, and MAP. PPGA, PBI, and MAP values showed significant differences between before and after intubation, as well as pre- and post-incision (P<0.05), but no significant changes in cerebral state index (P>0.05).

Conclusions

The present study shows that photoplethysmography-derived parameters appear to be more suitable in monitoring the nociceptive component of balanced general anesthesia, while CSI performs well in detecting the sedation or hypnotic component of balanced general anesthesia.

Different Propofol–Remifentanil or Sevoflurane–Remifentanil Bispectral Index Levels for Electrocorticographic Spike Identification during Epilepsy Surgery

Background:

Medical therapy, the cornerstone of managing epilepsy, still fails a substantial portion of patients. Little information is available regarding the potential impact of different bispectral index (BIS) levels on electrocorticographic spike identification for surgical epileptic foci resection.

Methods:

Twenty-two intractable epilepsy subjects were randomly allocated to the propofol–remifentanil or sevoflurane–remifentanil groups, and were further randomized to four BIS85 (BIS 71–85), BIS70 (BIS 56–70), BIS55 (BIS 41–55), and BIS40 (BIS ≤40) sequence order.

Results:

Two-way ANOVA revealed no differences between groups in spike frequency (P = 0.720), spike amplitude (P = 0.647), or number of spiking leads (P = 0.653). In the propofol and sevoflurane groups, decreasing BIS levels increased mean ± SD spike/min frequency (P &lt; 0.001 and P &lt; 0.001) at BIS85 (10 ± 12 and 10 ± 8), BIS70 (19 ± 17 and 17 ± 15), BIS55 (22 ± 17 and 18 ± 8), and BIS40 (25 ± 15 and 23 ± 17). Furthermore, in the propofol and sevoflurane groups, decreasing BIS levels increased spike microvolt amplitude (P = 0.006 and P = 0.009) at BIS85 (1,100 ± 400 and 750 ± 400), BIS70 (1,200 ± 460 and 850 ± 490), BIS55 (1,300 ± 560 and 940 ± 700), and BIS40 (1,400 ± 570 and 1,300 ± 700). Whereas, in the propofol and sevoflurane groups, there was no difference in the location or number of spiking leads (P = 0.057 and P = 0.109) at the four BIS levels. Compared with BIS85, spike frequency in the propofol and sevoflurane groups increased 100 and 170% at BIS70, 116 and 180% at BIS55, and 132 and 230% at BIS40. Compared with BIS85, spike amplitude increased 108 and 113% at BIS70, 121 and 125% at BIS55, and 128 and 170% at BIS40.

Conclusion:

Decreasing BIS levels in the propofol and sevoflurane groups enhanced epileptogenic spike frequency and amplitude with the same location and number of spiking leads.

Preoperative pregabalin does not reduce propofol ED(50): a randomized controlled trial

BACKGROUND

In many studies, gabapentinoids, such as pregabalin, have been shown to reduce preoperative anxiety. This anxiolysis is often accompanied by sedation, one of the most frequent side effects of pregabalin. We hypothesized that pregabalin taken preoperatively could reduce propofol requirements for induction of general anesthesia.

METHODS

A randomized double-blind placebo-controlled trial was conducted after approval by the local ethics committee. Fifty women aged 18-40 yr, American Anesthesiologists Society physical status I and II, and scheduled to undergo elective laparoscopic gynecologic procedures were enrolled after written consent. Treatment group patients were given pregabalin 150 mg po one hour before surgery while patients in the control group received a placebo. The primary outcome was the propofol dose required to achieve a targeted anesthetic depth in 50% of the population, i.e., effective dose (ED)50. The ED50 was estimated using Dixon's up-and-down methodology. The targeted anesthetic depth was defined based on predetermined entropy monitoring values (State Entropy [SE] < 50 and Response Entropy [RE]-SE < 10). As a secondary outcome, we tested if pregabalin reduced pre-induction anxiety levels which were measured on a 0-100 scale.

