Awareness in anaesthesia: incidence, consequences and prevention

Risk factor for intraoperative awareness

BACKGROUND AND OBJECTIVES

The intraoperative awareness is an adverse event in the general anesthesia, and may occur in approximately 20,000 cases per year, which justifies the study of the risk factors for this event. The objective of this study was to review this subject in order to reduce the incidence of intraoperative awareness and psychological sequelae incurring from this incident, which may result in post-traumatic stress disorder with negative repercussions on the surgical patient social, psychic and functional development.

CONTENT

It was conducted a review of the intraoperative awareness assessing its different phases during general anesthesia such as dreaming, wakefulness, explicit and implicit memory, as well as the analysis, consequences and prevention of its main related factors.

CONCLUSIONS

The reduction of awareness incidence during anesthesia is related to the anesthesiologist improved scientific and technical performance, involving issues such as monitoring, comprehension of the anesthesia activity components, hypnotic and analgesic drugs, neuromuscular blocking agents, autonomic and motor reflex control, in addition to the risk factors involved in this event.

Critical incident reports concerning anaesthetic equipment: analysis of the UK National Reporting and Learning System (NRLS) data from 2006-2008*

Anaesthetic equipment plays a central role in anaesthetic practice but brings the potential for malfunction or misuse. We aimed to explore the national picture by reviewing patient safety incidents relating to anaesthetic equipment from the National Reporting and Learning System for England and Wales between 2006 and 2008. We searched the database using the system's own classification and by scrutinising the free text of relevant incidents. There were 1029 relevant incidents. Of these, 410 (39.8%) concerned patient monitoring, most commonly screen failure during anaesthesia, failure of one modality or failure to transfer data automatically from anaesthetic room to operating theatre. Problems relating to ventilators made up 185 (17.9%) of the reports. Sudden failures during anaesthesia accounted for 142 (13.8%) of these, with a further 10 cases (0.9%) where malfunction caused a sustained or increasing positive pressure in the patient's airway. Leaks made up 99 (9.6%) of incidents and 53 (5.2%) of incidents arose from the use of infusion pumps. Most (89%) of the incidents caused no patient harm; only 30 (2.9%) were judged to have led to moderate or severe harm. Although equipment was often faulty, user error or unfamiliarity also played a part. A large variety of causes led to a relatively small number of clinical scenarios, that anaesthetists should be ready, both individually and organisationally, to manage even when the cause is not apparent. We make recommendations for enhancing patient safety with respect to equipment. You can respond to this article at http://www.anaesthesiacorrespondence.com.

The incidence of intra-operative awareness during general anesthesia in China: a multi-center observational study

BACKGROUND

The incidence of awareness in patients undergoing general anesthesia is 0.1-0.2% in Western countries. The medical literatures about awareness during general anesthesia are still rare in China, but some previous studies have reported a higher incidence (1.4-6%) of intra-operative awareness. To find out the reason why the incidence reported in China is much higher than that in Western countries, we performed a prospective, multicenter, non-randomized observational study to determine the true incidence of intra-operative awareness in China.

METHODS

This is a prospective, non-randomized descriptive cohort study that was conducted at 25 academic medical centers in China. Eleven thousand one hundred and eighty-five patients were interviewed by research staff for evaluation of awareness at the first and fourth day after general anesthesia with muscle relaxation. An independent blinded committee evaluated the responses and determined whether awareness occurred. Necessary data were collected for a binary logistic regression analysis.

RESULTS

Data from 11,101 patients were presented. Forty-six cases (0.41%) were reported as definite awareness and 47 additional cases (0.41%) as possible awareness. Three hundred and fifty-five patients (3.19%) had dreams during general anesthesia. Awareness was associated with increased American Society of Anesthesiologists (ASA) physical status, a previous anesthesia, and anesthesia methods of total intravenous anesthesia.

CONCLUSION

The incidence of intra-operative awareness in China is approximately 0.41%, two to three times higher than that widely cited in Western countries. Inappropriately light anesthesia, and the population proportion of surgery and general anesthesia in China may account for the difference. (ClinicalTrials.gov Identifier, NCT00693875.).

