Volume 57, Issue 6 p. 549-556
Free Access

Awareness during general anaesthesia: a review of 81 cases from the Anaesthetic Incident Monitoring Study

I. J. Bergman

I. J. Bergman

Department of Anaesthesia, Auckland Hospital, Auckland, New Zealand

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M. T. Kluger

M. T. Kluger

Department of Anaesthesia, North Shore Hospital, Private Bag 93-503, Takapuna, Auckland, New Zealand

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T. G. Short

T. G. Short

Department of Anaesthesia, Auckland Hospital, Auckland, New Zealand

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First published: 14 May 2002
Citations: 58
Correspondence to: Dr M. T. Kluger

Abstract

Summary Because of recent studies suggesting that awareness is still a major issue in anaesthetic practice, we reviewed 8372 incidents reported to the Anaesthetic Incident Monitoring Study. There were 81 cases in which peri-operative recall was consistent with awareness. There were 50 cases of definite awareness and 31 cases with a high probability of awareness. In 13 of the 81 incidents, the patients appeared to receive adequate doses of anaesthetic drugs. Where the cause could be determined, awareness was mainly due to drug error resulting in inadvertent paralysis of an awake patient (n = 32) and failure of delivery of volatile anaesthetic (n = 16). Less common causes included prolonged attempts at intubation of the trachea (n = 5), deliberate withdrawal of volatile anaesthetic (n = 4) or muscle relaxant apnoea with inadequate administration of hypnotic (n = 3). An objective central nervous system depth of anaesthesia monitor may have prevented 42 of these incidents and an improved drug administration system may have prevented 32. On the basis of these reports, we have developed guidelines that may have prevented the majority of these incidents.

The problem of awareness under anaesthesia is currently undergoing a re-evaluation because of recent studies suggesting that it is a relatively common event. A large Scandinavian prospective trial recently found a rate of awareness of 0.18% in patients in whom neuromuscular blockade was used, and 0.10% in patients who did not receive neuromuscular blockade [1]. Previous studies have reported an incidence of 0.2–0.4% [2]. Awareness has considerable potential for patient morbidity, including severe emotional distress and post-traumatic stress disorder [3]. It also has financial, professional and personal consequences for the anaesthetist [4]. Demand for ways of reducing the occurrence of awareness has led to the recent approval by the Federal Drug Administration in the USA of the Bispectral Index (BIS) monitor (Aspect Medical Systems Inc, Newton, MA, USA) which measures depth of anaesthesia [5]. Routine use of this monitor has significant resource implications.

The Anaesthetic Incident Monitoring Study (AIMS) is a well-described, internationally recognised reporting system that has been used for over 10 years for the collection and evaluation of anaesthetic-related incidents [6]. Awareness was reviewed in the 1993 symposium issue of the AIMS study that reported on the first 2000 incidents [7]. The objectives of this study were to assess the causes of awareness in the Australasian population based on the incidents reported to AIMS, and to suggest methods for reducing the occurrence of this problem.

Methods

The AIMS database is a voluntary anonymous reporting scheme in widespread use in Australasia, which reports incidents that affected or could have affected the safety of the patient while under anaesthetic care. Data were collected over the period 1988 to July 2001. The AIMS database, which has been reported previously, was searched under the keyword ‘awareness’[6]. This produced a data-sheet with all the information that was entered from the original AIMS form, as well as the narrative describing the incident.

Awareness was defined as postoperative recall of events occurring during general anaesthesia. Reports from the wards, the intensive care unit and postanaesthetic care unit were not analysed. All incidents were reviewed by two independent consultant anaesthetists, and initially separated into three categories:

  • 1

    definite awareness

  • 2

    high probability of awareness or high potential to cause awareness

  • 3

    no awareness of incident.

