Monitoring Of Auditory Evoked Potentials Research Articles

Frequency and Time Domain Analysis of EEG Based Auditory Evoked Potentials to Detect Binaural Hearing in Noise

Hearing loss is a prevalent health issue that affects individuals worldwide. Binaural hearing refers to the ability to integrate information received simultaneously from both ears, allowing individuals to identify, locate, and separate sound sources. Auditory evoked potentials (AEPs) refer to the electrical responses that are generated within any part of the auditory system in response to auditory stimuli presented externally. Electroencephalography (EEG) is a non-invasive technology used for the monitoring of AEPs. This research aims to investigate the use of audiometric EEGs as an objective method to detect specific features of binaural hearing with frequency and time domain analysis techniques. Thirty-five subjects with normal hearing and a mean age of 27.35 participated in the research. The stimuli used in the current study were designed to investigate the impact of binaural phase shifts of the auditory stimuli in the presence of noise. The frequency domain and time domain analyses provided statistically significant and promising novel findings. The study utilized Blackman windowed 18 ms and 48 ms pure tones as stimuli, embedded in noise maskers, of frequencies 125 Hz, 250 Hz, 500 Hz, 750 Hz, 1000 Hz in homophasic (the same phase in both ears) and antiphasic (180-degree phase difference between the two ears) conditions. The study focuses on the effect of phase reversal of auditory stimuli in noise of the middle latency response (MLR) and late latency response (LLR) regions of the AEPs. The frequency domain analysis revealed a significant difference in the frequency bands of 20 to 25 Hz and 25 to 30 Hz when elicited by antiphasic and homophasic stimuli of 500 Hz for MLRs and 500 Hz and 250 Hz for LLRs. The time domain analysis identified the Na peak of the MLR for 500 Hz, the N1 peak of the LLR for 500 Hz stimuli and the P300 peak of the LLR for 250 Hz as significant potential markers in detecting binaural processing in the brain.

Study of Correlation Between EEG Electrodes for the Analysis of Cortical Responses Related to Binaural Hearing

Binaural hearing is the ability of the human auditory system to integrate information received from both ears simultaneously. Binaural hearing is fundamental in understanding speech in noisy backgrounds. Any disfunction in one or both ears could cause a disruption in the processing mechanism. Auditory evoked potentials (AEPs) are electrical potentials evoked by externally presented auditory stimuli from any part of the auditory system. A non-invasive technology, electroencephalography (EEG) is used for the monitoring of AEPs. The research aims to identify the best suited electrode positions through correlation analysis and analyse the AEP signals from the selected electrodes in order to detect binaural sensitivity of the human brain. The study evaluates the time-averaged EEG responses of normal hearing subjects to auditory stimuli. The stimuli used for the study are 500 Hz Blackman windowed pure tones, presented in either homophasic (the same phase in both ears) or antiphasic (180-degree phase difference between the two ears) conditions. The study focuses on understanding the effect of phase reversal of auditory stimuli, an under interaural time difference (ITD) cue, on the middle latency response (MLR) region of the AEPs. A correlation analysis was carried out between the eight different locations and as a result, Cz and Pz electrode positions were selected as the best suited positions for further analysis. The selected electrode signals were further processed in the time domain and frequency domain analysis. In the time domain analysis, it was found that Cz electrode for eight subjects out of nine and Pz electrode for seven subjects out of nine, had the larger area under signal curve obtained in the antiphasic condition than in the homophasic signals. Frequency domain analysis showed that the frequency bands 20 to 25Hz and 25 to 30Hz had the most energy when elicited by antiphasic stimuli than by homophasic stimuli. The findings of this study can be further utilised for the detection of binaural processing in a human brain.

Electrophysiological predictors of hearing deterioration based on AEP monitoring during petroclival meningioma resection

The objective of this study was to calculate the risk of postsurgical hearing deterioration as a function of changes in the amplitude and latency of the most stable components (waves III and V) of the auditory evoked potential (AEP) during petroclival meningioma resection surgery. We retrospectively analyzed intraoperative AEP monitoring results and pre- and postsurgical hearing status in 40 consecutive patients who were surgically treated for petroclival meningiomas. Statistical analyses were conducted to identify the most sensitive and specific way to predict hearing dysfunction after surgery. Patients' mean age was 59 ± 10years, and 31 (77.5%) were women. Twelve (30%) patients presented with clinically detectable hearing impairment preoperatively. At the first postoperative assessment, four of those 12 patients reported subjective improvement, and eight reported hearing deterioration. Of those eight, four remained stable and four recovered hearing by the last assessment. Wave III latency reached its highest specificity (100%) and sensitivity (71.43%) at x = 143%. Wave V latency, on the other hand, reached its highest sensitivity (71%) and specificity (93%) at x = 124%. Finally, wave V amplitude reached its highest sensitivity (100%) and specificity (79%) at x = 74%. Intraoperative alterations of wave III latency and wave V amplitude seem to be highly sensitive and specific at predicting the risk of auditory dysfunction in patients undergoing petroclival meningioma resection and should be used to determine maximum resection with preservation of function.

