Comparative evaluation of the Datex-Ohmeda S/5 Entropy Module and the Bispectral Index monitor during propofol-remifentanil anesthesia

Current perioperative nociception monitoring and potential directions

Perioperative nociception-antinociception balance is essential for the prevention of adverse postoperative events. Estimating the nociception level helps optimize intraoperative management. In the past two decades, various nociception monitoring devices have been developed for the identification of intraoperative nociception. However, each type of nociception monitoring device has advantages and disadvantages, limiting their clinical application in particular patients and settings. Therefore, this review aimed to summarize the information on nociceptor monitoring in current clinical settings, explore each technique's particularities, and possible future directions to provide a reference for clinicians and researchers.

Evaluation of Quality of Recovery With Quality of Recovery-15 Score After Closed-Loop Anesthesia Delivery System-Guided Propofol Versus Desflurane General Anesthesia in Patients Undergoing Transabdominal Robotic Surgery: A Randomized Controlled Study

BACKGROUND

Robotic technique of surgery allows surgeons to perform complex procedures in difficult-to-access areas of the abdominal/pelvic cavity (eg, radical prostatectomy and radical hysterectomy) with improved access and precision approach. At the same time, automated techniques efficiently deliver propofol total intravenous anesthesia (TIVA) with lower anesthetic consumption. As both above are likely to bring benefit to the patients, it is imperative to explore their effect on postanesthesia recovery. Quality of Recovery-15 (QoR-15) is a comprehensive patient-reported measure of the quality of postanesthesia recovery and assesses compendious patients' experiences (physical and mental well-being). This randomized study assessed the effect of automated propofol TIVA versus inhaled desflurane anesthesia on postoperative quality of recovery using the QoR-15 questionnaire in patients undergoing elective robotic surgery.

METHODS

One hundred twenty patients undergoing robotic abdominal surgery under general anesthesia (GA) were randomly allocated to receive propofol TIVA administered by closed-loop anesthesia delivery system (CLADS) (CLADS group) or desflurane GA (desflurane group). Postoperative QoR-15 score on postoperative day 1 (POD-1) and postoperative day 2 (POD-2) (primary outcome variables), individual QoR-15 item scores (15 nos.), intraoperative hemodynamics (heart rate, mean blood pressure), anesthesia depth consistency, anesthesia delivery system performance, early recovery from anesthesia (time-to-eye-opening, and time to tracheal extubation), and postoperative adverse events (sedation, postoperative nausea and vomiting [PONV], pain, intraoperative awareness recall) (secondary outcome variables) were analyzed.

RESULTS

On POD-1, the CLADS group scored significantly higher than the desflurane group in terms of "overall" QoR-15 score (QoR-15 score: 114.5 ± 13 vs 102.1 ± 20.4; P = .001) and 3 individual QoR-15 "items" scores ("feeling rested" 7.5 ± 1.9 vs 6.4 ± 2.2, P = .007; "good sleep" 7.8 ± 1.9 vs 6.6 ± 2.7, P = .027; and "feeling comfortable and in control" 8.1 ± 1.7 vs 6.9 ± 2.4, P = .006). On the POD-2, the CLADS group significantly outscored the desflurane group with respect to the "overall" QoR-15 score (126.0 ± 13.6 vs 116.3 ± 20.3; P = .011) and on "5" individual QoR-15 items ("feeling rested" 8.1 ± 1.4 vs 7.0 ± 2.0, P = .003; "able to return to work or usual home activities" 6.0 ± 2.2 vs 4.6 ± 2.6, P = .008; "feeling comfortable and in control" 8.6 ± 1.2 vs 7.7 ± 1.9, P = .004; "feeling of general well-being" 7.8 ± 1.6 vs 6.9 ± 2.0, P = .042; and "severe pain" 9.0 ± 1.9 vs 8.1 ± 2.5, P = .042).

CONCLUSIONS

Automated propofol TIVA administered by CLADS is superior to desflurane inhalation GA with respect to early postoperative recovery as comprehensively assessed on the QoR-15 scoring system. The effect of combined automated precision anesthesia and surgery (robotics) techniques on postoperative recovery may be explored further.

Determination of the 50% and 95% Effective Dose of Remimazolam Combined with Propofol for Intravenous Sedation During Day-Surgery Hysteroscopy

Purpose

Remimazolam has demonstrated the potential as a valuable medication for procedural sedation. However, there were some shortcomings for higher doses of remimazolam during hysteroscopy in spite of less frequent adverse events. The aim of this study was to find the 50% and 95% effective dose (ED₅₀ and ED₉₅) of remimazolam when combined with propofol for intravenous sedation during day-surgery hysteroscopy.

Patients and Methods

Patients were randomly assigned evenly (20 per group) to one of five different dosage of remimazolam: group A (0.05mg/kg), group B (0.075mg/kg), group C (0.1mg/kg), group D (0.125mg/kg) or group E (0.15mg/kg). Intravenous injection of sufentanil 0.1µg/kg was administered before sedative medication. Intravenous anesthesia was commenced with remimazolam. Subsequently, propofol was administered at 1mg/kg and maintained at 6mg/kg/h. Success was defined when the patient did not move during cervical dilation, had sufficient sedation as judged by SE <60 and no requirement for rescue doses. The success rate, induce and average dosage of propofol, the induction time, total surgery time, recovery time, and adverse events were recorded. Estimate of ED₅₀ and ED₉₅ with 95% confidence interval (CI) was performed by probit regression.

Results

The mean (95% CI) values for ED₅₀ and ED₉₅ of remimazolam in patients were 0.09 (0.08-0.11) mg/kg and 0.21 (0.16-0.35) mg/kg, respectively. There was no difference in the induction time, total surgery time, and recovery time among groups. No serious adverse events occurred in all patients.

Conclusion

The dose-response effects of remimazolam were evaluated for intravenous sedation during hysteroscopy. A combination of remimazolam and propofol was recommended to produce stabler sedation, reduce the total dosage and have less effect on cardiovascular and respiratory depression.