Psychological consequences of awareness and their treatment

Intraoperative awareness with subsequent recall is a rare but serious complication with an incidence of 0.1-0.2%. In approximately one third of the patients who have experienced awareness, late severe psychiatric sequelae may develop. The psychiatric symptoms in these patients fulfil the diagnostic criteria for post traumatic stress disorder. To prevent awareness as a negative outcome after anaesthesia, a thorough perioperative management of anaesthesia is necessary. The definite risk for post traumatic stress disorder following awareness indicates the necessity of postoperative clinical routines to identify awareness patients. The problem must be acknowledged. Professional psychiatric assessment and follow up should constitute standard practice. The treatments of choice are Eye Movement Desensitisation Reprocessing and Cognitive Behaviour Therapy.

Monitoring Oxygenator Expiratory Isoflurane Concentrations and the Bispectral Index to Guide Isoflurane Requirements During Cardiopulmonary Bypass

OBJECTIVE

The purpose of this study was to measure the changes in isoflurane requirements during the rewarming phase of cardiopulmonary bypass with moderate hypothermia.

DESIGN

An observational study.

SETTING

University hospital, single center.

PARTICIPANTS

Forty patients undergoing elective coronary artery bypass surgery with cardiopulmonary bypass.

INTERVENTIONS

Isoflurane requirements were quantified by measuring the concentrations in the oxygenator expiratory gas. Anesthesia was guided by bispectral index monitoring.

MEASUREMENTS AND MAIN RESULTS

Isoflurane concentrations required to maintain the bispectral index between 40 and 50 during the rewarming phase of cardiopulmonary bypass were measured. There was a progressive increase in expiratory isoflurane requirements during rewarming from 30 degrees C to 37 degrees C, with a Pearson correlation coefficient of 0.78. There was a significant difference in the concentration required at 30 degrees C (0.41% +/- 0.14%) compared with 37 degrees C (1.00% +/- 0.12%).

CONCLUSION

Isoflurane requirements are reduced during hypothermic cardiopulmonary bypass. Monitoring anesthetic concentrations in the oxygenator expiratory gas may be a useful adjunct to monitoring the depth of anesthesia.

Palmar skin conductance compared to a developed stress score and to noxious and awakening stimuli on patients in anaesthesia

BACKGROUND

The number of fluctuations in the skin conductance per s (NFSC) as a measure of the sympathetic nervous system may be a tool for monitoring physiological stress during surgery and general anaesthesia. The purpose of this study was to find the sensitivity and specificity of the NFSC when compared to a peroperative clinical stress score. Moreover, different patterns of skin conductance responses were compared with the BIS score to find out if the mean level of skin conductance (SC) and NFSC monitoring could differentiate between awakening and noxious stimuli.

METHODS

Fourteen patients were studied during stressful or non-stressful registration periods. During each registration period, the NFSC was compared to a five-point clinical stress score (CSS) (systolic blood pressure >130 mmHg, cough, tears, EMG in the forehead >50 or movements) and BIS score.

RESULTS

The NFSC and the CSS both indicated physiological stress at 12 registrations and no stress at 186 registrations. The NFSC indicated physiological stress without signs of clinical stress (CSS = 0) in 28 registrations, whereas signs of clinical stress (CSS > 0) were indicated on two occasions without signs of stress in the NFSC. The sensitivity of the NFSC when compared to the CSS was 86% and the specificity was 86%. Moreover, in all situations (n = 16) where NFSC indicated stress and the BIS score >50, the SC increased. This was different from situations (n = 13) where NFSC indicated stress and the BIS score <50, then the SC did not increase (P < 0.001).

CONCLUSION

The NFSC is sensitive to clinical stress during surgical stimulation. Moreover, the combined use of SC and NFSC may have a potential to differentiate between situations of stress due to inadequate hypnotic effect vs. inadequate analgesic effect.

Prolonged (more than ten hours) neuromuscular blockade after cardiac surgery: report of two cases

PURPOSE

We examine two cases of prolonged neuromuscular blockade (NMB) after cardiac surgery. To the best of our knowledge, these are the first reported cases of complete paralysis lasting more than ten hours after surgery.

CLINICAL FEATURES

We attribute the extended durations of NMB (more than ten hours) to high doses of NMB drugs in combination with magnesium sulphate and moderate renal failure. Advanced age, hepatic disease, aminoglycoside exposure, hypocalcemia, and possible interaction between rocuronium and pancuronium may have played minor roles.

CONCLUSION

We should avoid administering large doses of NMB agents, even in the context of planned postoperative ventilation. If NMB is not monitored intraoperatively in patients who are at risk of prolonged NMB, then train-of-four response should be measured in the intensive care unit. Adequate sedation should be provided until proper recovery of neuromuscular function is documented.