RESULTS

The propofol ED50 was not statistically different between the pregabalin group (mean 1.33 mg·kg(-1); 95% confidence interval [CI] 1.23 to 1.43) vs the placebo group (mean 1.37 mg·kg(-1); 95% CI 1.28 to 1.46); P = 0.19. Also, pre-induction anxiety level was not different between groups (median 31; interquartile range [IQR] [10-52] vs median 42; IQR [4-71], respectively; P = 0.41).

CONCLUSIONS

Preoperative pregabalin does not reduce propofol requirements in a population of healthy young women undergoing laparoscopic gynecologic procedures. This study failed to show a pre-induction anxiolytic effect of pregabalin in such a population. This trial was registered at www.clinicaltrials.gov (NCT01158859).

Limitations of anaesthesia depth monitoring

PURPOSE OF REVIEW

We critically review brain function monitors based on the processed electroencephalogram with regards to signal quality, artefacts and other limitations in clinical performance.

RECENT FINDINGS

Several studies have been showing that depth of anaesthesia monitors based on processed electroencephalogram has limitations that can lead to a wrong interpretation of the level of anaesthesia. Processed electroencephalogram indices can be altered by nonanaesthetic influences ranging from artefacts that affect signal quality and signal processing, adverse effects of some anaesthetic and nonanaesthetic drugs, neuromuscular blocking agents to conditions inherent to the patient such as cerebral tumours, brain ischemia and temperature.

SUMMARY

Clinicians should be aware of the several limitations of the commercial devices intending to monitor the depth of anaesthesia, which may not reflect the real underlying level of unconsciousness.

The effects of mivacurium-induced neuromuscular block on Bispectral Index and Cerebral State Index in children under propofol anesthesia - a prospective randomized clinical trial

BACKGROUND

In adults anesthetized with propofol, muscle relaxants may decrease the Bispectral Index (BIS). The aim of this prospective randomized trial was to detect the influence of a muscle relaxant bolus on the BIS and the Cerebral State Index (CSI) in children under propofol anesthesia.

METHODS

Forty pediatric patients, age 6.6 +/- 3.3 years, weight 24 +/- 9 kg, scheduled for surgical procedures requiring general anesthesia were enrolled. Two minutes after i.v. injection of 0.3 mcg.kg(-1) of sufentanil, general anesthesia was induced by an initial bolus of 3 mg.kg(-1) of propofol, followed by a continuous infusion titrated to achieve a stable BIS value of 50 +/- 5. Patients received either mivacurium 0.25 mg.kg(-1) (Group Miva) or NaCl 0.9% 0.12 ml.kg(-1) (Group Control). Mean BIS and CSI values per minute were compared between (Miva vs. Control) and within groups (Baseline vs 5 min. after study drug administration).

RESULTS

The observed changes in BIS and CSI values before and after administration of study drugs revealed no differences between the study groups. Mean baseline BIS and CSI values were lower than 5 min after study drug administration. There were no intergroup differences with respect to BIS and CSI values at any time point.

CONCLUSIONS

These data suggest that in pediatric patients anesthetized with propofol, administration of mivacurium has no impact on BIS and CSI values.

Surgical stress index as a measure of nociception/antinociception balance during general anesthesia

BACKGROUND

No validated monitoring method is available for evaluating the nociception/antinociception balance. We assessed the surgical stress index (SSI), computed from finger photoplethysmographic waveform amplitudes and pulse-to-pulse intervals, in patients undergoing shoulder surgery under general anesthesia (GA) and interscalene plexus block and in patients with GA only.

METHODS

In this prospective, randomized study in 26 patients, increased blood pressure (BP) or heart rate, movement, and coughing were considered to be signs of intraoperative nociception and were treated with alfentanil. GA was maintained with desflurane aiming at a State Entropy level of 50. Photoplethysmographic waveforms were collected from the contra-lateral arm to the surgery and SSI values from 0 (no surgical stress) to 100 (maximal surgical stress) were calculated off-line.