Consciousness and complexity: a consilience of evidence

Over the last years, a surge of empirical studies converged on complexity-related measures as reliable markers of consciousness across many different conditions, such as sleep, anesthesia, hallucinatory states, coma, and related disorders. Most of these measures were independently proposed by researchers endorsing disparate frameworks and employing different methods and techniques. Since this body of evidence has not been systematically reviewed and coherently organized so far, this positive trend has remained somewhat below the radar. The aim of this paper is to make this consilience of evidence in the science of consciousness explicit. We start with a systematic assessment of the growing literature on complexity-related measures and identify their common denominator, tracing it back to core theoretical principles and predictions put forward more than 20 years ago. In doing this, we highlight a consistent trajectory spanning two decades of consciousness research and provide a provisional taxonomy of the present literature. Finally, we consider all of the above as a positive ground to approach new questions and devise future experiments that may help consolidate and further develop a promising field where empirical research on consciousness appears to have, so far, naturally converged.

The role of entropy monitoring in reducing propofol requirements during open heart surgeries. A prospective randomized study

Background:

Hypotension, which is commonly associated with propofol induction of general anesthesia in coronary artery bypass grafting (CABG) surgery, may cause adverse consequences in patients with coronary artery diseases undergoing this type of surgeries. The clinical absence of verbal response and eyelash reflex was used as an endpoint for hypnosis. Spectral entropy, as a novel monitoring method for the endpoint of hypnosis, affect the dose of required anesthetic agents for induction as well as the hemodynamic profile during general anesthesia in CABG surgery.

Aims:

We hypothesized that entropy monitoring might reduce the dose of propofol required for induction of anesthesia during CABG surgery and could maintain hemodynamic stability when compared with the conventional clinical monitoring.

Materials and Methods:

Sixty adult patients of both sexes, aged 30–60 years, ASA II and III, and scheduled for CABG surgery were enrolled in this prospective, controlled, randomized, double-blind study. These patients were randomly divided into two equal groups to receive intravenous propofol for induction of anesthesia guided by either the patients’ clinical response (Group I) or by entropy monitoring (Group II). The total dose of propofol used for induction of anesthesia was recorded. Hemodynamic parameters and entropy values were also recorded.

Results:

Propofol consumption was significantly reduced in Group II than Group I (P = 0.000*). Heart rate showed no statistical significance between the two groups, whereas the mean arterial pressure significantly decreased at induction in group I compared to Group II (P = 0.000*). The entropy values were significantly lower in Group I than Group II at induction (P = 0.036* for state entropy; 0.002* for response entropy). However, during intubation, and after 1 and 5 min, entropy indices displayed a significant increase in Group I than Group II.

Conclusions:

Entropy monitoring significantly reduced the dose of propofol required for induction of anesthesia and maintained hemodynamic stability compared to the conventional clinical monitoring during CABG surgeries.

Comparative evaluation of the bispectral index (BIS) and BISpro during propofol anaesthesia

OBJECTIVE

To compare the accuracy, correlation and agreement between the bispectral index (BIS) and BISpro during propofol anaesthesia.

METHODS

The BIS, BISpro, heart rate, target-concentration of propofol and Observer's Assessment of Alertness and Sedation (OAA/S) score were recorded every 30 s in female patients scheduled for hysteroscopic surgery. Propofol anaesthesia was induced by an initial target-controlled concentration (1.0 μg/ml) followed by a stepwise increase (0.5 μg/ml) until the patient was unresponsive. Spearman's correlation coefficient and prediction probability were calculated for the association between sedation levels and the above parameters. The ability of investigated parameters to distinguish between OAA/S scores was analysed. Bland-Altman analysis was used to compare the agreement between BIS and BISpro. The BIS and BISpro cut-off values for lost response were also determined.

RESULTS

Out of 30 patients in total, a high correlation was found between BIS and BISpro, and both correlated well with OAA/S score. Only BIS was able to distinguish all investigated OAA/S states accurately, but the ability to predict OAA/S score 5 to loss of response was comparable between BIS and BISpro. The calculated cut-off values were 68 for BIS and 70 for BISpro.

CONCLUSION

BISpro and BIS are reliable monitors of general anaesthesia during sedation.Trial registration number: Chinese Clinical Trial Registry (URL: www.chictr.org.cn): ChiCTR1900024037 (retrospectively registered).

Influence of Potential Gene Polymorphisms on Propofol Dosage Regimen in Patients Undergoing Abdominal Hysterectomy

Propofol (2,6-diisopropylphenol) is the most common intravenous anesthetic used in modern medicine. It is postulated that individual differences in genetic factors [polymorphism of single nucleotide polymorphisms (SNPs)] in the genes encoding metabolic enzymes, molecular targets and molecular binding sites of propofol can be responsible for susceptibility to propofol effects. The aim of our study was to investigate the influence of the cytochrome P450 2B6 isozyme CYP2B6 (rs3745274), γ-aminobutyric acid type A (GABAA) receptor α1 subunit GABRA1 (rs2279020) and ATP-binding cassette subfamily B member 1 ABCB1 (rs1045642) gene polymorphisms on propofol therapeutic outcomes in the patients undergoing abdominal hysterectomy. Ninety patients aged 29-74 years, with different ethnicities were included in this study. The presence of polymorphisms was analyzed using TaqMan SNP genotype analysis on Stratagene MxPro 3005P real-time polymerase chain reaction (qPCR). The distribution of all three genetic variants was within the Hardy-Weinberg equilibrium. There was no significant difference (p >0.05) in the allelic frequencies of polymorphic variants and genotype distributions between adult and older patients and between patients of different ethnicities. Our study did not detect a statistically significant influence of the CYP2B6 (c.516G>A), GABRA1 (c.1059+15G>A) and ABCB1 (c.3435T>C) variants on the variability of clinical parameters (doses for induction in anesthesia, additional doses, induction time and wake time after anesthesia and side effects of propofol). However, the observed trend on the possible influence of the CYP2B6 (c.516G>A) and GABRA1 (c.1059+15G>A) variants warrant an extension of these studies on a larger number of patients.

Offline comparison of processed electroencephalogram monitors for anaesthetic-induced electroencephalogram changes in older adults

BACKGROUND

Several devices record and interpret patient brain activity via electroencephalogram (EEG) to aid physician assessment of anaesthetic effect. Few studies have compared EEG monitors on data from the same patient. Here, we describe a set-up to simultaneously compare the performance of three processed EEG monitors using pre-recorded EEG signals from older surgical patients.

METHODS

A playback system was designed to replay EEG signals into three different commercially available EEG monitors. We could then simultaneously calculate indices from the SedLine® Root (Masimo Inc., Irvine, CA, USA; patient state index [PSI]), bilateral BIS VISTA™ (Medtronic Inc., Minneapolis, MN, USA; bispectral index [BIS]), and Datex Ohmeda S/5 monitor with the Entropy™ Module (GE Healthcare, Chicago, IL, USA; E-entropy index [Entropy]). We tested the ability of each system to distinguish activity before anaesthesia administration (pre-med) and before/after loss of responsiveness (LOR), and to detect suppression incidences in EEG recorded from older surgical patients receiving beta-adrenergic blockers. We show examples of processed EEG monitor output tested on 29 EEG recordings from older surgical patients.