An anaesthetic protocol in the young domestic pig allowing neuromuscular blockade for studies of cardiac function following cardioplegic arrest and cardiopulmonary bypass

BACKGROUND

Neuromuscular blockade should, for ethical reasons, not be allowed in animal experiments unless the use is strongly motivated. Beforehand, the anaesthetic protocol must be documented without muscle relaxation in the species studied. Documentation is difficult to obtain from the scientific literature. When focusing on cardiac function over time, in particular, the ideal anaesthetic protocol should cause no or minor alterations in cardiac variables.

METHODS

We intended to document an anaesthetic protocol involving ventilation with N(2)O combined with loading doses and continuous infusions of pentobarbital, fentanyl and midazolam in seven pigs by applying potentially painful stimuli every 15 min for 7 h. Subsequently, left ventricular global and regional function was studied with conductance catheter and strain rate imaging by echocardiography in eight pigs with pancuronium included.

RESULTS

Pigs without pancuronium were completely immobilized and unresponsive to potentially painful stimuli and sternotomy, with no accumulation or degradation of anaesthetic agents. With pancuronium included, left ventricular preload gradually decreased together with reduction of cardiac index from 3.52 +/- 0.14 at 2 h to 2.84 +/- 0.11 L min(-1). m(-2) (+/-SEM) after 7 h of observation. Preload recruitable stroke work decreased after 7 h, whereas peak systolic strain in the anterior left ventricular wall and load-independent indices of diastolic function were not significantly altered.

CONCLUSION

In specific experimental protocols, the anaesthetic protocol described could allow the use of muscular paralysis in young domestic pigs, for instance when involving hypothermic cardiopulmonary bypass, cardioplegic arrest and reperfusion.

[Unwanted wakefulness during general anesthesia]

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

Human auditory steady-state responses: the effects of recording technique and state of arousal

There is some controversy in the literature about whether auditory steady-state responses (ASSRs) can be reliably recorded in all subjects and whether these responses consistently decrease in amplitude during drowsiness. In 10 subjects, 40-Hz ASSRs became significantly different from background electroencephalogram activity with a probability of P < 0.01 and an average time of 22 s (range, 2-92 s), provided that the responses were analyzed with time-domain averaging rather than spectral averaging. In a second experiment with 10 subjects, 40-Hz ASSRs recorded between the vertex and posterior neck consistently decreased in amplitude during drowsiness and sleep. Findings that the ASSR may occasionally increase during drowsiness may be explained by postauricular muscle responses recorded from a mastoid reference. These may occur during drowsiness in association with rolling-eye movements. ASSRs recorded between the vertex and posterior neck are not distorted by these reflexes. These findings combine with previous literature on the effects of general anesthetics on the ASSR to confirm that the ASSR is a valid option for monitoring the hypnotic effects of general anesthetics.

IMPLICATIONS

Auditory steady-state responses to stimuli presented at rates near 40 Hz can be used to monitor anesthesia. These responses can be quickly and reliably recorded during both sleep and wakefulness, provided that appropriate averaging techniques are used.

Consent and anaesthetic risk

The incidences of mortality and morbidity associated with anaesthesia were reviewed. Most of the published incidences for common complications of anaesthesia vary considerably. Where possible, a realistic estimate of the incidence of each morbidity has been made, based on the best available data. Perception of risk and communication of anaesthetic risk to patients are discussed. The incidences of anaesthetic complications are compared with the relative risks of everyday events, using a community cluster logarithmic scale, in order to place the risks in perspective when compared with other complications and with the inherent risks of surgery. Documentation of these risks and discussion with patients should allow them to be better informed of the relative risks of anaesthetic complications. Depending on specific comorbidities and the severity of operation, these risks associated with anaesthesia may increase for any one individual.

Relationship between awareness and middle latency auditory evoked responses during surgical anaesthesia

BACKGROUND

Some studies support the view that meaningful auditory input can be processed by the brain during apparent surgical anaesthesia. Consequently, patients may be able to remember some information implicitly after anaesthesia as well through a 'dream-like process' (subconscious awareness). The aim of this study was to investigate the presence of subconscious awareness during anaesthesia and to examine its relationship to the mid-latency auditory evoked responses (MLAERs).