RESULTS

Two minutes after skin incision, SSI had not increased in the plexus group and was lower in the plexus group (38 +/- 13) compared with the controls (58 +/- 13, P<0.005). Among the controls, 1 min before alfentanil administration, the SSI value was higher than during periods of adequate antinociception, 59 +/- 11 vs. 39 +/- 12 (P<0.01). The total cumulative need for alfentanil was higher in controls (2.7 +/- 1.2 mg) compared with the plexus group (1.6 +/- 0.5 mg; P=0.008). Tetanic stimulation to the ulnar region of the hand increased SSI significantly only among the patients with plexus block not covering the site of the stimulation.

CONCLUSION

SSI values were lower in patients with plexus block covering the sites of nociceptive stimuli. In detecting nociceptive stimuli, SSI had better performance than heart rate, BP, or response entropy.

Spectral entropy as a monitor of depth of propofol induced sedation

OBJECTIVE

The aim of this prospective, observational study was to evaluate State and Response entropy (Entropy(TM) Monitor, GE Healthcare, Finland), indices as measures of moderate ("conscious") sedation in healthy adult patients receiving a low dose propofol infusion. Sedation was evaluated using: (I) the responsiveness component of the OAA/S scale (Observer's Assessment of Alertness/Sedation scale) and (II) multi-channel electroencephalogram (EEG) interpretation by a clinical expert.

METHODS

12 ASA I patients were recruited. A target-controlled infusion of propofol was administered (using Schnider's pharmacokinetic model) with an initial effect site concentration set to 0.5 microg ml(-1). A 4 minute equilibrium period was allowed. This concentration was increased at 4 minute intervals by 0.5 microg ml(-1) to a maximum of 2.0 microg ml(-1). State (SE) and Response (RE), entropy values were recorded for each 4 minute epoch together with clinical sedation scores (OAA/S) and continuous multi-channel EEG. The multi-channel EEG recorded during the final minute of each 4 minute epoch or "patient/time unit" was presented to a neurophysiologist who assigned a label "sedated/not sedated". SE/RE values were compared in patient/time units with clinical or EEG evidence of sedation versus those without.

RESULTS

Mean SE and RE values were less in patient/time units when clinical evidence of sedation was present, [mean = 86.8 (95% CI, 84.0-88.3) and 94.3 (95%CI, 92-96.1)], P = 0.002 and P = 0.001, respectively. In patient/time units assigned the label "sedated" by the clinical neurophysiologist assessing the multi-channel EEG, SE and RE values were less [mean = 87.5 (95% CI, 86.3-88.4) and 95.0 (95% CI, 93.8-96.1)] P = 0.001 and P < 0.001, respectively.

CONCLUSIONS

A statistically significant decrease in SE and RE values was demonstrated in patient/time units in which clinical or EEG evidence of sedation was present. We conclude that spectral entropy offers potential as a monitor of propofol induced sedation.

Power analysis of gamma frequencies (30 - 47Hz), adjusting for muscle activity (80 - 97Hz), in anesthesia: a comparison between young adults, middle-aged and the elderly

This study looks at the role of EEG gamma activity, and the influence of facial EMG (80-97 Hz), in predicting consciousness during anesthesia. It also studies the association between the conventional depth of anesthesia index, BIS (Aspect Medical Systems), and EEG gamma and EMG activity. Data has been collected from 21 adult patients and grouped into young adults (18 - 39 yrs, n=3), middle-aged (40 - 64 yrs, n=10) and the elderly (65+ yrs, n=8). The power of the EEG gamma activity was recorded from Fpz - Mastoid and the power of the EMG was recorded from Fpz - Mastoid and Masseter - Mastoid. It has been found that when considered alone, EEG gamma power is associated with both BIS index and consciousness versus unconsciousness, showing a decrease in power as consciousness is lost. When the effect of EEG gamma power is adjusted for EMG, it is found that generally these associations can be explained by the EMG power alone. There are two exceptions to this. In the young adults group there is a stronger association between BIS index and EEG gamma than there is between BIS index and EMG. In the elderly group, the state of consciousness is equally associated with EEG gamma and EMG recorded from the Masseter, but not with the EMG recorded from Fpz.