RESULTS

All monitors showed significantly different indices and high effect sizes between comparisons pre-med to after LOR and before/after LOR. Both PSI and BIS showed the highest percentage of deeply anaesthetised indices during periods with suppression ratios (SRs) > 25%. We observed significant negative correlations between percentage of suppression and indices for all monitors (at SR >5%).

CONCLUSIONS

All monitors distinguished EEG changes occurring before anaesthesia administration and during LOR. The PSI and BIS best detected suppressed periods. Our results suggest that the PSI and BIS monitors might be preferable for older patients with risk factors for intraoperative awareness or increased sensitivity to anaesthesia.

Evaluation of Automated Delivery of Propofol Using a Closed-Loop Anesthesia Delivery System in Patients Undergoing Thoracic Surgery: A Randomized Controlled Study

OBJECTIVE

Automated propofol total intravenous anesthesia (TIVA) administered by a closed-loop anesthesia delivery system (CLADS) exhibits greater efficiency than conventional manual methods, but its use in major thoracic surgery is limited.

DESIGN

Prospective, single-blind, randomized controlled study.

SETTING

Single-center tertiary care hospital.

PARTICIPANTS

Patients undergoing thoracic surgery.

INTERVENTIONS

Patients were randomly allocated to receive CLADS-driven (CLADS group) or manually controlled (manual group) propofol TIVA.

MEASUREMENTS AND MAIN RESULTS

Anesthesia depth consistency (primary objective) and anesthesia delivery performance, propofol usage, work ergonomics, intraoperative hemodynamics, and recovery profile (secondary objectives) were analyzed. No differences were found for anesthesia depth consistency (percentage of time the bispectral index was within ± 10 of target) (CLADS group: 82.5% [78.5%-87.2%] v manual group: 86.5% [74.2%-92.5%]; p = 0.581) and delivery performance, including median performance error (CLADS group: 3 [-4 to 6] v manual group: 1 [-2.5 to 6]); median absolute performance error (CLADS group: 10 [10-12] v manual group:10 [8-12]); wobble (CLADS group: 10 [8-12] v manual group: 9 [6-10.5]); and global score (CLADS group: 24.2 [21.2-29.3] v manual group: 22.1 [17.3-32.3]) (p > 0.05). However, propofol requirements were significantly lower in the CLADS group for induction (CLADS group: 1.27 ± 0.21] mg/kg v manual group: 1.78 ± 0.51 mg/kg; p = 0.014) and maintenance (CLADS group: 4.02 ± 0.99 mg/kg/h v manual group: 5.11 ± 1.40 mg/kg/h; p = 0.025) of TIVA. Ergonomically, CLADS-driven TIVA was found to be significantly superior to manual control (infusion adjustment frequency/h) (manual infusion: 9.6 [7.8-14.9] v CLADS delivery [none]).

CONCLUSIONS

In thoracic surgery patients, CLADS-automated propofol TIVA confers significant ergonomic advantage along with lower propofol usage.

Variations in Values of State, Response Entropy and Haemodynamic Parameters Associated with Development of Different Epileptiform Patterns during Volatile Induction of General Anaesthesia with Two Different Anaesthetic Regimens Using Sevoflurane in Comparison with Intravenous Induct: A Comparative Study

BACKGROUND AND OBJECTIVES

Raw electroencephalographic (EEG) signals are rarely used to monitor the depth of volatile induction of general anaesthesia (VIGA) with sevoflurane, even though EEG-based indices may show aberrant values. We aimed to identify whether response (RE) and state entropy (SE) variations reliably reflect the actual depth of general anaesthesia in the presence of different types of epileptiform patterns (EPs) in EEGs during induction of general anaesthesia.

MATERIALS AND METHODS

A randomized, prospective clinical study was performed with 60 patients receiving VIGA using sevoflurane with the increasing concentrations (group VIMA) or the vital capacity (group VCRII) technique or an intravenous single dose of propofol (group PROP). Facial electromyography (fEMG), fraction of inspired sevoflurane (FiAA), fraction of expired sevoflurane (FeAA), minimal alveolar concentration (MAC) of sevoflurane, RE and SE, and standard electroencephalographic evaluations were performed in these patients.

RESULTS

In contrast to periodic epileptiform discharges, erroneous SE and RE values in the patients' EEGs were associated with the presence of polyspikes (PS) and rhythmic polyspikes (PSR), which were more likely to indicate toxic depth rather than false emergence from anaesthesia with no changes in the FiAA, FeAA, and MAC of sevoflurane.

CONCLUSION

Calculated RE and SE values may be misleading during VIGA when EPs are present in patients' EEGs. During VIGA with sevoflurane, we recommend monitoring raw EEG data in scientific studies to correlate it with potentially erroneous RE and SE values and the end-tidal concentration of sevoflurane in everyday clinical practice, when monitoring raw EEG is not available, because they can mislead anaesthesiologists to reduce sevoflurane levels in the ventilation gas and result in unintentional true emergence from anaesthesia. Further studies are required to investigate the behaviour of EEG-based indices during rapid changes in sevoflurane concentrations at different stages of VIGA and the influence of polyspikes and rhythmic polyspikes on the transformation of EEG signals into a digital form.

INSMA: An integrated system for multimodal data acquisition and analysis in the intensive care unit

Modern intensive care units (ICU) are equipped with a variety of different medical devices to monitor the physiological status of patients. These devices can generate large amounts of multimodal data daily that include physiological waveform signals (arterial blood pressure, electrocardiogram, respiration), patient alarm messages, numeric vitals data, etc. In order to provide opportunities for increasingly improved patient care, it is necessary to develop an effective data acquisition and analysis system that can assist clinicians and provide decision support at the patient bedside. Previous research has discussed various data collection methods, but a comprehensive solution for bedside data acquisition to analysis has not been achieved. In this paper, we proposed a multimodal data acquisition and analysis system called INSMA, with the ability to acquire, store, process, and visualize multiple types of data from the Philips IntelliVue patient monitor. We also discuss how the acquired data can be used for patient state tracking. INSMA is being tested in the ICU at University Hospitals Cleveland Medical Center.