METHODS

We studied 40 patients, ASA I-II, undergoing laparoscopic cholecystectomy. General anaesthesia was induced with thiopental 5 mg kg(-1), fentanyl 3 micro g kg(-1), and vecuronium 0.08 mg kg(-1). For the maintenance of anaesthesia, patients were randomly assigned to one of four anaesthetic regimen groups: sevoflurane+air in oxygen 40%; sevoflurane+nitrous oxide 60%; isoflurane+air in oxygen 40%; and isoflurane+nitrous oxide 60%. MLAERs were recorded before anaesthesia, at 1 MAC of inhaled anaesthetic and then 30 min after awakening. An audiotape with one of four stories was played immediately after intraoperative MLAER recording. Explicit and implicit memory was assessed 24 h after awakening.

RESULTS

None of the patients had explicit recall. One of the patients from the isoflurane-air group showed implicit memory of listening to the audiotape. A dream-like process, in which they remembered implicitly the story played during anaesthesia, occurred in one of the patients from the sevoflurane-nitrous oxide group. In the patients with subconscious awareness, MLAERs were similar to that of the awake state with a Pa latency increase of less than 8.87. When there was a marked increase in Pa latency during anaesthesia, no subconscious awareness was observed. No statistically significant differences were found between Pa latency before and after anaesthesia.

CONCLUSIONS

MLAERs may help to predict subconscious cerebral processing of auditory inputs during anaesthesia.

Propofol - not thiopental or etomidate - with remifentanil provides adequate intubating conditions in the absence of neuromuscular blockade

PURPOSE

Administration of remifentanil followed by propofol provides adequate conditions for tracheal intubation without muscle relaxants. Other hypnotic drugs have not been thoroughly investigated in this regard. Intubating conditions with remifentanil followed by propofol, thiopentone or etomidate are compared in this study.

METHODS

In a randomized, double-blind study 45 healthy males were assigned to one of three groups (n = 15). After iv atropine, remifentanil 3 microg x kg(-1) were injected over 90 sec followed by propofol 2 mg x kg(-1) (Group I), thiopentone 6 mg x kg(-1) (Group II) or etomidate 0.3 mg x kg(-1) (Group III). Ninety seconds after the administration of the hypnotic agent, laryngoscopy and intubation were attempted. Intubating conditions were assessed as excellent, good or poor on the basis of ease of ventilation, jaw relaxation, position of the vocal cords, and patient response to intubation and slow inflation of the endotracheal tube cuff.

RESULTS

One patient in Group I, three patients in Group II and five patients in Group III could not be intubated on the first attempt. Clinically acceptable intubating conditions were observed in 93.3%, 66.7%, 40.0% of patients in Groups I, II and III, respectively. Overall conditions at intubation were significantly (P < 0.05) better, and the frequency of excellent conditions was significantly (P < 0.05) higher in the propofol group compared with the thiopentone and etomidate groups. No patient was treated for hypotension or bradycardia.

CONCLUSION

Propofol 2 mg x kg(-1) was superior to thiopentone 6 mg x kg(-1) and etomidate 0.3 mg x kg(-1) for tracheal intubation when combined with remifentanil 3 microg x kg(-1) and no muscle relaxant.

Remifentanil-sevoflurane anaesthesia for laparoscopic cholecystectomy: comparison of three dose regimens

The objective of this study was to determine a dosing regimen for remifentanil-sevoflurane anaesthesia that achieves an optimal balance between quality of anaesthesia and time to recovery. Patients undergoing elective laparoscopic cholecystectomy were randomly allocated to receive 0.4, 0.8 or 1.2 MAC (minimal alveolar concentration) of sevoflurane combined with remifentanil as required to maintain stable anaesthesia. For induction of anaesthesia, the remifentanil dose was 25 microg x kg(-1) x h(-1) and the mean propofol dose which was required to obtain loss of consciousness was 1.59 mg x kg(-1). During the maintenance phase, the mean remifentanil dose was 16.0, 14.1 and 13.0 microg x kg(-1) x h(-1) for the 0.4, 0.8 and 1.2 MAC groups, respectively. The mean sevoflurane maintenance dose was 0.91, 1.24 and 2.1% end-tidal for the 0.4, 0.8 and 1.2 MAC groups, respectively. The incidence of somatic responses was significantly higher in the 0.4 MAC sevoflurane group. Recovery times were significantly faster in the 0.4 compared to the 0.8 and 1.2 MAC groups and in the 0.8 compared to the 1.2 MAC group. The combination of 14 microg x kg(-1) x h(-1) remifentanil and 1.24% end-tidal sevoflurane achieved the optimal balance between the quality, and recovery from anaesthesia.