Narcotrend-assisted propofol/remifentanil anaesthesia vs clinical practice: does it make a difference?

BACKGROUND

The Narcotrend is a computer-based EEG monitor designed to measure the depth of anaesthesia. The aim of the present study is to test the hypothesis that the intraoperative level of anaesthetic depth differs if decision-making is guided by Narcotrend monitoring or not.

METHODS

Forty-eight patients undergoing elective surgery were randomized to receive a Narcotrend-controlled propofol/remifentanil anaesthetic regimen or standard clinical practice. In the EEG group, anaesthesia was adjusted to achieve a Narcotrend level of D2-E0, which is recommended for moderate to deep anaesthetic depth for surgery. EEG values were recorded continuously every 20 s in both groups. Depending on data distribution, group comparisons of the EEG parameters, propofol plasma concentration, and recovery characteristics were performed by analysis of variance for repeated measurements or non-parametric statistics.

RESULTS

About 62 (sd 29)% of the Narcotrend values were within the target level in the EEG group during maintenance of anaesthesia; this was true for 64 (26)% of the data in the non-EEG group. The variance of the Narcotrend data was significantly lower in the EEG group compared with the non-EEG group [median: 0.4 (range: 3.5) vs 0.6 (2.5); P = 0.048]. There was no difference in propofol or remifentanil dosage, propofol plasma concentrations, and time for extubation. Ten minutes after extubation, visual analogue scores for nausea indicated a lower incidence in the Narcotrend group [7 (15) vs 24 (34); P = 0.005].

CONCLUSIONS

Guidance of anaesthesia with the Narcotrend-monitor leads to fewer deviations from a defined target than clinical assessment of anaesthetic depth only. This results in lower scores of nausea in the immediate period after anaesthesia.

The effects of electromyographic activity on the accuracy of the Narcotrend monitor compared with the Bispectral Index during combined anaesthesia

The Narcotrend is a monitor system for the assessment of depth of anaesthesia. The objective of this trial was to investigate the susceptibility of the Narcotrend to electromyographic (EMG) activity when compared with the Bispectral Index (BIS). We enrolled 33 patients undergoing major urological procedures under combined anaesthesia (thoracic epidural analgesia and general anaesthesia). Anaesthetic depth was assessed simultaneously by the BIS XP and Narcotrend. The intended anaesthetic depth ranged between 40 and 55 in the BIS and between D2 and D0 in the Narcotrend. BIS, but not Narcotrend, values correlated significantly (p < 0.0001) with EMG. BIS values between 70 and 80 occurred intermittently above an EMG activity of 35 dB, whereas the Narcotrend and the clinical signs remained unchanged during the period of elevated BIS values. None of the patients reported intra-operative awareness. Increased electromyographic activity does not affect Narcotrend values. Under combined anaesthesia, the Narcotrend monitor is more reliable when compared with the BIS regarding susceptibility to increased EMG activity.

Changes in a surgical stress index in response to standardized pain stimuli during propofol–remifentanil infusion

BACKGROUND

The surgical stress index (SSI) is based on a sum of the normalized pulse beat interval (PBI) and the pulse wave amplitude (PPGA) time series of the photoplethysmography. As a measure of the nociception-anti-nociception balance in response to a standardized pain stimulus, SSI was compared with EEG changes in state and response entropy (SE and RE), PPGA, and heart rate (HR) during various targeted pseudo-steady-state concentrations of propofol and remifentanil.