Comparative Evaluation of a New Depth of Anesthesia Index in ConView® System and the Bispectral Index during Total Intravenous Anesthesia: A Multicenter Clinical Trial

The performance of a new monitor for the depth of anesthesia (DOA), the Depth of Anesthesia Index (Ai) based on sample entropy (SampEn), 95% spectral edge frequency (95%SEF), and burst suppression ratio (BSR) was evaluated compared to Bispectral Index (BIS) during total intravenous anesthesia (TIVA). 144 patients in six medical centers were enrolled. General anesthesia was induced with stepwise-increased target-controlled infusion (TCI) of propofol until loss of consciousness (LOC). During surgery propofol was titrated according to BIS. Both Ai and BIS were recorded. Primary outcomes: the limits of agreement between Ai and BIS were -17.68 and 16.49, which were, respectively, -30.0% and 28.0% of the mean value of BIS. Secondary outcomes: prediction probability (Pk) of BIS and Ai was 0.943 and 0.935 (p=0.102) during LOC and 0.928 and 0.918 (p=0.037) during recovery of consciousness (ROC). And the values of BIS and Ai were 68.19 and 66.44 at 50%LOC, and 76.65 and 78.60 at 50%ROC. A decrease or an increase of Ai was significantly greater than that of BIS when consciousness changes (during LOC: -9.13±10.20 versus -5.83±9.63, p<0.001; during ROC: 10.88±11.51 versus 5.32±7.53, p<0.001). The conclusion is that Ai has similar characteristic of BIS as a DOA monitor and revealed the advantage of SampEn for indicating conscious level. This trial is registered at Chinese Clinical Trial Registry with ChiCTR-IOR-16009471.

Electrocorticographic delineation of human auditory cortical fields based on effects of propofol anesthesia

The functional organization of human auditory cortex remains incompletely characterized. While the posteromedial two thirds of Heschl's gyrus (HG) is generally considered to be part of core auditory cortex, additional subdivisions of HG remain speculative. To further delineate the hierarchical organization of human auditory cortex, we investigated regional heterogeneity in the modulation of auditory cortical responses under varying depths of anesthesia induced by propofol. Non-invasive studies have shown that propofol differentially affects auditory cortical activity, with a greater impact on non-core areas. Subjects were neurosurgical patients undergoing removal of intracranial electrodes placed to identify epileptic foci. Stimuli were 50Hz click trains, presented continuously during an awake baseline period, and subsequently, while propofol infusion was incrementally titrated to induce general anesthesia. Electrocorticographic recordings were made with depth electrodes implanted in HG and subdural grid electrodes implanted over superior temporal gyrus (STG). Depth of anesthesia was monitored using spectral entropy. Averaged evoked potentials (AEPs), frequency-following responses (FFRs) and high gamma (70-150Hz) event-related band power were used to characterize auditory cortical activity. Based on the changes in AEPs and FFRs during the induction of anesthesia, posteromedial HG could be divided into two subdivisions. In the most posteromedial aspect of the gyrus, the earliest AEP deflections were preserved and FFRs increased during induction. In contrast, the remainder of the posteromedial HG exhibited attenuation of both the AEP and the FFR. The anterolateral HG exhibited weaker activation characterized by broad, low-voltage AEPs and the absence of FFRs. Lateral STG exhibited limited activation by click trains, and FFRs there diminished during induction. Sustained high gamma activity was attenuated in the most posteromedial portion of HG, and was absent in all other regions. These differential patterns of auditory cortical activity during the induction of anesthesia may serve as useful physiological markers for field delineation. In this study, the posteromedial HG could be parcellated into at least two subdivisions. Preservation of the earliest AEP deflections and FFRs in the posteromedial HG likely reflects the persistence of feedforward synaptic activity generated by inputs from subcortical auditory pathways, including the medial geniculate nucleus.

Drug-induced sleep endoscopy with target-controlled infusion using propofol and monitored depth of sedation to determine treatment strategies in obstructive sleep apnea

BACKGROUND

Drug-induced sleep endoscopy (DISE) has become an important diagnostic examination tool in the treatment decision process for surgical therapies in the treatment of obstructive sleep apnea (OSA). Currently, there is a variety of regimes for the performance of DISE, which renders comparison and assessment across results difficult. It remains unclear how the different regimes influence the findings of the examination and the resulting conclusions and treatment recommendations. This study aimed to investigate the correlation between increasing levels of sedation (i.e., light, medium, and deep) induced by propofol using a target-controlled infusion (TCI) pump, with the obstruction patterns at the levels of the velum, oropharynx, tongue base, and epiglottis (i.e., VOTE classification). A second goal was the establishment of a sufficient sedation level to enable a reliable decision regarding treatment recommendations.

MATERIAL AND METHODS

Forty-three patients with OSA underwent a DISE procedure using propofol TCI. Three levels of sedation were defined, depending on entropy levels and assessment of sedation: light sedation, medium sedation, and deep sedation. The evaluation of the upper airway at each level, with increasing sedation, was documented using the VOTE classification. The elapsed time at which each assessment was performed was recorded.

RESULTS

Upper airway changes occurred and were measured throughout the DISE procedure. Clinically useful determinations of airway closure occurred at medium sedation; this level of sedation was most probably achieved with a blood propofol concentration of 3.2 μg/ml. In all 43 patients, definite treatment decisions could be made at medium sedation level. Increasing sedation did not result in changes in the treatment decision.

CONCLUSIONS

Changes in upper airway collapse during DISE with propofol TCI occur at levels of medium sedation. Decisions regarding surgical treatment could be made at this level of sedation.

CLINICAL TRIAL NAME

Upper Airway Collapse in Patients with Obstructive Sleep Apnea Syndrome by Drug Induced Sleep Endoscopy (URL: https://clinicaltrials.gov/ct2/results?term=NCT02588300&Search=Search ) REGISTRATION NUMBER: NCT02588300.

Spectral entropy in monitoring anesthetic depth

Monitoring the brain response to hypnotics in general anesthesia, with the nociceptive and hemodynamic stimulus interaction, has been a subject of intense investigation for many years. Nowadays, monitors of depth of anesthesia are based in processed electroencephalogram by different algorithms, some of them unknown, to obtain a simplified numeric parameter approximate to brain activity state in each moment. In this review we evaluate if spectral entropy suitably reflects the brain electric behavior in response to hypnotics and the different intensity nociceptive stimulus effect during a surgical procedure.