Victims of awareness

BACKGROUND

Intraoperative awareness with explicit recall may be followed by long-lasting mental symptoms. However, the average risk for developing mental sequelae after awareness, and the average severity and the duration of symptoms has not previously been illustrated in a consecutive series of awareness cases.

METHODS

Nine patients among 18 consecutive, prospectively identified cases of intraoperative awareness with recall could be located after approximately 2 years and agreed to an interview about possible persisting problems.

RESULTS

Four of the nine interviewed patients were still severely disabled due to psychiatric/psychological sequelae. All of these patients had experienced anxiety during the period of awareness, but only one had complained about pain. Another three patients had less severe, transient mental symptoms, although they could cope with these in daily life. Two patients denied any sequelae from their awareness episode.

CONCLUSIONS

Up to 3 weeks after their unsuccessful anesthetic, repeated information and discussions had been offered. Despite the fact that all patients at that time claimed to be satisfied with this management, and eventually considered no further contacts necessary, this was obviously inaccurate. Therefore, professional psychiatric assessment, treatment and long-term follow-up should constitute standard practice for all patients who have experienced intraoperative awareness.

Postoperative reading speed does not indicate implicit memory in elderly cardiac patients after propofol and remifentanyl anaesthesia

BACKGROUND

A recent study in young patients undergoing propofol-alfentanil-nitrous oxide anaesthesia demonstrated implicit memory for stories presented during operation using a postoperative reading speed task. In this study we investigated whether patients who tolerate only small amounts of anaesthetics are prone to develop implicit and explicit memories about intraoperative events.

METHODS

Thirty patients with poor physical status (ASA III-IV) undergoing cardioverter defibrillator implantation were included in the study. Patients were premedicated with intravenous midazolam and anaesthesia was maintained using propofol and remifentanil infusions. During surgery one of two audio-tapes containing two short stories was played to the patients. Reading speed for the stories played during surgery and two similar stories from the other tape was tested 4 h later. Explicit memory was tested at 4 h and 24 h after audiotape presentation using a structured interview and a forced-choice recognition test pertaining to the story content. Thirty additional awake subjects served as controls.

RESULTS

Although half of the patients seemed to be awake one or more times during the operation, no explicit memories of intraoperative events were reported. The forced-choice recognition of the stories was at chance level. No effect on reading speed was found in either the patients or the control subjects.

CONCLUSIONS

The possible reasons for reduced explicit and implicit memory performance in elderly patients are age and poor physical status of the patients and the modality change between study and test phases. A non-anaesthetised control group of the same age and physical status should therefore be included in all studies of implicit memory.

Routine intra-operative assessment of pain and/or depth of anaesthesia by nurse anaesthetists in clinical practice

Patient safety and comfort during general anaesthesia and surgery are to a considerable extent dependent on the capability of anaesthesia personnel to interpret directly monitored as well as indirect clinical signs of pain and/or depth of anaesthesia. The aim of the present study was to evaluate how nurse anaesthetists in their clinical routine work assess and interpret intra-operative responses evoked by pain stimuli and/or insufficient depth of anaesthesia. A questionnaire was designed to assess the perceived relevance and validity of cardiovascular, respiratory, mucocutaneous, eye-associated, and muscular responses for routine assessment of intra-operative pain and/or insufficient depth of anaesthesia in patients undergoing surgery under general anaesthesia. Data were obtained from 223 nurse anaesthetists working at nine different university anaesthesia departments in Sweden. A number of significant indicators for pain and depth of anaesthesia could be identified for spontaneously breathing as well as for mechanically ventilated patients. No variable was considered entirely specific for either intra-operative pain or depth of anaesthesia. Changes in breathing rate/volume, central haemodynamics (BP, HR), lacrimation, and presence of moist and sticky skin were given higher score values as indicators of pain than as indicators of depth of anaesthesia. Occurrence of grimaces, attempted movements, and presence of non-centred pupils were variables considered more indicative of insufficient depth of anaesthesia than intra-operative pain. In conclusion, it is obvious from the present data that indirect physiological signs of intra-operative pain and depth of anaesthesia are still considered of importance by Swedish anaesthesia nurses in the anaesthetic management of surgical patients.