METHODS

Forty ASA I patients were allocated to one of the four groups to receive a remifentanil step-up/-down effect-compartment target-controlled infusion (Ce(remi)) of 0, 2, 6, 2, 0 ng ml(-1), or 6, 2, 0, 2, 6 ng ml(-1), and an effect-compartment target-controlled propofol infusion (Ce(prop)) to keep the SE between 30 and 50 or 15 and 30, respectively. At each steady-state Ce(remi), maximum change in SSI, SE, RE, PPGA, and HR after a noxious stimulus was compared with the baseline value. A correlation and prediction probability (P(K)) with Ce(prop) and Ce(remi) was measured.

RESULTS

Static and dynamic values of SSI correlated to Ce(remi) better than SE, RE, HR, and PPGA. SSI was independent of Ce(prop), in contrast to SE and RE. The P(K) for Ce(remi) both before and during a noxious stimulus was better with SSI.

CONCLUSIONS

SSI appeared to be a better measure of nociception-anti-nociception balance than SE, RE, HR, or PPGA.

Closed-loop anaesthesia delivery system (CLADS) using bispectral index: a performance assessment study

The development of electroencephalographic indices of anaesthetic depth has in turn generated interest in automated anaesthesia delivery systems using these as the input variable. In this paper, one patented closed loop anaesthesia delivery system (CLADS) (502/DEL/2003) is compared to manual control of propofol delivery titrated to the bispectral index (BIS). Forty ASA I-II patients undergoing elective surgery under general anaesthesia were enrolled in the study. The study participants were randomised using computer generated random numbers to two equal groups. One group received propofol titrated by the CLADS while in the other group (control), anaesthetic delivery was manually titrated to BIS. Closed loop anaesthetic delivery using our patented system led to lower induction doses of propofol (P < 0.05) and less overshoot of the target BIS (P < 0.05). The closed loop system maintained BIS to within +/-10 of target for a significantly longer time during the maintenance phase of anaesthesia (P < 0.01). Smaller amounts of anaesthetic agent were required (P < 0.01) and there was faster postoperative recovery (P < 0.05). Manual delivery of propofol required the infusion rate to be changed a median of 30 times (IQR 12-45), which required considerable time and attention by the anaesthetist. In conclusion, automated delivery of propofol adjusted to the bispectral index using our CLADS was both effective and efficient as compared to manual control.

Measure of the electroencephalographic effects of sevoflurane using recurrence dynamics

This paper proposes a novel method to interpret the effect of anesthetic agents (sevoflurane) on the neural activity, by using recurrence quantification analysis of EEG data. First, we reduce the artefacts in the scalp EEG using a novel filter that combines wavelet transforms and empirical mode decomposition. Then, the determinism in the recurrence plot is calculated. It is found that the determinism increases gradually with increasing the concentration of sevoflurane. Finally, a pharmacokinetic and pharmacodynamic (PKPD) model is built to describe the relationship between the concentration of sevoflurane and the processed EEG measure ('determinism' of the recurrence plot). A test sample of nine patients shows the recurrence in EEG data may track the effect of the sevoflurane on the brain.

Using EEG to monitor anesthesia drug effects during surgery

The use of processed electroencephalography (EEG) using a simple frontal lead system has been made available for assessing the impact of anesthetic medications during surgery. This review discusses the basic principles behind these devices. The foundations of anesthesia monitoring rest on the observations of Guedel with ether that the depth of anesthesia relates to the cortical, brainstem and spinal effects of the anesthetic agents. Anesthesiologists strive to have a patient who is immobile, is unconscious, is hemodynamically stable and who has no intraoperative awareness or recall. These anesthetic management principles apply today, despite the absence of ether from the available anesthetic medications. The use of the EEG as a supplement to the usual monitoring techniques rests on the observation that anesthetic medications all alter the synaptic function which produces the EEG. Frontal EEG can be viewed as a surrogate for the drug effects on the entire central nervous system (CNS). Using mathematical processing techniques, commercial EEG devices create an index usually between 0 and 100 to characterize this drug effect. Critical aspects of memory formation occur in the frontal lobes making EEG monitoring in this area a possible method to assess risk of recall. Integration of processed EEG monitoring into anesthetic management is evolving and its ability to characterize all of the anesthetic effects on the CNS (in particular awareness and recall) and improve decision making is under study.