Differences between state entropy and bispectral index during analysis of identical electroencephalogram signals: a comparison with two randomised anaesthetic techniques

BACKGROUND

It is claimed that bispectral index (BIS) and state entropy reflect an identical clinical spectrum, the hypnotic component of anaesthesia. So far, it is not known to what extent different devices display similar index values while processing identical electroencephalogram (EEG) signals.

OBJECTIVE

To compare BIS and state entropy during analysis of identical EEG data. Inspection of raw EEG input to detect potential causes of erroneous index calculation.

DESIGN

Offline re-analysis of EEG data from a randomised, single-centre controlled trial using the Entropy Module and an Aspect A-2000 monitor.

SETTING

Klinikum rechts der Isar, Technische Universität München, Munich.

PATIENTS

Forty adult patients undergoing elective surgery under general anaesthesia.

INTERVENTIONS

Blocked randomisation of 20 patients per anaesthetic group (sevoflurane/remifentanil or propofol/remifentanil). Isolated forearm technique for differentiation between consciousness and unconsciousness.

MAIN OUTCOME MEASURES

Prediction probability (PK) of state entropy to discriminate consciousness from unconsciousness. Correlation and agreement between state entropy and BIS from deep to light hypnosis. Analysis of raw EEG compared with index values that are in conflict with clinical examination, with frequency measures (frequency bands/Spectral Edge Frequency 95) and visual inspection for physiological EEG patterns (e.g. beta or delta arousal), pathophysiological features such as high-frequency signals (electromyogram/high-frequency EEG or eye fluttering/saccades), different types of electro-oculogram or epileptiform EEG and technical artefacts.

RESULTS

PK of state entropy was 0.80 and of BIS 0.84; correlation coefficient of state entropy with BIS 0.78. Nine percent BIS and 14% state entropy values disagreed with clinical examination. Highest incidence of disagreement occurred after state transitions, in particular for state entropy after loss of consciousness during sevoflurane anaesthesia. EEG sequences which led to false 'conscious' index values often showed high-frequency signals and eye blinks. High-frequency EEG/electromyogram signals were pooled because a separation into EEG and fast electro-oculogram, for example eye fluttering or saccades, on the basis of a single EEG channel may not be very reliable. These signals led to higher Spectral Edge Frequency 95 and ratio of relative beta and gamma band power than EEG signals, indicating adequate unconscious classification. The frequency of other artefacts that were assignable, for example technical artefacts, movement artefacts, was negligible and they were excluded from analysis.

CONCLUSION

High-frequency signals and eye blinks may account for index values that falsely indicate consciousness. Compared with BIS, state entropy showed more false classifications of the clinical state at transition between consciousness and unconsciousness.

EEG Signals Analysis Using Multiscale Entropy for Depth of Anesthesia Monitoring during Surgery through Artificial Neural Networks

In order to build a reliable index to monitor the depth of anesthesia (DOA), many algorithms have been proposed in recent years, one of which is sample entropy (SampEn), a commonly used and important tool to measure the regularity of data series. However, SampEn only estimates the complexity of signals on one time scale. In this study, a new approach is introduced using multiscale entropy (MSE) considering the structure information over different time scales. The entropy values over different time scales calculated through MSE are applied as the input data to train an artificial neural network (ANN) model using bispectral index (BIS) or expert assessment of conscious level (EACL) as the target. To test the performance of the new index's sensitivity to artifacts, we compared the results before and after filtration by multivariate empirical mode decomposition (MEMD). The new approach via ANN is utilized in real EEG signals collected from 26 patients before and after filtering by MEMD, respectively; the results show that is a higher correlation between index from the proposed approach and the gold standard compared with SampEn. Moreover, the proposed approach is more structurally robust to noise and artifacts which indicates that it can be used for monitoring the DOA more accurately.

Awareness and recall during general anesthesia

Anesthesia awareness is defined as both consciousness and recall of surgical events. New research has been conducted out to test this phenomenon. However, testing methods have not proven reliable, including those using devices based on electroencephalographic techniques to detect and prevent intraoperative awareness. The limitations of a standard intraoperative brain monitor reflect our insufficient understanding of consciousness. Moreover, patients who experience an intraoperative awareness can develop serious post-traumatic stress disorders that should not be overlooked. In this review, we introduce the incidence of intraoperative awareness during general anesthesia and discuss the mechanisms of consciousness, as well as risk factors, various monitoring methods, outcome and prevention of intraoperative awareness.

Neural network-based classification of anesthesia/awareness using Granger causality features

This article investigates the signal processing part of a future system for monitoring awareness during surgery. The system uses features from the patients' electrical brain activity (EEG) to discriminate between "anesthesia" and "awareness." We investigate the use of a neural network classifier and Granger causality (GC) features for this purpose. GC captures anesthetic-induced changes in the causal relationships between pairs of signals from different brain areas. The differences in the pairwise causality estimated from the EEG activity are used as features for subsequent classification between "awake" and "anesthetized" states. EEG data from 31 subjects obtained during surgery and maintenance of anesthesia with propofol, sevoflurane, or desflurane, are classified using a neural network with one layer of hidden units. An average accuracy of 96% is obtained.

Time-varying spectral analysis revealing differential effects of sevoflurane anaesthesia: non-rhythmic-to-rhythmic ratio

BACKGROUND

Heart rate variability (HRV) may reflect various physiological dynamics. In particular, variation of R-R peak interval (RRI) of electrocardiography appears regularly oscillatory in deeper levels of anaesthesia and less regular in lighter levels of anaesthesia. We proposed a new index, non-rhythmic-to-rhythmic ratio (NRR), to quantify this feature and investigated its potential to estimate depth of anaesthesia.

METHODS

Thirty-one female patients were enrolled in this prospective study. The oscillatory pattern transition of RRI was visualised by the time-varying power spectrum and quantified by NRR. The prediction of anaesthetic events, including skin incision, first reaction of motor movement during emergence period, loss of consciousness (LOC) and return of consciousness (ROC) by NRR were evaluated by serial prediction probability (PK ) analysis; the ability to predict the decrease of effect-site sevoflurane concentration was also evaluated. The results were compared with Bispectral Index (BIS).

RESULTS

NRR well-predicted first reaction (PK  > 0.90) 30 s ahead, earlier than BIS and significantly better than HRV indices. NRR well-correlated with sevoflurane concentration, although its correlation was inferior to BIS, while HRV indices had no such correlation. BIS indicated LOC and ROC best.