Intraoperative Auditory Evoked Potential Monitoring for Anaesthesia Depth and Utilization of Inhaled Isoflurane

Background: Hypnosis monitoring has been shown to reduce the incidence of awareness. A-line ARX-IndexTM (AAI) derived from auditory evoked potentials (AEP) represents as a numerical variable depth of anaesthesia. Objectives: To study the efficacy of AEP as an indicator of anaesthetic depth and monitor intraoperative awareness in neurosurgical patients by using the AAI scale. Design: Prospective cohort study is used. Setting: The study is in Neurosurgical centre of Tertiary care hospital. Participants: Neurosurgical patients requiring general anaesthesia with duration of surgery between 90 - 150 minutes were enrolled for the study. Intervention: Patients in Group 1 (control) were monitored by conventional methods. Patients in Group 2 (study) underwent intraoperative monitoring by using the AEP monitor. Primary outcome: To study the efficacy of AEP monitoring and AAI index for monitoring the depth of anaesthesia and reducing the incidence of awareness. Results: There was no significant difference in the intraoperative haemodynamic responses measured between the two study groups (p > 0.5). There was no significant difference in the identification of intraoperative awareness by using conventional parameters between the two groups (p > 0.5). There was also a significantly faster time to recovery for patients in Group 2 (p < 0.05). Conclusion: Hypnosis monitoring using AEP monitor/AAI in neurosurgical patients under general anaesthesia did not show any significant difference in haemodynamic response and intraoperative awareness but had significant lower consumption of volatile anaesthetic with cost sparing effect and a faster recovery time as compared to conventional monitoring.

Monitoring sedation for bronchoscopy in mechanically ventilated patients by using the Ramsay sedation scale versus auditory-evoked potentials

BackgroundAppropriate sedation benefits patients by reducing the stress response, but it requires an appropriate method of assessment to adjust the dosage of sedatives. The aim of this study was to compare the difference in the sedation of mechanically ventilated patients undergoing flexible bronchoscopy (FB) monitored by auditory-evoked potentials (AEPs) or the Ramsay sedation scale (RSS).MethodsIn a prospective, randomized, controlled study, all patients who underwent FB with propofol sedation were monitored and their sedation adjusted. During FB, one group was monitored by AEP and another group was monitored by RSS. The propofol dosage was adjusted by the nursing staff during examination to maintain the Alaris AEP index (AAI) value between 25 and 40 in the AEP group and the RSS at 5 or 6 in the RSS group. Before FB and during FB, the AAI, heart rate (HR), and mean arterial pressure (MAP) were recorded every 5 min. The percentages of time at the sedation target and the propofol dosages were calculated.ResultsNineteen patients received AEP monitoring and 18 patients received RSS monitoring. The percentage of time at the sedation target during FB was significantly higher in the AEP monitoring group (51.3%; interquartile range [IQR], 47.0–63.5%) than in the RSS group (15.4%; IQR, 9.5–23.4%), (P < 0.001). During FB, the RSS group had a significantly higher AAI (P = 0.011), HR (P < 0.001), and MAP (P < 0.001) than the AEP group.ConclusionsIn mechanically ventilated patients undergoing FB, AEP monitoring resulted in less variation in AAI, HR, and MAP, and a higher percentage of time at the sedation target than RSS monitoring.Trial registrationClinicalTrials.gov NCT01448811.