Comparison of auditory evoked potential parameters for predicting clinically anaesthetized state

BACKGROUND

Most of the research efforts to monitor the depth of anaesthesia using the mid-latency auditory evoked potential (MLAEP) signal in humans are based on the detection of the amplitudes and latencies of the signal peaks. Attempts have also been made to combine different time-domain and frequency-domain parameters. A comparison of different parameters is required to identify those which best discriminate the awake state from the anaesthetized state.

METHODS

Although the sensitivity of MLAEP signal peaks is appreciable in awake and light anaesthesia states, it is reduced considerably at the moderate anaesthesia level, rendering this method unsuitable for predicting the surgical stage of anaesthesia. To overcome this problem, a numerically derived quantity--the morphology index--was used which does not require location of the peaks of the signal, but, at the same time, reflects the changes in both the latency and amplitude of the peaks. AEPs were recorded in the hospital for 18 patients during various states, i.e. awake, induction, unconscious and after regaining consciousness from halothane anaesthesia. The peak latencies, amplitudes, morphology index and peak power frequency (PPF) were calculated.

RESULTS

The sensitivity and specificity of PPF (89% and 95%, respectively) were found to be better than those for Pa and Nb peak amplitudes, their latencies and the morphology index. In addition, PPF showed minimum inter-patient variation. The mean value (standard deviation) of this parameter was 26.9 (0.67) during the awake state, decreased to 17.1 (1.2) during the anaesthetized state, and increased again to 26.1 (0.93) when the patients regained full consciousness.

CONCLUSION

PPF is the best of the four studied MLAEP parameters for the clinical characterization of the anaesthetized state during surgery.

Forecasting the Unresponsiveness to Verbal Command on the Basis of EEG Frequency Progression During Anesthetic Induction With Propofol

The objective of this study is to model the association between the electroencephalogram (EEG) spectral features and the novel r scale representing the sedative effects of the propofol anesthetic drug. On the basis of the r scale, the unresponsiveness to the verbal command (LVC) is forecasted. EEG recordings are taken from a 16-patient study population undergoing propofol anesthetic induction. EEG was filtered into consecutive 4-Hz passbands up to 28 Hz. Of these time-series, the amplitude envelopes were extracted and used as input features to the first and the second-order polynomial multiple linear regression models. The values r epsilon [0.4, 1] were predicted with the R2 value of 0.775 with a cross validation. The LVC times were forecasted with the median error of 5%-7% or equivalently 10-13 s. In contrast, using the median of the measured LVC times of the training population as a forecast, the corresponding error was 12% or 26 s. The results suggest an acceptable correlation between the r scale and the EEG spectrum in the studied range. Moreover, the r values of an individual can be predicted using a population model. The suggested framework enables forecasting the LVC, which may open new possibilities for steering the drug administration.

Cerebral state index response to incision: a clinical study in day-surgical patients

BACKGROUND

Inadequate anaesthesia, with somatic/autonomic response or awareness, is often revealed at intubation and surgical incision. Anaesthetic depth monitors should be able to prevent this risk. This explorative study examined the ability of the cerebral state monitor to predict autonomic/somatic responses to incision.

METHODS

Forty-two ASA I-II day-surgical patients [19 men and 23 females; mean age 52 (29-79) years, mean weight 77 (50-118) kg] were induced clinically with fentanyl/propofol with sevoflurane after placement of the laryngeal mask airway. The cerebral state index (CSI) was blindly recorded 4 min prior to and 4 min after incision.

RESULTS

During the 4 min prior to incision, the mean CSI was 45 (16-62) and increased by 9 (-13-40) when the mean value for the first 4 min after incision was subtracted from the value prior to incision, corresponding to a relative change of 21% (-21-118). The change in CSI did not show any consistent relation to the value before incision. Five patients showed minor movements after incision and six patients had > 25% increase in blood pressure. Neither CSI nor the change in index differed between patients who did or did not respond somatically or autonomically to incision. The last CSI value just prior to incision was 44 for non-responders and 40 and 42 for somatic and autonomic responders, respectively.