The attitude of the general public towards preoperative assessment and risks associated with general anesthesia

PURPOSE

To survey the general public's attitude towards preoperative assessment and commonly perceived fears about general anesthesia.

METHODS

A province wide telephone survey was conducted in Alberta. General and regional anesthesia were defined, a scenario involving major knee surgery was described, and participants were asked to choose between regional and general anesthesia. Respondents used a seven-point scale to rate the importance of seeing an anesthesiologist preoperatively and were questioned about the timing of such a visit. Attitudes towards commonly perceived fears associated with anesthesia were also assessed.

RESULTS

A total of 1,216 people were surveyed. Over 30% of respondents felt that it was very important to see an anesthesiologist preoperatively, with a total of over 60% attributing a high degree of importance to this. Fifty percent felt that this assessment should occur on the day prior to surgery. A preference for regional or general anesthesia was not expressed in the situation. Approximately 20% of respondents were very concerned about brain damage, waking up intraoperatively and memory loss. Twelve percent were concerned about dying intraoperatively. Nine percent expressed concern about postoperative pain, with 12% reporting being concerned about nausea and vomiting.

CONCLUSIONS

The general public considers anesthetic assessment on the day prior to surgery an important part of preoperative preparation. Fears of brain damage, death and intraoperative awareness associated with general anesthesia remain prevalent, suggesting that preoperative education of patients should address these concerns. The general population was less concerned about realistic fears such as nausea, vomiting and postoperative discomfort.

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

We assessed the effect of propofol on the auditory steady-state response (ASSR), bispectral (BIS) index, and level of consciousness in two experiments. In Experiment 1, propofol was infused in 11 subjects to obtain effect-site concentrations of 1, 2, 3, and 4 microg/mL. The ASSR and BIS index were recorded during baseline and at each concentration. The ASSR was evoked by monaural stimuli. Propofol caused a concentration-dependent decrease of the ASSR and BIS index values (r(2) = 0.76 and 0.93, respectively; P<0.0001). The prediction probability for loss of consciousness was 0.89, 0.96, and 0.94 for ASSR, BIS, and arterial blood concentration of propofol, respectively. In Experiment 2, we compared the effects of binaural versus monaural stimulus delivery on the ASSR in six subjects during awake baseline and propofol-induced unconsciousness. During baseline, the ASSR amplitude with binaural stimulation (0.47+/-0.13 microV, mean +/- SD) was significantly (P<0.002) larger than with monaural stimulation (0.35+/-0.11 microV). During unconsciousness, the amplitude was 0.09+/-0.09 microV with monaural and 0.06+/-0.04 microV with binaural stimulation (NS). The prediction probability for loss of consciousness was 0.97 (0.04 SE) for monaural and 1.00 (0.00 SE) for binaural delivery. We conclude that the ASSR and BIS index are attenuated in a concentration-dependent manner by propofol and provide a useful measure of its sedative and hypnotic effect. BIS was easier to use and slightly more sensitive. The ASSR should be recorded with binaural stimulation. The ASSR and BIS index are both useful for assessing the level of consciousness during sedation and hypnosis with propofol. However, the BIS index was simpler to use and provided a more sensitive measure of sedation.

IMPLICATIONS

We have compared two methods for predicting whether the amount of propofol given to a human subject is sufficient to cause unconsciousness, defined as failure to respond to a simple verbal command. The two methods studied are the auditory steady-state response, which measures the electrical response of the brain to sound, and the bispectral index, which is a number derived from the electroencephalogram. The results showed that both methods are very good predictors of the level of consciousness; however, bispectral was easier to use.

Adverse drug errors in anesthesia, and the impact of coloured syringe labels

PURPOSE

To describe the frequency and pattern of drug errors in clinical anesthesia, and to evaluate whether a change to colour coded syringe labels, along with education, could reduce the problem of drug errors.

METHODS

We prospectively recorded anesthesia-related information from all anesthetic cases for 36 mo, totally 55,426 procedures. Intraoperative problems, including drug errors, were recorded. After eighteen months we changed to colour coded syringe labels, and the effect of this change and education on drug errors was assessed. Errors were divided into four groups: syringe swap, ampoule swap, other 'wrong drug' errors, and wrong dose errors. The problems were graded into four levels, according to severity.