CONCLUSIONS

Our findings suggest that NRR provides complementary information to BIS regarding the differential effects of anaesthetics on the brain, especially the subcortical motor activity.

Effects of caudal sufentanil supplemented with levobupivacaine on blocking spermatic cord traction response in pediatric orchidopexy

PURPOSES

Caudal block is one of the most commonly used anesthetic techniques in subumbilical and genitourinary procedures. However, traditional administration of caudal levobupivacaine was inadequate on blocking peritoneal response during spermatic cord traction. The aim of this study was to evaluate whether the addition of caudal sufentanil to levobupivacaine provided better analgesia for children undergoing orchidopexy.

METHODS

Sixty-two patients, scheduled for right orchidopexy, received caudal block after induction. Group LS (n = 31) received levobupivacaine 0.25% 1 ml/kg plus sufentanil 0.5 μg/kg, and group L (n = 31) received levobupivacaine 0.25% 1 ml/kg only. HR or MAP fluctuation >20% or entropy increase >15% during spermatic cord traction was defined as inadequate anesthesia and was treated with increasing sevoflurane concentration. The number of children who needed sevoflurane rescue was counted, and postoperative side effects and quality of sleep were also recorded.

RESULTS

There were no statistically significant differences between the two groups in age, weight, and duration of surgery. Two (6.45%) children in group LS required inspired sevoflurane rescue to block hemodynamic fluctuation during spermatic cord traction, as compared with 12 (38.71%) patients in group L (P < 0.001). At the time of exerting spermatic cord traction, the median HR was, respectively, 134 and 145 (P < 0.001); the corresponding response entropy (RE) and state entropy (SE) was 65 and 54, respectively, in group LS versus 76 and 65 in group L (P < 0.001).

CONCLUSION

In pediatric orchidopexy, the addition of sufentanil to levobupivacaine for caudal blockade offers clinical benefit over levobupivacaine alone in blocking the spermatic cord traction response.

Influence of esmolol on requirement of inhalational agent using entropy and assessment of its effect on immediate postoperative pain score

Background and Context:

Beta-blockers have been used for attenuation of stress response, decreasing anaesthetic requirement and augmentation of the effect of opioids during general anaesthesia.

Aims and Objectives:

The present study aims to evaluate the influence of esmolol on the requirement of an inhalational agent while monitoring the depth of anaesthesia by entropy and also its effect on immediate postoperative pain score.

Methods:

Fifty American Society of Anaesthesiologists (ASA) I and II patients, between 25 and 65 years of age who underwent lower abdominal surgeries were randomly allocated to two groups: Group E and Group S of 25 patients each. Group E received esmolol infusion while Group S received the same volume of saline infusion. Demographic data, haemodynamics, amount of isoflurane used, end-tidal isoflurane concentration, postoperative pain score and total dose of morphine consumed in immediate postoperative period of 30 min were analyzed by using appropriate statistical tests. Value of P<0.05 was considered significant and P<0.001 as highly significant.

Results:

The two groups were comparable with respect to age, weight, ASA physical status, duration of surgery and amount of isoflurane used during anaesthesia. Assessment of postoperative pain was assessed by Visual Analogue Scale (VAS) which showed significant difference at 30 min. The total dose of morphine consumption was significantly less (P<0.05) in Group E for relief of postoperative pain.

Conclusions:

We conclude that in light of depth of anaesthesia monitor esmolol has no effect on requirement of isoflurane, but it decreases the postoperative pain as well as postoperative requirement of morphine without increasing the risk of awareness.

EEG-based automatic classification of 'awake' versus 'anesthetized' state in general anesthesia using Granger causality

BACKGROUND

General anesthesia is a reversible state of unconsciousness and depression of reflexes to afferent stimuli induced by administration of a "cocktail" of chemical agents. The multi-component nature of general anesthesia complicates the identification of the precise mechanisms by which anesthetics disrupt consciousness. Devices that monitor the depth of anesthesia are an important aide for the anesthetist. This paper investigates the use of effective connectivity measures from human electrical brain activity as a means of discriminating between 'awake' and 'anesthetized' state during induction and recovery of consciousness under general anesthesia.

METHODOLOGY/PRINCIPAL FINDINGS

Granger Causality (GC), a linear measure of effective connectivity, is utilized in automated classification of 'awake' versus 'anesthetized' state using Linear Discriminant Analysis and Support Vector Machines (with linear and non-linear kernel). Based on our investigations, the most characteristic change of GC observed between the two states is the sharp increase of GC from frontal to posterior regions when the subject was anesthetized, and reversal at recovery of consciousness. Features derived from the GC estimates resulted in classification of 'awake' and 'anesthetized' states in 21 patients with maximum average accuracies of 0.98 and 0.95, during loss and recovery of consciousness respectively. The differences in linear and non-linear classification are not statistically significant, implying that GC features are linearly separable, eliminating the need for a complex and computationally expensive non-linear classifier. In addition, the observed GC patterns are particularly interesting in terms of a physiological interpretation of the disruption of consciousness by anesthetics. Bidirectional interaction or strong unidirectional interaction in the presence of a common input as captured by GC are most likely related to mechanisms of information flow in cortical circuits.

CONCLUSIONS/SIGNIFICANCE

GC-based features could be utilized effectively in a device for monitoring depth of anesthesia during surgery.

Spectral entropy for assessing the depth of propofol sedation

Background

For patients in the intensive care unit (ICU) or under monitored anesthetic care (MAC), the precise monitoring of sedation depth facilitates the optimization of dosage and prevents adverse complications from underor over-sedation. For this purpose, conventional subjective sedation scales, such as the Observer's Assessment of Alertness/Sedation (OAA/S) or the Ramsay scale, have been widely utilized. Current procedures frequently disturb the patient's comfort and compromise the already well-established sedation. Therefore, reliable objective sedation scales that do not cause disturbances would be beneficial. We aimed to determine whether spectral entropy can be used as a sedation monitor as well as determine its ability to discriminate all levels of propofol-induced sedation during gradual increments of propofol dosage.

Methods

In 25 healthy volunteers undergoing general anesthesia, the values of response entropy (RE) and state entropy (SE) corresponding to each OAA/S (5 to 1) were determined. The scores were then analyzed during each 0.5 mcg/ml- incremental increase of a propofol dose.

Results

We observed a reduction of both RE and SE values that correlated with the OAA/S (correlation coefficient of 0.819 in RE-OAA/S and 0.753 in SE-OAA/S). The RE and SE values corresponding to awake (OAA/S score 5), light sedation (OAA/S 3-4) and deep sedation (OAA/S 1-2) displayed differences (P < 0.05).