The definite awareness category required a narrative stating that the patient was aware, or a description of the awareness incident. The high probability of awareness or high potential to cause awareness category was used for those reports that did not meet the criteria for definite awareness, but did describe incidents that were related to awareness. There was either a history only suggestive of awareness at the post-operation interview or there was insufficient information in the narrative of the AIMS data-sheet to confirm that awareness had actually occurred. Incidents that had the potential to cause awareness if not corrected in time, and were coded as awareness by the reporting anaesthetist, were also included in this category. Where there was disagreement as to categorisation, all three authors discussed the report and an assignment was made according to the above criteria.

Each group was further subdivided into incidents with no obvious preventable cause, incidents with a clearly documented incident resulting in awareness and incidents caused by drug error. The reports were then entered into a Microsoft Excel spreadsheet. Fields entered included time of day, whether the surgery was elective or emergency and the operation undertaken. Also entered was the monitoring in use at the time of the incident and the factors contributing to the incident as recorded by the anaesthetist submitting the report. The ability of an anaesthesia depth monitor such as the BIS or auditory-evoked potentials to prevent the incident was also assessed [8–10]. This was a subjective assessment completed at the time of categorisation, using an assumption that the depth of anaesthesia monitor would have been completely effective at detecting inadequate depth of anaesthesia in sufficient time to allow additional anaesthetic agent to be administered. Also assessed was the ability of a drug administration tool capable of preventing administration of an incorrect drug to prevent awareness in those incidents in which a drug error occurred. This was done subjectively by reviewing each awareness incident and assuming that the drug administration tool was completely effective in preventing drug error.

Results

The AIMS database contained 8372 reports at the time of analysis, 116 of which were extracted using the keyword `awareness'. Of these, 35 were found to not describe an awareness event, or the narratives contained insufficient information to make a determination and were discarded.

Of the remaining 81 reports, there were 50 cases of definite awareness and 31 with a high probability of awareness. The 31 cases with a high probability of awareness have been combined with the definite awareness cases in the results below because it is equally relevant to develop strategies to prevent these cases occurring.

The total of 81 incidents was divided into:

  • 1

    incidents with no obvious cause

  • 2

    incidents due to low inspired volatile concentration or inadequate hypnosis

  • 3

    incidents due to drug error.

Incidents with no obvious cause (n = 13)

Thirteen incidents were found to have occurred with apparently normal conduct of anaesthesia and had no obvious cause (Table 1). Of these, 12 were of definite awareness and one was of high probability of awareness or high potential to cause awareness. Two of these 13 cases had end-tidal agent monitoring that confirmed that normally adequate concentrations of volatile anaesthetic had been given, but this did not prevent awareness. In four cases the awareness appeared to be due to inadequate doses of anaesthetic induction agent to prevent awareness occurring before a sufficient volatile concentration was reached. In these four cases, the narratives all describe recall at the beginning of the procedure including intubation of the trachea and positioning.

Table 1. Incidents involving awareness under general anaesthesia with no obvious cause.
Procedure Event
Cardio-thoracic surgery Felt chest discomfort
Cardiac surgery Felt sternotomy
Burn debridement Recall of eye taping, paralysis
Electroconvulsive therapy Recall of gag and electrode placement
Electroconvulsive therapy Recall of gag and electrode placement
Cervical spine manipulation Recall of manipulation
Nephrectomy in trauma patient Recall of intubation, talking
Dilatation and curettage Recall of lithotomy positioning, airway insertion, talking
Incisional hernia repair Recall of voices and incision
Bronchoscopy and bronchial dilatation Aware of entire procedure
Gynaecological Heard talking throughout procedure
Laparoscopy, dilatation and curettage Heard talking throughout procedure
Laparotomy Felt some of procedure

Incidents due to low inspired volatile concentration or inadequate hypnosis (n = 36)