CONCLUSION

The CSI in the majority of patients was within acceptable ranges during clinically adjusted anaesthesia prior to incision but seems not to be able to reliably predict an autonomic or somatic response to incision.

Depth of anaesthesia monitoring: what's available, what's validated and what's next?

Depth of anaesthesia monitors might help to individualize anaesthesia by permitting accurate drug administration against the measured state of arousal of the patient. In addition, the avoidance of awareness or excessive anaesthetic depth might result in improved patient outcomes. Various depth of anaesthesia monitors based on processed analysis of the EEG or mid-latency auditory-evoked potentials are commercially available as surrogate measures of anaesthetic drug effect. However, not all of them are validated to the same extent.

Subjective assessment of depth of anaesthesia by experienced and inexperienced anaesthetists

BACKGROUND AND OBJECTIVE

To measure 'depth of anaesthesia', anaesthesiologists use a combination of observable end-points such as immobility and autonomic stability. Unconsciousness and amnesia are not reliably observable end-points, but correlate with parameters derived from the electroencephalogram. We investigated the association of subjective assessment and electroencephalographic measures of anaesthetic depth in a group of experienced (>4 yr of experience) and a group of inexperienced (<2 yr of experience) anaesthesiologists.

METHODS

One hundred ASA I or II patients were assigned to either group. Anaesthesiologists assessed 'anaesthetic depth' using an 11-point numeric and a 5-point verbal scale. Bispectral index and spectral entropy were recorded as electroencephalogram parameters. The association between the subjective assessment and the electroencephalogram parameters was calculated using the prediction probability, PK.

RESULTS

Association between subjective assessment and electroencephalographic parameters showed a tendency to a better prediction probability in the experienced group. The difference was significant (P < 0.05) for the bispectral index (PK 0.76 +/- 0.01 for experienced and 0.71 +/- 0.01 for inexperienced anaesthesiologists). In both groups, a large percentage of the data points recorded during surgery showed bispectral index values above the recommended value of 60 (13.2% in the experienced and 34.3% in the inexperienced group) despite a subjective assessment of 'deep' or 'very deep' anaesthetic depth.

CONCLUSION

The study demonstrates that the association between subjectively assigned values of anaesthetic depth and electroencephalographic parameters of anaesthetic depth is better for anaesthesiologists with more clinical experience. However, in the 'inexperienced' as well as 'experienced' group a high percentage of bispectral index and entropy values above 60 occurred despite a subjective assessment of adequate anaesthetic depth. Although there was no evidence for explicit memory, this may indicate a risk for memory formation.

Bispectral Index Monitoring in the Intensive Care Unit Provides More Signal Than Noise

The bispectral index (BIS) is processed electroencephalographic technology used in concert with clinical evaluations to objectively evaluate a patient's level of consciousness and probability of recall. Although the BIS has been extensively studied in the operating room setting, differences in patient populations, goals of treatment, and the environments themselves necessitate the development of BIS data specific to the intensive care unit. Data have evolved over the last several years, but for many reasons, the results and conclusions have varied. Yet within the data are important consistencies that help define the usefulness of BIS in patients who cannot be evaluated with subjective assessment tools such as the Sedation-Agitation Scale or the Richmond Agitation-Sedation Scale. Some of these patients cannot be evaluated with such tools because they lack motor responsiveness due to therapeutic paralysis or because they are receiving deep sedation. Bispectral index scores that are higher than expected in clinically sedated patients can often be traced to electromyographic activity or to the possibility of inadequate sedation and analgesia. The BIS must not be regarded as the sole indicator of level of consciousness, but should be used as part of an integrated approach to the evaluation of carefully selected patients with critical illness.