Conclusions

The results indicate that spectral entropy can be utilized as a reliable objective monitor to determine the depth of propofol-induced sedation.

Sex differences in conscious sedation during upper gastrointestinal panendoscopic examination

BACKGROUND/PURPOSE

Sex differences in response to noxious stimuli or analgesia have been demonstrated. We investigated sex differences in conscious sedation during upper gastrointestinal panendoscopic examination with regard to drug dose and entropy scores.

METHODS

We investigated sex differences in 30 men and 30 women who were undergoing conscious sedation during upper gastrointestinal panendoscopic examination. The drug mixture was prepared as 5 mg midazolam plus 1 mg alfentanil diluted with normal saline to a volume of 10 mL. An initial injection of 4 mL was followed by an additional 1 mL every 1 minute, until the modified Observer Assessment of Alertness and Sedation (OAAS) rating scale was ≤ 3 when the panendoscope was inserted. Further injection was allowed thereafter. Entropy values, including state entropy (SE) and response entropy (RE), were monitored from baseline to full recovery.

RESULTS

The volume of mixture needed to achieve an OAAS score of ≤ 3 was significantly lower in men than in women (4.4 ± 0.7 mL vs. 4.8 ± 0.8 mL, p = 0.034). The initial drug demand was not significantly influenced by age, body weight, or body height. The RE and SE values at the time of panendoscope insertion were not significantly different between men and women. The total volume for men was also significantly lower than that for women (5.7 ± 1.1 mL vs. 6.5 ± 1.4 mL, p < 0.01). The lowest RE and SE values during the procedure were not significantly different between men and women.

CONCLUSION

Women need more analgesic agents than men during panendoscopic examination. There was no significant difference between men and women with regard to anesthetic depth and response to noxious stimuli, as revealed by similar SE and RE values.

Effect of remifentanil on consumption of sevoflurane in entropy monitored general anesthesia

BACKGROUND

Monitoring of anesthetic depth is important for successful general anesthesia. It is well known that entropy or BIS monitoring assisted titration of anesthetic drugs decreases their consumption. This study evaluated the effect of remifentanil on consumption of sevoflurane during entropy monitored general anesthesia.

METHODS

Patients were randomly assigned to two groups. The R group was administered 0.1 µg/kg/min of remifentanil and inhaled sevoflurane, while the S group was administered only inhaled sevoflurane. Anesthesia was maintained using sevoflurane with nitrous oxide, and entropy was monitored. In both groups, the concentration of sevoflurane was adjusted to keep the state entropy (SE) value between 40 and 60. End-tidal sevoflurane concentration (ET), entropy value, systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (HR) were measured at 5 minute intervals, during a 25 minute period after intubation, skin suture and the end of surgery.

RESULTS

ET was significantly lower in the R group than the S group. There were no significant differences in entropy value between R and S groups.

CONCLUSIONS

Entropy monitored titration of sevoflurane with remifentanil administration decreased ET with stable hemodynamics.

Determining entropy values equivalent to the bispectral index values during sevoflurane anaesthesia

INTRODUCTION

We aimed to identify the entropy levels that would be equivalent to the bispectral index (BIS) levels in sevoflurane anaesthesia in patients who are to undergo elective lumbar disc surgery.

MATERIAL AND METHODS

Thirty cases in ASA groups I-II who underwent lumbar disc surgery under general anaesthesia were included in our study after obtaining the consent of the patients and Ethics Committee of our medical school between January 1, 2005 and October 30, 2006. BIS and entropy electrodes were applied at the same time in 30 cases in the study group. The depth of the anaesthesia was regulated so that 10 min after beginning the general anaesthesia the BIS values were between 40 and 60. At the same time equivalent entropy values corresponding to BIS values were obtained.

RESULTS

At the end of the study, entropy values corresponding to general anaesthesia BIS values were identified. General anaesthesia BIS and RE/SE values at 10 min were lower than the values of the control, which was statistically significant (p < 0.05). Equivalent entropy values were obtained lower than the BIS values during general anaesthesia and these values were found to be statistically significant (p < 0.05). The blood pressure (BP) values obtained were very low at equivalent BIS values and when these BP values were compared with the initial BP values they were found to be statistically significant (p < 0.05).

CONCLUSIONS

Based on this observation, we think that general anaesthetic agents that might cause severe hypotension could be more safely administered under entropy monitoring.

Correlation and agreement between the bispectral index vs. state entropy during hypothermic cardio-pulmonary bypass

BACKGROUND

The bispectral index (BIS) and spectral entropy enable monitoring the depth of anaesthesia. Mild hypothermia has been shown to affect the ability of electroencephalography monitors to reflect the anaesthetic drug effect. The purpose of this study was to investigate the effect of hypothermia during a cardio-pulmonary bypass on the correlation and agreement between the BIS and entropy variables compared with normothermic conditions.

METHODS

This prospective clinical study included coronary artery bypass grafting patients (n=25) evaluating correlation and agreement (Bland-Altman analysis) between the BIS and both spectral and response entropy during a hypothermic cardio-pulmonary bypass (31-34 degrees C) compared with nomothermic conditions (34-37.5 degrees C). Anaesthesia was maintained with propofol and sufentanil and adjusted clinically, while the anaesthetist was blinded to the monitors.

RESULTS

The BIS and entropy values decreased during cooling (P<0.05), but the decrease was more pronounced for entropy variables compared with BIS (P<0.05). The correlation coefficients (bias+/-SD; percentage error) between the BIS vs. spectral state entropy and response entropy were r(2)=0.56 (1+/-11; 42%) and r(2)=0.58 (-2+/-11; 43%) under normothermic conditions, and r(2)=0.17 (10+/-12; 77%) and r(2)=0.18 (9+/-11; 68%) under hypothermic conditions, respectively. Bias was significantly increased under hypothermic conditions (P<0.001 vs. normothermia).

CONCLUSION

Acceptable agreement was observed between the BIS and entropy variables under normothermic but not under hypothermic conditions. The BIS and entropy variables may therefore not be interchangeable during a hypothermic cardio-pulmonary bypass.