Thirty-six cases had a definite incident that caused awareness (Table 2). Of these incidents, 22 were of definite awareness and 14 were of high probability of awareness or high potential to cause awareness. Five reports documented pain during the procedure. Sixteen cases involved a failure of volatile anaesthetic or nitrous oxide delivery due to equipment malfunction. Vaporiser problems were the most frequently reported (13/16) followed by breathing circuit problems (3/16). In 14 of these 16 cases no agent monitor was present, and in one further case the anaesthetist was unfamiliar with the agent monitoring system. Prolonged attempts at tracheal intubation contributed to five awareness incidents. Hypotension or cardiovascular instability necessitating a reduction in the inspired volatile concentration caused four awareness incidents. Less common causes included suxamethonium or mivacurium apnoea with inadequate hypnosis while awaiting return of muscle power (three cases), early cessation of volatile, or no volatile given deliberately (two), no thiopentone in fresh ampoule (one) and inadequate reversal (one).

Table 2. Incidents involving awareness under general anaesthesia due to low inspired volatile concentration or inadequate hypnosis.
Procedure Event Cause
Debridement of chest wall Recall of talking intra-operatively N2O turned off along with volatile after pleural puncture
Incision pilonidal abscess Bad dreams/paranoia post op Volatile turned off before end of case
Gynaecological Bad dream during anaesthesia Low inspired volatile concentration due to
vaporiser problems
Gynaecological Coughing and bucking N2O failure at induction
Gynaecological Patient movement Low inspired volatile concentration with
vaporiser not seated correctly
Rhinoplasty Patient movement Low inspired volatile concentration with
vaporiser not seated correctly
Gynaecological Probable awareness Post-operation laryngospasm requiring suxamethonium
Opthalmological Probable awareness Inadequate reversal/laryngospasm
Possible myasthenia gravis
Adrenalectomy Probable awareness Inadequate reversal leading to respiratory arrest
Partial hepatectomy Probable awareness Dilution of N2O, volatile
Elevation orbital fracture Probable awareness Low inspired volatile concentration with
vaporiser not seated properly
Swallowed acid, emergency
intubation
Probable awareness Light anaesthesia with failed intubation and
emergency tracheostomy
General surgery Probable awareness Fresh gas line disconnection
Drainage peri-anal abscess Probable awareness Suxamethonium apnoea
Dental/maxillo-facial Probable awareness Low inspired volatile concentration with
vaporiser filling port jammed
Cardioversion Recall of paralysis, voices Suxamethonium apnoea, inadequate hypnosis
while awaiting muscle power return
Laparotomy Felt some of procedure, paralysis Hypotensive/hypovolaemic, run deliberately light
Ophthalmological-unspecified Recall of intubation, talking Hypotensive, run deliberately light
Caesarean section Felt incision No volatile, 50/50 O2:N20
Laparotomy Recall of paralysis, voices Low inspired volatile concentration using circle
plus low flows
Laminectomy Recall of paralysis, voices Low inspired volatile concentration due
to circuit disconnection
Manual removal of placenta Recall of intubation Difficult intubation, second dose of suxamethonium
required
Laparotomy Awareness − unspecified Low inspired volatile concentration due to vaporiser
malpositioning
Ear, nose and throat-unspecified Recall of paralysis, voices Prolonged intubation leading to low initial volatile
concentrations
Open reduction and internal
fixation of femur
Awareness − unspecified Low inspired volatile concentration due to vaporiser
missing stop-cock
Laparotomy Felt some of procedure, paralysis Low inspired volatile concentration due to vaporiser
malfunction
Caesarean Section Recall of intubation No thiopentone in fresh ampoule
Excision lip carcinoma Recall of local anaesthetic injection Hypotensive, run deliberately light
Laparoscopic fundoplication Aware at beginning of case Graseby pump failure leading to no total intravenous
infusion anaesthesia infusion
Laparoscopic cholecystectomy Awareness −unspecified Low inspired volatile concentration due to vaporiser leak
Laparoscopic sterilisation Awareness − unspecified with
psychological sequalae
Mivacurium apnoea from cholinesterase deficiency,
inadequate hypnosis while awaiting muscle power return
Skin grafting Aware of insertion of cannula,
mask ventilation
IV line tissued after suxamethonium given, with propofol
in subcutaneous tissues
Carotid endarterectomy Awareness − unspecified with
moderate pain
IV line tissued during total intravenous infusion
anaesthesia with propofol/remifentanil
Appendicectomy Could see people but paralysed No volatile while tracheal tube position checked
Not specified Awareness − unspecified Prolonged intubation leading to low initial volatile
concentrations
Laparoscopy Aware of entire procedure Unspecified vaporiser failure