A Telehealth System for Remote Auditory Evoked Potential Monitoring

A portable, Internet-based EEG/Auditory Evoked Potential (AEP) monitoring system was developed for remote electrophysiological studies during sleep. The system records EEG/AEP simultaneously at the subject’s home for increased comfort and flexibility. The system provides simultaneous recording and remote viewing of EEG, EMG and EOG waves and allows on-line averaging of auditory evoked potentials. The design allows the recording of all major AEP components (brainstem, middle and late latency EPs) and constituent single sweep EEG epochs. Auditory stimulation is remotely controlled from any internet-connected PC. The developed platform consists of a portable server computer for data acquisition and secured data transmission to the Internet, a web server for temporary storage, and a remote viewing station. During operation, signals are acquired during sleep and transmitted to the secured web server for controlled access viewing. The remote viewer station allows the user to continuously visualize the auditory evoked potentials, modify auditory stimuli at any time and adjust system parameters for its online computation. The developed internet-based EEG/EP system has been tested in a laboratory environment. Preliminary results demonstrate feasibility of remote real-time acquisition and viewing of EEG and EPs and future potential for home monitoring and telemedicine applications.

Comparison of auditory evoked potential index and clinical signs as indicator for laryngeal mask airway insertion

Objective Auditory evoked potential (AEP) index is one of the several physiological parameters for assessing the depth of anesthesia. The purpose of this study was to investigate whether the AEP monitoring could provide a better information for assessment of anesthesia level in classic laryngeal mask airway (C-LMA) insertion than the use of clinical signs in general anesthesia with single standard dose of intravenous propofol and fentanyl. Methods One hundred and seventy adult patients requiring general anesthesia for minor surgery were recruited and randomized to receive AEP monitoring (group A) or judgment of clinical signs (group B) for assessment of anesthesia depth and optimal condition to insert the C-LMA. The insertion conditions, including jaw relaxation, movements, presence of airway trauma and airway reflex, successful insertion rate and induction time were recorded and compared. Results The two groups were demographically similar. In group A, baseline heart rate was slower than group B (74 ± 14 vs. 78 ± 14 beats/min, p = 0.0267) and persisted throughout the whole study period. There was no significant difference in the change of heart rate during induction of general anesthesia between both groups. The incidence of movement was reduced in group A patients with AEP monitoring in comparison with group B patients (2.4% vs. 28.2%, p < 0.0001); of the unwanted events, swallowing was 0% versus 7.1%, p = 0.0126; laryngospasm was 0% versus 4.7%, p = 0.0430 and emergence of airway reflex was 1.2% versus 11.8%, p = 0.0050; the successful insertion rate was 100% versus 94.1%, p = 0.0232; and jaw relaxation was 83.5% versus 70.6%, p = 0.0448. There were no differences between both groups in trauma and induction time. Conclusion This study demonstrated that AEP index provided better information for C-LMA insertion with higher successful rate, less emergence of airway reflex and lower incidence of movement during induction of general anesthesia with single dose of intravenous propofol and fentanyl.

Effect of Auditory Evoked Potential-Guided Anaesthesia on Consumption of Anaesthetics and Early Postoperative Cognitive Dysfunction: a randomised controlled trial

Post-operative cognitive dysfunction (POCD) after non-cardiac surgery is a well known problem in some categories of patients. This study aims to evaluate the influence of auditory evoked potential (AEP)-guided anaesthesia on the requirement for anaesthetic drugs and their influence on POCD. Four hundred and fifty patients aged between 18 and 92 years scheduled for ophthalmic surgery under general anaesthesia were assigned randomly to one of two groups. In group A (AEP group), the depth of anaesthesia (DoA) was aimed at an AEP index (AAI) between 15 and 25. In group C (control group), DoA was guided by clinical signs. Hypotension was treated with fluids and vasopressors using a standardised algorithm. A mini-mental test and the Cognitive Failure Questionnaire were used to evaluate cognitive function. Anaesthetic drug requirements were significantly lower in group A than in group C: propofol 92.5 ± 26.5 vs. 103.8 ± 39.5 mg (P = <0.001) and desflurane end-tidal concentration 2.5 ± 0.58 vs. 3.3 ± 0.79% (P < 0.001). In group A, 36 patients (16%) received additional fluids and vasopressors compared to 65 patients (29%) in group C (P < 0.01). AAI values differed significantly between the groups: 18 (11-21) in group A vs. 12 (10-19) in group C (P < 0.001). The number of patients with POCD was 16 in group C compared to two in group A (P < 0.001) at day 1 post-operation. AEP monitoring allows dose reduction of anaesthetic agents, leading to better cardiovascular stability and decreased requirements for intra-operative fluids and vasopressors. Cognitive decline seen following minor ophthalmic surgery, even when anaesthesia is assessed clinically, is short-lived with no long-term sequelae.