Comparison of Entropy and Bispectral Index during Propofol and Fentanyl Sedation in Monitored Anaesthesia Care

Comparison of entropy (state entropy [SE] and response entropy [RE]) with the bispectral index (BIS) during propofol sedation in monitored anaesthesia care (MAC) was carried out in patients undergoing hand surgery. Thirty candidates for elective hand surgery were pre-medicated with midazolam 0.06 mg/kg and atropine 0.01 mg/kg. Sedation was induced with intravenous propofol and fentanyl was also administered. The Modified Observer's Assessment of Alertness/Sedation Scale (MOAA/S) was used to determine sedation level and pain was maintained at < 4 on a 0-10 verbal rating scale. The BIS, entropy, MOAA/S and pain values were recorded before initiation of sedation (control), during initiation of sedation, during surgery, and for 30 min after the end of surgery and anaesthesia. On initiation of sedation, entropy decreased more rapidly than BIS. At 10 min after initiation of sedation, the mean ± SD values for MOAA/S, BIS, RE and SE were 3.00 ± 0.36, 85.45 ± 0.15, 74.00 ± 0.60 and 72.02 ± 0.12, respectively. During recovery, BIS and RE and SE increased in parallel with MOAA/S. It is concluded that entropy monitoring is as reliable as BIS monitoring in MAC.

Auditory event-related potentials, bispectral index, and entropy for the discrimination of different levels of sedation in intensive care unit patients

BACKGROUND

Sedation protocols, including the use of sedation scales and regular sedation stops, help to reduce the length of mechanical ventilation and intensive care unit stay. Because clinical assessment of depth of sedation is labor-intensive, performed only intermittently, and interferes with sedation and sleep, processed electrophysiological signals from the brain have gained interest as surrogates. We hypothesized that auditory event-related potentials (ERPs), Bispectral Index (BIS), and Entropy can discriminate among clinically relevant sedation levels.

METHODS

We studied 10 patients after elective thoracic or abdominal surgery with general anesthesia. Electroencephalogram, BIS, state entropy (SE), response entropy (RE), and ERPs were recorded immediately after surgery in the intensive care unit at Richmond Agitation-Sedation Scale (RASS) scores of -5 (very deep sedation), -4 (deep sedation), -3 to -1 (moderate sedation), and 0 (awake) during decreasing target-controlled sedation with propofol and remifentanil. Reference measurements for baseline levels were performed before or several days after the operation.

RESULTS

At baseline, RASS -5, RASS -4, RASS -3 to -1, and RASS 0, BIS was 94 [4] (median, IQR), 47 [15], 68 [9], 75 [10], and 88 [6]; SE was 87 [3], 46 [10], 60 [22], 74 [21], and 87 [5]; and RE was 97 [4], 48 [9], 71 [25], 81 [18], and 96 [3], respectively (all P < 0.05, Friedman Test). Both BIS and Entropy had high variabilities. When ERP N100 amplitudes were considered alone, ERPs did not differ significantly among sedation levels. Nevertheless, discriminant ERP analysis including two parameters of principal component analysis revealed a prediction probability PK value of 0.89 for differentiating deep sedation, moderate sedation, and awake state. The corresponding PK for RE, SE, and BIS was 0.88, 0.89, and 0.85, respectively.

CONCLUSIONS

Neither ERPs nor BIS or Entropy can replace clinical sedation assessment with standard scoring systems. Discrimination among very deep, deep to moderate, and no sedation after general anesthesia can be provided by ERPs and processed electroencephalograms, with similar P(K)s. The high inter- and intraindividual variability of Entropy and BIS precludes defining a target range of values to predict the sedation level in critically ill patients using these parameters. The variability of ERPs is unknown.

Same-patient reproducibility of state entropy: a comparison of simultaneous bilateral measurements during general anesthesia

BACKGROUND

State Entropy (SE) is an index of anesthetic depth similar to Bispectral Index (BIS). Both indices use a single-channel electroencephalogram, recorded from a unilaterally applied electrode on the forehead, as their input. Intrapatient reproducibility of BIS was questioned in a recent study in which simultaneous measurements from two electrodes applied to the same patient showed conflicting anesthetic depths. Our purpose was to determine whether SE results are similarly reproducible, even though their computation uses a different algorithm than BIS. In this study, we investigated the reproducibility of SE measurements simultaneously obtained from bilaterally applied electrodes in the same patient.

METHODS

Entropy electrodes were applied bilaterally on 21 patients under general inhaled anesthesia. Simultaneous SE measurements from both electrodes were recorded every 10 s from each patient. Data were analyzed with Bland-Altman statistics.

RESULTS

We obtained 14,379 pairs of SE measurements. Four percent of the individual measurements suggested conflicting anesthetic depth along with a numeric difference more than 10 points. Bias was not clinically significant (-0.3). Ninety-five percent limits of agreement were -11.7 and +11.6.

CONCLUSIONS

SE showed a clinically significant degree of disagreement when probes were applied on both sides of the forehead in the same patient. Bland-Altman statistics showed better same-patient reproducibility in SE than did a similar study on BIS. Nevertheless, 4% of the simultaneously measured pairs of SE suggested different anesthetic depths and differed by more than 10 points. Caution is advised when using SE as a clinical index of anesthetic depth.

[Measurement of the depth of anaesthesia]

One of the most important mandates of the anaesthesiologist is to control the depth of anaesthesia. An unsolved problem is that a straight definition of the depth of anaesthesia does not exist. Concerning this it is rational to separate hypnosis from analgesia, from muscle relaxation and from block of cardiovascular reactions. Clinical surrogate parameters such as blood pressure and heart rate are not well-suited for a valid statement about the depth of hypnosis. To answer this question the brain has become the focus of interest as the target of anaesthesia. It is possible to visualize the brain's electrical activity from anelectroencephalogram (EEG). The validity of the spontaneous EEG as an anesthetic depth monitor is limited by the multiphasic activity, especially when anaesthesia is induced (excitation) and in deep anaesthesia (burst suppression). Recently, various commercial monitoring systems have been introduced to solve this problem. These monitoring systems use different interpretations of the EEG or auditory-evoked potentials (AEP). These derived and calculated variables have no pure physiological basis. For that reason a profound knowledge of the algorithms and a validation of the monitoring systems is an indispensable prerequisite prior to their routine clinical use. For the currently available monitoring systems various studies have been reported. At this time it is important to know that the actual available monitors can only value the sedation and not the other components of anaesthesia. For example, they cannot predict if a patient will react to a painful stimulus or not. In the future it would be desirable to develop parameters which allow an estimate of the other components of anaesthesia in addition to the presently available monitoring systems to estimate sedation and muscle relaxation. These could be sensoric-evoked potentials to estimate analgesia and AEPs for the detection of awareness.