Incidents due to drug error (n = 32)

There were 32 cases of inadvertent paralysis of an awake patient (Table 3). Of these incidents, 16 were of definite awareness and 16 were of high probability of awareness or high potential to cause awareness. At least two of the patients had their trachea intubated awake before the error was recognised. In the remainder, the error was apparently recognised because the patients were given a hypnotic after commencing bag-mask ventilation.

Table 3. Inadvertent paralysis.
Procedure Event Cause
Orthopaedic, patient with spinal Paralysis before induction with awareness Suxamethonium given instead of antibiotic
Percutaneous nephrolithotomy Paralysed before induction Atracurium infusion started prematurely
General surgical-unspecified Paralysed before induction Suxamethonium given instead of fentanyl
Ear, nose and throat Paralysis before induction with awareness Suxamethonium given instead of fentanyl
Incision of peri-anal abscess Paralysis before induction with awareness Suxamethonium given instead of fentanyl
Gynaecological Paralysed during case Suxamethonium left in injection port
Gynaecological Paralysed before induction Suxamethonium given instead of fentanyl
Manipulation fractured nose Paralysis before induction with awareness Suxamethonium given instead of fentanyl
Appendicectomy Paralysed before induction Suxamethonium given instead of pethidine
Incision umbilical abscess Paralysed before induction Suxamethonium given instead of narcotic
Urological Paralysed before induction Suxamethonium given instead of atropine
Colonoscopy Paralysed before induction Suxamethonium given instead of fentanyl
Electroconvulsive therapy Paralysis before induction with awareness Suxamethonium given instead of methohexitone
Repair crushed finger Paralysed during case Atracurium given instead of midazolam
Urological Paralysis before induction with awareness Cefuroxime given instead of thiopentone
General surgery Paralysed before induction Suxamethonium given instead of morphine
Caesarean section Paralysis before induction with awareness Suxamethonium given instead of syntocinon
Gynaecological Paralysed before induction Suxamethonium given instead of fentanyl
Caesarean section Paralysis before induction Suxamethonium given instead of metoclopramide
Orthopaedic Paralysis before induction Suxamethonium given instead of fentanyl
Achilles tendon repair Paralysis before induction Suxamethonium given instead of midazolam
General surgery Paralysis before induction with awareness Rocuronium given instead of midazolam
Appendicectomy Paralysis before induction Rocuronium given instead of midazolam
Basal cell carcinoma excision Paralysis before induction Atracurium given instead of midazolam
Open reduction and internal
fixation of ankle
Paralysis before induction with awareness,
including intubation
Suxamethonium given instead of fentanyl
Endoscopy Paralysis before induction with awareness,
including intubation
Rocuronium given instead of midazolam
Open reduction and internal
 fixation of hand fracture
Paralysis before induction with awareness Rocuronium given instead of midazolam
Axillary node dissection Paralysis before induction Rocuronium given instead of midazolam
Vaginal hysterectomy Paralysis before induction Suxamethonium given instead of fentanyl
Gynaecological Paralysis before induction Vecuronium given instead of fentanyl
Trans-cervical polyp resection Paralysis before induction Suxamethonium given instead of midazolam
General surgery Paralysis before induction Suxamethonium given instead of metoclopramide

When factors contributing to the incident for the drug error group were reviewed, inattention or distraction contributed to the incident in 20 cases, haste in 14 cases and fatigue in five cases. In the incidents due to low inspired volatile concentration or inadequate hypnosis, inattention or distraction contributed to the incident in four cases, haste in six cases and fatigue in only one case. More commonly cited in this group were failure to check equipment (nine cases), inadequate pre-operative assessment (six) or preparation (three), inexperience (seven), an error in judgement (six) or a fault of technique (five).