RESULTS

A drug error was recorded in 63 cases (0.11%). There were 28 syringe swaps, and muscle relaxants were erroneously given in 15. There were nine ampoule swaps. There were eight 'other wrong drug' cases, and 18 cases where a wrong dose of the correct drug was given. Three of the drug errors were classified as serious, and 27 were of moderate severity. We found no differences between the two periods except for decreased number of ampoule swaps (P = 0.04).

CONCLUSION

Drug errors are uncommon, and represent a small part of anesthesia problems but still have the potential for serious morbidity. Syringe swaps occurred most often between syringes of equal size, and were not eliminated by colour coding of labels. As muscle relaxant drugs are most commonly involved, and can cause lasting morbidity, special preventive measures should be taken for this group of drugs.

Auditory evoked responses and learning and awareness during general anesthesia

BACKGROUND

There is a major distinction between conscious and unconscious learning. Monitoring the mid-latency auditory evoked responses (AER) has been proposed as a measure to ascertain the adequacy of the hypnotic state during surgery. In the present study, we investigated the presence of explicit and implicit memories after anesthesia and examined the relationships of such memories to the AER.

METHODS

We studied 180 patients scheduled for elective surgical procedures. After a thiopental induction, one of four anesthetics were studied: Opioid bolus: 7.5 microg x kg(-1) fentanyl, 70% N2O, with 2.5 microg x kg(-1) supplements as needed (n=100); Opioid infusion: Alfentanil 50 microg x kg(-1) bolus, 1-1.5 microg x kg(-1) x min(-1) infusion, 70% N2O (n=40); Isoflurane 0.3%: Fentanyl 1 microg x kg(-1), 70% N2O, isoflurane 0.3% expired (n=16); Isoflurane 0.7%: Fentanyl 1 microg x kg(-1), 70% N2O, isoflurane 0.7% expired (n=23). AER were recorded before anesthesia, 5 min after surgical incision and then every 30 min until the end of surgery. A tape of either the story of the "Three Little Pigs" or the "Wizard of Oz" was played continuously between the recordings. Explicit memory was assessed postoperatively by tests of recall and recognition, and implicit memory was assessed by the frequency of story-related free associations to target words from the stories, which were solicited twice during a structured interview.

RESULTS

Six patients showed explicit recall of intraoperative events: All received the opioid bolus regimen. About 7% of patients reported dreaming during anesthesia. The incidence of picking the correct story that had been presented during anesthesia averaged 49%, i.e., very close to chance level. Overall, priming occurred only at the second association tests for the opioid bolus regimen, for which the frequency of an association to the presented story among those not giving an association to the control story was 26%, which was double the frequency (13%) of an association to the control story among those not giving an association to the presented story. This was significant by McNemar's test, P=0.02. There were significant associations between awareness, priming and AER, e.g., recall was associated with higher Nb amplitudes during anesthesia and priming was associated with shorter wave latencies.

CONCLUSIONS

The incidence of awareness in patients anesthetized with nitrous oxide and bolus supplementation was 6%. Thus, this anesthetic technique did not reduce the risk of awareness compared with the use of nitrous oxide alone. Implicit memory occurred during nitrous oxide and bolus supplementation. Recording AER during anesthesia may help to predict awareness and implicit memory, particularly the former. The short contents of most of the dreams which were recalled could hamper future studies in this area.

Monitoring for intraoperative awareness

Despite advances in anesthetic technique and technology, intraoperative awareness continues to occur with alarming regularity. The psychological effects on patients and the medicolegal consequences to providers have created the need for a reliable monitor of the hypnotic component of anesthesia. A new technology known as bispectral analysis (BIS) has the potential to put an end to the devastating occurrence of intraoperative awareness, as well as improve the overall effectiveness of anesthetic delivery and the surgical experience. Applicable to almost any situation requiring sedation or general anesthesia, BIS's reliability is a real-time measure of hypnosis levels.

1 MAC-incision sevoflurane prevents explicit awareness during surgical skin incision and tracheal intubation

BACKGROUND

This study tested the hypothesis that 1 MAC-incision anesthesia secures unconsciousness during surgical skin incision and tracheal intubation.

METHODS

Twenty patients scheduled for gynecological laparotomy were anesthetized with sevoflurane as the sole agent. At 1 MAC-incision steady-state conditions, the patients were observed for autonomic/movement responses and wakefulness (response to verbal commands) in the 1-min period following surgical skin incision and tracheal intubation.