Comparison of A-Line Autoregressive Index and Observer Assessment of Alertness/Sedation Scale for Monitored Anesthesia Care With Target-controlled Infusion of Propofol in Patients Undergoing Percutaneous Vertebroplasty

Percutaneous vertebroplasty (PV) with monitored anesthesia care (MAC) is a growing trend. Without adequate sedation, patient movement can affect and even interrupt the procedure during MAC. The aim of this study was to compare the performance of the auditory-evoked potential (AEP) index and the Observer Assessment of Alertness/Sedation (OAA/S) scale as indicators of depth of sedation in patients undergoing PV. Two hundred and twenty patients in ASA II to III, aged 43 to 92 years, undergoing elective PV with MAC, were randomly allocated to the AEP or the OAA/S group (n = 110 each). Initially, all patients received 1 μg/kg of fentanyl and 0.02 mg/kg of midazolam intravenously and sedation with a target-controlled infusion (TCI) of propofol at a target concentration of 1.2 μg/mL. The concentration for the propofol TCI was adjusted in 0.2 μg/mL increments or decrements according to the A-Line autoregressive index (AAI) or the OAA/S scale. A blinded study nurse recorded the measured parameters. Some parameters were significantly different in the AEP group compared with the OAA/S group: lower AAI, lower OAA/S score, lower respiratory rates, and higher end-tidal carbon dioxide pressure were noted from local anesthetic infiltration to bone cement implantation, fewer patients whose movements affected the procedure (10 vs. 36, respectively, P < 0.001), and more adjustments of TCI (twice vs. once, respectively, P < 0.006). The surgeons' satisfaction was greater for the AEP group than for the OAA/S group. TCI propofol with AEP monitoring can provide less patient movement, better sedation, and higher surgeon satisfaction in patients during prone-position PV procedures than can TCI propofol with OAA/S monitoring.

The Effects of Auditory Evoked Potential Click Sounds on Bispectral Index and Entropy

The click sounds of auditory evoked potentials (AEP) might have some effect on electroencephalogram indices and Bispectral Index (BIS) but many studies, unconcerned about this effect, have measured both indices simultaneously. In this study, I examined the effects, of AEP click sounds on the BIS, and also on the response entropy (RE) and state entropy (SE) of the entropy monitor. Forty patients aged 40-70 yr and scheduled for surgery of lower extremities under spinal anesthesia were anesthetized with 0.5% bupivacaine or tetracaine. Patients were sedated with midazolam 1 mg followed by propofol infusion started at 1 mg.kg(-1).h(-1). Propofol infusion was controlled to keep BIS or SE at 80, 60, or 40 for several minutes, and then click sounds (65 dB) of the AEP were given for 60 s. The changes in BIS, RE, and SE were observed continuously for 60 s after the click sounds had stopped. BIS, SE, and RE significantly increased during the click sounds. The longest duration of increase was at BIS or SE 60. AEP monitor click sounds transiently increased the simultaneously measured BIS, RE, and SE during different levels of sedation by propofol infusion during spinal anesthesia. Therefore, the effects of the click sounds should be considered when these monitors are used simultaneously in the same patient.

Utilidad del BIS® y AEP Monitor/2® para el mantenimiento de la anestesia combinada sevoflurano-regional, comparada con la práctica estándar en población pediátrica

To compare the indices computed by a bispectral (BIS) monitor and an auditory evoked potential (AEP) monitor during maintenance of anesthesia in pediatric patients. A secondary objective was to compare anesthetic consumption and recovery times. Patients aged 6 months to 12 years under inhaled anesthesia with sevoflurane were administered a penile or caudal nerve block. BIS and AEP monitoring was then started. The patients were randomized to 3 groups in which the sevoflurane dosage was guided by the BIS index, the A-line ARX index (AAI), or standard procedures (control group). The BIS index, the AAI, the fraction of expired sevoflurane, hemodynamic parameters, and recovery time were the variables recorded for all patients. Thirty-five patients were enrolled. Significant differences between the 2 monitor groups were observed. The mean (SD) AAI was lower in patients whose dosage was controlled by the BIS index (19 [2]) than in those whose dosage was based on the AEP monitor's reading (22 [4]) (P=.04). According to a Bland and Altman plot, there was good agreement between the BIS index and the AAI except for the highest and, in particular, the lowest values. The volumes of sevoflurane used and the recovery times were similar between groups. There was a positive correlation between the 2 indices (Pearson correlation coefficient, 0.207; P<.001). BIS monitoring provided a more stable index than did AEP monitoring and BIS-index guidance achieved a deeper level of hypnosis. There was good correlation between the 2 indices during maintenance of anesthesia. Neither monitor offered advantages over standard procedures with regard to movement, volume of anesthetic consumed, or recovery time.