Anaesthesia occurred between 0800 and 1700 in 78% (58/74, seven not specified) of the cases. The procedure was classed as an emergency in 25 cases (31%).

When preventability was assessed, it appeared that objective central nervous system monitoring (e.g. BIS or auditory-evoked potentials) may have prevented the incident of awareness in 42 of 81 cases. If a drug administration tool capable of detecting administration of an incorrect drug was used, awareness may have been prevented in 32 of 81 cases.

Discussion

The largest group of incidents was of drug error resulting in inadvertent paralysis of an awake patient. This rate of 32 from the total of 8372 incidents (0.38%) is similar to that of Osborne et al. [7] from their previous study of 2000 incidents from the AIMS database (0.30%). This suggests that this potentially avoidable complication is not being reported less often, and it therefore may be not decreasing in frequency. Most of the errors involved swapping syringes when two drugs were drawn up in the same size syringe. This suggests that a more rigorous checking of syringes before injection would limit this error. Suxamethonium is commonly drawn up as an emergency drug prior to an anaesthetic being given. Stopping this practice may reduce this error. Some areas in Australia utilise a syringe with a red plastic plunger for neuromuscular blocking drugs, but two narratives state that they were not using this even though it was available. Many recommendations for reducing drug errors have been suggested including checking by two people, prefilled syringes, bar-coding of syringes and automated delivery systems [11–13]. Recently, Merry et al. [14] reported a system with colour- and bar-coded labelling of syringes, along with automatic visual and auditory confirmation of contents. In a recent closed claims analysis from the USA there were 18 claims for awake paralysis, with a median payment of US $9500, suggesting that the financial implications are not trivial [4]. The ways in which the anaesthetist reported dealing with this complication differed widely. At least two anaesthetists told the patient that the drugs had worked more quickly than expected, rather than admit to their error. Although that may be understandable to avoid medico-legal consequences, it could also cause increased problems. The suggested way of dealing with an awareness incident has been reviewed by Ghonheim [15], and includes full disclosure, an explanation of events with an apology and early referral to a liaison psychiatry service.

The group of patients who had awareness despite the apparently normal conduct of anaesthesia is of particular interest and concern to the practising anaesthetist. It is possible that these incidents are due to an unrecognised failure to give what was intended, as often the awareness does not become obvious until the ability to check the drugs given or any monitor memory has passed. It is also possible that these represent a group who were given what was intended but require a higher concentration of hypnotic to prevent awareness. Two of the 13 cases had end-tidal agent monitoring which confirmed that an apparently adequate volatile concentration had been given, but this did not prevent awareness. Both were paralysed and in neither incident did the patient display any signs of sympathetic stimulation (tachycardia, hypertension, lacrimation, sweating); these are unreliable markers of awareness [16]. The electroconvulsive therapy (ECT) cases appear to come from the same operating theatre list, which suggests a problem with the induction agent. In procedures such as ECT or manipulations, often no volatile agent is given and end-tidal monitoring is of no benefit in preventing awareness.

In those events with a known cause, the most common was that of accidental delivery of insufficient volatile anaesthetic agent due to vaporiser or circuit problems. An earlier review of the AIMS database concluded that equipment failure could be minimised by meticulous adherence to equipment checking guidelines [17]. These incidents often occur after the beginning of the day, implying that the development of a quick machine check to be performed before every case would be useful in reducing this problem. The requirement for end-tidal agent monitoring in some countries should reduce the occurrence of awareness, but this monitor only works if used correctly. This involves frequent checking of the end-tidal volatile concentration and appropriate use of the low anaesthetic volatile alarm. Of note, the alarm for end-tidal volatile agents usually defaults to 0%. It is interesting to note that the Australian and New Zealand College of Anaesthetists and the Association of Anaesthetists of Great Britain and Ireland minimum monitoring standards now include end-tidal agent monitoring, whereas those of the Royal College of Anaesthetists and the American Society of Anaesthetists do not [18–21].