RESULTS

Blood pressure and heart rate increased significantly secondary to both stimuli, and significantly more after tracheal intubation than skin incision. Ten and 19 patients moved in response to skin incision and tracheal intubation, respectively. None of the patients showed wakefulness.

CONCLUSION

It is concluded that 1MAC-incision sevoflurane secures unconsciousness during surgical skin incision and tracheal intubation.

Induction of anaesthesia with sevoflurane, preprogrammed propofol infusion or combined sevoflurane/propofol for laryngeal mask insertion: cardiovascular, movement and EEG bispectral index responses

Inhalation induction with sevoflurane was compared with propofol or sevoflurane/propofol in 60 unpremedicated adults. Target concentrations for the three groups (with 60% nitrous oxide) were 3% end-tidal sevoflurane, 12 mg/l propofol and 1.5% sevoflurane/6 mg/l propofol respectively, prior to insertion of a laryngeal mask airway (LMA) at 10 minutes. Induction of anaesthesia was satisfactory in each group, but movement response to LMA insertion was observed in 20 patients (least in the sevoflurane group). Cardiovascular responses were similar except for a lower heart rate in the sevoflurane group. EEG bispectral index suggested a greater depth of anaesthesia in the inhalation induction group. A bispectral index of 60 separated patients responding to LMA insertion from nonresponders (P = 0.006), and had a sensitivity of 68% and specificity 70%. Movement response was not predicted by cardiovascular changes.

Clinical usefulness of the bispectral index for titrating propofol target effect-site concentration

The bispectral index, a new processed electroencephalographic parameter which may give information on depth of anaesthesia, was used in 58 patients undergoing outpatient gynaecological surgery in order to study if the addition of bispectral index monitoring to standard clinical monitoring could improve the titration of target propofol concentration when using effect-site target-controlled propofol infusion for sedation. In Group 1 (n = 30), the bispectral index was recorded but the anaesthetist was unaware of the readings and therefore only classical signs of depth of anaesthesia were used to guide the anaesthetist in controlling the effect-site concentration. In Group 2 (n = 28), bispectral index readings were available to the anaesthetist and effect-site concentration was adjusted to ensure that bispectral index was maintained between 40 and 60. Similar propofol induction and maintenance doses, blood and effect-site concentrations and mean bispectral index were found in the two groups. A greater percentage of bispectral index readings lying outside the target range (i.e. < 40 or > 60) and more movement at incision and during maintenance were found in Group 1. There was a trend towards more implicit awareness in patients in Group 1. Bispectral index was found to be useful for measuring depth of sedation when using propofol target-controlled infusion. Propofol dosage could not be decreased but a more consistent level of sedation could be maintained due to a more satisfactory titration of target effect-site concentration.

Incidence of awareness in total i.v. anaesthesia based on propofol, alfentanil and neuromuscular blockade

BACKGROUND

There is no reliable technique for monitoring drug concentrations in total i.v. anaesthesia (TIVA) with muscle relaxation. An increased risk of awareness with TIVA has been stated as a possible drawback. The present study was conducted in order to assess the incidence of conscious awareness in TIVA based on propofol, alfentanil and neuromuscular blockade.

METHODS

One thousand patients anaesthetized with TIVA based on propofol, alfentanil, and neuromuscular blockade were subjected postoperatively to a structured interview for conscious awareness on two separate occasions: on discharge from the post-anaesthesia care unit, and the day after anaesthesia. Five hundred of these patients were also interviewed a third time, approximately 1 week later.

RESULTS

Two cases of awareness were detected (0.2%). One of these was identified immediately after extubation. The second patient had no memory of intraoperative events or dreams at the first interview, recalled a bad dream on the day after, and had explicit recall of intraoperative events at the interview 8 days later. In both cases, haemodynamic signs of inadequate anaesthesia were present. The incidence of conscious awareness in this study is similar to the lowest previously reported incidence of awareness following general anaesthesia in patients who had been paralyzed and thereafter subjected to a structured postanesthesia interview.

CONCLUSIONS

If the true incidence of conscious awareness is to be determined, interviews must be extended beyond the first postoperative day. Our study indicates that if appropriate dosing of propofol and alfentanil are adhered to, and proper action is taken in case of haemodynamic alterations suggestive of inadequate anaesthesia, the incidence of conscious awareness in non-cardiac TIVA with neuromuscular blockade is low.