Hemifacial Spasm: A Neurosurgical Perspective

Hemifacial spasm (HFS) is characterized by tonic clonic contractions of the muscles innervated by the ipsilateral facial nerve. Compression of the facial nerve by an ectatic vessel is widely recognized as the most common underlying etiology. HFS needs to be differentiated from other causes of facial spasms, such as facial tic, ocular myokymia, and blepharospasm. To understand the overall craniofacial abnormalities and to perform the optimal surgical procedures for HFS, we are to review the prevalence, pathophysiology, differential diagnosis, details of each treatment modality, usefulness of brainstem auditory evoked potentials monitoring, debates on the facial EMG, clinical course, and complications from the literature published from 1995 to the present time.

Concomitant assessment of depth of sedation with auditory evoked potentials monitor and Richmond Agitation-Sedation Scale in traumatic brain injury patients

Concomitant assessment of depth of sedation with auditory evoked potentials monitor and Richmond Agitation-Sedation Scale in traumatic brain injury patients

Efficacy of A-line™ AEP Monitor as a tool for predicting acceptable tracheal intubation conditions during sevoflurane anaesthesia

It is essential for the clinical anaesthetist to know whether patients are sufficiently anaesthetized to tolerate direct laryngoscopy and endotracheal intubation. Because of the lack of an accurate objective method to determine the level of general anaesthesia, under- or overdosing of anaesthetics may occur. Auditory evoked potential (AEP) is one of several physiological parameters under investigation. We aimed to determine the clinically required depth of anaesthesia, measured by the A-line AEP Monitor and expressed as A-Line ARX Index (AAI) for 90% probability of acceptable conditions for endotracheal intubation. We studied 108 patients anaesthetized by mask with increasing concentration of sevoflurane in 30% oxygen and 70% nitrous oxide. Fentanyl 1.5 microg kg(-1) and glycopyrrolate 0.2 mg were administered intravenously immediately before starting induction of anaesthesia. The monitor was programmed to give an alarm at AAI 10, 15, 20, 25 or 30 according to randomization. When the alarm sounded, the end-expiratory sevoflurane concentration was registered and endotracheal intubation was attempted. Intubation conditions were assessed by an observer blinded to the AAI. At AAI 10 we found acceptable conditions in 91% (confidence interval [CI 72-99%]) of patients. The prediction probability value P(K) of AAI was 0.69 (CI 0.59-0.79) and the P(K) of end-expiratory sevoflurane concentration was 0.93 (CI 0.87-0.99). ED(90) (the AAI with a 90% probability of acceptable intubation conditions) was calculated as 8.5 (CI 0-17.5). AAI indicates the depth of anaesthesia necessary for acceptable endotracheal intubation conditions. Under the conditions of the present study, end-expiratory sevoflurane concentration was a better predictor and may turn out to be more useful in the clinical setting.

Mid-latency auditory evoked potentials monitor (AEP) and brain death

Badenes, R.; Maruenda, A.; García-Pérez, M. L.; Chisbert, V.; Belda, F. J. Author Information

Combined general‐epidural anesthesia decreases the desflurane requirement for equivalent A‐line ARX index in colorectal surgery

The present study used the A-line ARX index, derived from auditory evoked potential measurements, to examine the effect of epidural lidocaine on the end-tidal concentration of desflurane during general anesthesia. Thirty ASA I-II patients scheduled for elective colorectal surgery were included and randomized, in a double-blinded fashion, to receive general anesthesia, and 15 ml of either 2% lidocaine (group GE, n=15) or normal saline (group GS, n=15) was administered epidurally with a maintenance infusion rate of 6 ml h-1. After a 10-min high-flow oxygen wash-in period, desflurane was titrated to a target A-line ARX index (AAI) of 20+/-5. Epidural lidocaine reduced the end-tidal concentration of desflurane required to maintain an adequate clinical effect by 42% compared to general anesthesia alone (2.6% vs. 4.5%, respectively; P<0.001). The initial mean value of AAI was 87.8 (range 78-99) in group GE and 88.13 (79-99) in group GS before general anesthesia induction, the AAI values were approximately 19.7 (15-25) in group GE and 20.2 (16-25) in group GS during anesthesia maintenance, and returned to 84.53 (77-98) in group GE and 86.87 (79-98) in group GS when the patients regained consciousness in the recovery room. No statistical difference in the AAI values was observed either before, during, or after emergence of anesthesia. No patient reported intraoperative awareness. Lower-than-expected concentrations of volatile anesthetics are sufficient to maintain appropriate a clinical anesthesia effect during combined general-epidural anesthesia under auditory-evoked potential monitoring.