Awareness incidents involving prolonged attempts at intubation of the trachea (5/49) were common. Two reports commented on how the addition of a premedication or co-induction with midazolam when confronted with a possible difficult airway may have attenuated the awareness. There is no evidence from controlled trials to support this proposition although it is commonly cited as a means of preventing awareness [15]. Awareness has occurred in patients given benzodiazepines [1]. There is no substitute for thorough assessment of the airway pre-operatively to prevent an unexpected difficult intubation but, when repeated failed intubation attempts occur, consideration of providing an additional dose of hypnotic is warranted. A frequent event in anaesthesia is the trade off between hypnotic concentration and blood pressure in severe hypotension. It is common teaching for the volatile to be reduced or even turned off in these situations. Bogetz & Katz [22] found an incidence of awareness of 11% in trauma patients given minimal hypnosis and 43% in those who were given none for periods of up to 20 min. There are few guidelines on the ways to avoid awareness in these situations. It may be argued that these awareness incidents are unavoidable as, even if a depth of anaesthesia monitor detects the awareness, administration of a hypnotic may prove to be too hazardous.

Cholinesterase deficiency is rare, but this condition was associated with three awareness incidents in this series. In all three cases, a nerve stimulator was not used until the anaesthetist was alerted by the patient's failure to breathe despite low hypnotic concentrations. The objective assessment of neuromuscular blockade should be used whenever paralysis has occurred, and certainly hypnosis should be maintained until resumption of spontaneous ventilation or definitive demonstration of return of normal neuromuscular function using a peripheral nerve stimulator.

There is currently no evidence that use of the BIS monitor to titrate hypnotic requirements will prevent awareness under anaesthesia. A large multicentre trial is currently underway in Australasia to investigate the ability of this monitor to actually prevent awareness. Assuming the BIS monitor is proven to be effective, then 42 of the 81 awareness incidents may have been prevented. BIS would be ineffective in the cases of inadvertent paralysis of an awake patient. Awareness monitoring in association with end-tidal agent monitoring could provide a method of assessing sufficient depth of anaesthesia, a situation analogous to the two-tier system provided by the pulse oximeter and the inspired oxygen monitor.

Although not included in this study, review of the data-sheets also detailed two incidents of paralysis of patients on the ward postoperatively resulting in awareness. This occurred because of suxamethonium left in injection ports. As much as 0.3 ml (15 mg) can remain in an intravenous cannula port, and this represents a paralysing dose. These ports should be flushed after giving a muscle relaxant.

In a previous study of the first 2000 incidents [7], and as part of a complete review [6], each incident in the entire database was studied for possible awareness data. In this analysis, only data-sheets with the keyword ‘awareness’ were used, so the data set is different. It is possible that there are several awareness incidents in the database that have been coded incorrectly, and these would not be found by this search strategy. Most of the awareness incidents from the review of the first 2000 cases analysed previously were included in our study as it was not possible to accurately separate these reports out. As AIMS reporting is voluntary, and has a considerable variation in reporting rates within Australasia, it not possible to estimate a prevalence rate for awareness from this data. However, if the rate in Australasia is similar to that reported by Sandin et al. [1] these incidents represent only a small fraction of the total number.

As with any voluntary, narrative-driven incident reporting system, there is variability in data due to nonuniform reporting, incomplete follow-up of patients and subjective interpretation of the data-sheets. In a number of cases it was unclear whether awareness had actually occurred, or that the incident simply had the potential to cause awareness if not corrected. This latter problem was evident for incidents in the ‘high probability of awareness’ category, in which the report was submitted because the incident had the potential to cause awareness and it was not clear whether awareness actually occurred. This was due to a discrepancy between the narrative from the AIMS form and the outcome box that was coded as awareness. For some incidents, the only data suggesting awareness were in the ‘Patient Outcome’ tick-box of the AIMS form. Some of the discarded forms that had the ‘Patient Outcome’ box ticked as awareness clearly described an incident completely unrelated to this outcome. This suggests that some clarification is required in the form to make this category well defined. Despite these limitations, our study represents a large series of awareness incidents, and useful information can be gained from analysing these reports.

Previously, awareness has possibly been perceived as a less important problem by anaesthetists when compared with other problems involving more serious morbidity and mortality. However, it is a problem that patients frequently worry about. Few practising anaesthetists will not have had to reassure patients they will not be awake during their surgery. Although there can be significant morbidity to the patient, incidents of awareness also provide a source of stress for the anaesthetist. Some of the narratives certainly convey the considerable distress the incident has caused the practitioners as well as the patients.

The major reason for analysing a critical incident reporting database such as AIMS is to provide strategies for prevention. We have produced a set of guidelines that may have prevented 68 of the 81 incidents, assuming they were completely effective (Table 4). The 13 incidents in which the cause of awareness was not confirmed could not be tested against the guidelines. The machine check prior to use and after any changes to the machine is an integral part of anaesthetic practice, and would reduce or prevent the number of incidents with failure to deliver the intended volatile anaesthetic. An end-tidal agent monitor whose alarm was set with a concentration sufficient to prevent awareness would further prevent this failure. This could be confirmed by depth of anaesthesia monitoring, as well as helping to prevent awareness in other situations such as trauma, in which low concentrations of volatile anaesthetic are purposefully given. Drug error would be reduced by double-checking syringe contents that were labelled immediately upon drawing up. There are numerous more complex methods of avoiding drug error. A patent intravenous line is essential for total intravenous anaesthesia. When using a syringe pump, it is vital to check that the syringe plunger is advancing. Anaesthetists should have a low threshold for a further dose of hypnotic with prolonged attempts at intubation. They should also check that sufficient muscular power has returned before allowing patients to regain consciousness by using a nerve stimulator routinely.

Table 4. Suggestions for reducing the incidence of awareness.
Guidelines
1. Check machine before each use, ensuring a correctly mounted vaporiser.
2. Use an end-tidal agent monitor, with the low alarm set for a sufficient volatile concentration to prevent awareness.
3. Provide a further dose of hypnotic for repeated intubation attempts.
4. Be aware of the potential for awareness in hypovolaemic patients with low concentrations of hypnotic. Introduce these as soon as is practical.
5. Routinely use a peripheral nerve stimulator, and ensure sufficient hypnosis until muscle power returns.
6. When using total intravenous anaesthesia, ensure a patent intravenous line and periodically check the volume in the syringe to ensure the
barrel is advancing.
7. Clearly label all drug syringes immediately they are drawn up. Check this label carefully, and do not rely on recognition of syringe size to
confirm its contents. Consider other methods of ensuring correct drug given.
8. Consider use of depth of anaesthesia monitor, if not routinely, then for selected cases.

The main finding of this study is that awareness continues to be a problem during general anaesthesia. Reports that indicated there was no discernible cause for the awareness incident, along with those reports that suggested there was an inadequate titration of anaesthetic agents support the development of a valid, reliable monitor of anaesthetic depth.

In summary, this series of 81 incidents suggests that significant numbers of preventable incidents do occur. This study found only that 13 incidents had no obvious cause. Inadvertent paralysis of an awake patient was the most common cause of an awareness incident, followed by vaporiser malfunction. A number of measures to prevent awareness have been discussed, and these measures could have prevented the vast majority of these incidents.

Acknowledgements

The authors would like to thank Monika Bullock of the Australian Patient Safety Foundation for her work in extracting the data for this study. Professor Alan Merry provided valuable advice while reviewing the document. We would also like to thank the anaesthetists who took the time to submit a report to the AIMS database.