Does the A-line ARX-Index provide a reasonable assessment of anaesthetic depth in dogs undergoing routine surgery?

The monitoring of anaesthetic depth is usually based on the subjective assessment of the patient. An objective assessment of anaesthesia has only recently become possible. The auditory-evoked response has predictable changes in response to increasing doses of anaesthetic agents. Recent advances have brought about a regression model with exogenous input of the auditory-evoked response, the A-line ARX-Index (AAI Index). The AAI Index is a dimensionless number between 0 and 100. This technology has been incorporated into the AEP (auditory-evoked potential) monitor that is utilised to assess anaesthetic depth in humans. This study was undertaken to determine if the AEP monitor was useful in dogs. Ten dogs were enrolled in the study. After a full clinical and otoscopic examination, dogs were premedicated with acetylpromazine and morphine. Anaesthesia was induced with thiopentone and maintained with halothane. End-tidal carbon dioxide, temperature, pulse oximetry, blood pressure and the electrocardiogram were monitored and recorded every 5 minutes. Anaesthetic depth was assessed as either being adequate or inadequate by the anaesthetist during surgery. An AEP monitor was attached to the patient and automatically collected AAI Index data. The anaesthetist was blinded to the AEP monitor. Following the completion of the surgical procedure, the patient was allowed to wake up with the AEP monitor attached. The AAI Index was analysed to compare adequate with inadequate anaesthesia during the period of surgery and awake with sleep data during recovery. All AAI Index values associated with inadequate anaesthesia were greater than 31 while adequate values were less than 35. The difference between the groups was statistically significant and the power was 0.97. Statistically, the awake and sleep values were significantly different with a power of 0.99. From this study it can be concluded that the AAI Index shows good prospect for the evaluation of anaesthetic depth in dogs undergoing surgery. A larger study is needed to confirm these results.

Awareness During Anesthesia

Awareness During Anesthesia M. M. Ghoneim (ed.) Butterworth-Heinemann; Oxford, UK, 2001, 181 pp ISBN: 0-7506-7201-3; Price £40.00 This is the first definitive book on awareness during anaesthesia edited by the world's foremost expert on the subject. The other 12 contributors hail from the disciplines of anaesthesiology, psychology, medicine and obstetrics, and are top authorities in their fields. The book comprises nine chapters covering all aspects of awareness during anaesthesia - implicit and explicit memory, learning, monitoring (BIS, auditory-evoked responses, isolated forearm techniques), and psychological and medicolegal consequences. It starts with a long Preface that could probably be renamed as the summary of the book. The first two chapters are the heaviest part, accounting for more than one-third of the publication. They deal with the history, concepts and clinical aspects of implicit and explicit recall during anaesthesia. For those who have followed Ghoneim's previous reviews of the subject, these two chapters should be welcomed because they contain more updated information and are organized in a very logical order. Four chapters summarize the currently available techniques for monitoring the depth of anaesthesia. All the advanced tools, including the isolated forearm technique, bispectral index and auditory-evoked potential monitoring, are described in detail. The review on 'Learning during sedation, anesthesia and surgery' is particularly interesting. Of note, Andrade's hypothesis on the relationship between plasma catecholamine concentrations and the probability of learning during anaesthesia will certainly stimulate lots of interest. The chapter on the psychological consequences of intraoperative awareness is illustrated with lively case studies (as is also the case in the other chapters), and is very helpful in describing the scope of the problem. The final chapter presents balanced views from both sides of the Atlantic on the medico-legal issues of intraoperative awareness. On the one hand, all the chapters can be read independently of one another. On the other hand, there has been much repetition of material. Furthermore, the terminology used is not necessarily consistent among chapters. Overall, the book has accomplished its aim of providing a succinct and up-to-date review of the subject. It will become a valuable resource for researchers starting in this field. Clinicians, lawyers and patients will also find it an excellent reference. As more data on intraoperative awareness will undoubtedly become available in the near future, the next edition is eagerly awaited. A. M.-H. Ho M. T. V. Chan Hong Kong, SAR