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Chilling accounts of intraoperative awareness abound in the medical literature and lay press.^3,8,9 Patients who have experienced awareness during anaesthesia report the perception of paralysis, conversations, and surgical manipulations, accompanied by feelings of helplessness, fear and pain. While patients usually recognise the event as real, few are willing to report the experience to their anaesthetist for fear of being disbelieved or ridiculed. Post-traumatic stress disorder may develop as a devastating sequel of awareness.⁹

Why does awareness occur? Anaesthetic requirement is a balance between the amount of anaesthetic administered and the state of arousal of the patient. During any operation, the intensity of stimulation varies markedly, with the most potent noxious stimulus, endotracheal intubation, occurring at the beginning of the procedure. At the same time, the haemodynamic effects of the anaesthetic drugs may limit the amount that can be safely given. Thus, critical imbalances between anaesthetic requirement and delivery may occur. Marked interindividual variation in anaesthetic requirement, the use of muscle relaxants, and lack of a proven monitor for awareness compound the problem. In addition, awareness may occur as a result of anaesthetist error or technical mishaps.¹⁰

In an era of sophisticated intraoperative monitoring, it may surprise many non-anaesthetists (and our patients) that we are unable to guarantee loss of consciousness during surgery. A definitive monitor for awareness has been described as the "Holy Grail" of anaesthesia.¹¹

Clinical signs of somatic or autonomic responsiveness have always been the mainstay of anaesthetic depth monitoring, but they lack proven utility in detecting awareness.⁵ Other techniques (the isolated forearm technique,¹² frontalis electromyogram¹³ and lower-oesophageal contractility¹⁴) have similarly been unreliable. Promising technologies such as auditory evoked potential¹⁵ and heart-rate variability¹⁶ monitoring await wide availability and acceptance into clinical practice.

Many attempts have been made to produce a simplified interpretation of the electroencephalograph (EEG) that predicts anaesthetic depth,¹¹ but, in most cases, these were unsatisfactory.¹⁷ Recently, sophisticated pattern recognition systems that assess multiple features of the EEG have been developed. One such monitor, the bispectral index (BIS) (Aspect Medical Systems Inc, MA, USA), displays a single number derived from bispectral analysis of the EEG. The BIS ranges from 0 to 100, values below 60 being associated with unconsciousness. It has been shown to be a reliable indicator of level of consciousness^18,19 and to improve operating room utilisation and reduce costs (by allowing faster patient turnover and reducing the use of drugs).¹⁸

Is BIS monitoring a reliable method of detecting and preventing awareness during anaesthesia? According to Aspect Medical Systems' product information, more than 1.2 million patients have been monitored with BIS and only 41 have reported awareness. Many of those experiencing awareness recorded a BIS value of greater than 65. This low reporting rate either represents an underestimate of the true incidence of awareness or reflects the effectiveness of BIS monitoring in preventing awareness.

A suitably designed randomised trial could help to answer this clinically important question,²⁰ although it has been argued that the low incidence of awareness under anaesthesia would necessitate a prohibitively large trial (about 50 000 patients).^7,21 However, if a high-risk group could be identified, adequate power could be achieved with a much smaller sample size.²⁰ Obstetric, cardiac and trauma patients are among those who are more likely to report awareness during anaesthesia.² A study of 2300 patients in this group would be sufficient to reliably detect a decrease in incidence of awareness from 1% to 0.1% resulting from a more effective monitor (a = 0.05; b = 0.2). This large treatment effect is realistic for two reasons: (i) the acceptance of a monitor into routine anaesthetic practice would require a convincing demonstration of benefit; and (ii) the rate of awareness during BIS monitoring is thought to be extremely low. We are currently undertaking such a trial in Australia, New Zealand and Hong Kong (for further details, see our website at <http://www.b-aware-trial.org>).

While widespread use of BIS would certainly incur considerable cost for acquisition and ongoing expenses, this should be balanced against the ability of BIS monitoring to improve anaesthetic drug titration (thereby decreasing drug and recovery-room costs¹⁸), the potential to prevent costly litigation, and the possibility of reducing the level of patient anxiety about awareness (up to 54% of patients due to undergo surgery are concerned about awareness⁶). We believe that if a monitor was proven to decrease the incidence of awareness in an appropriately designed and conducted trial, the costs would be justified. Our patients expect nothing less.

Kate Leslie
Anaesthetist, Royal Melbourne Hospital, Melbourne, VIC
Honorary Senior Fellow, Department of Pharmacology, University of Melbourne
kate.leslieATmh.org.au

Paul S Myles
Head of Anaesthesia Research
Alfred Hospital, Melbourne, VIC
Associate Professor
Department of Epidemiology and Preventative Medicine, Monash University

Competing interests: Aspect Medical Systems Inc have provided some funding for a multicentre awareness monitoring trial (the B-Aware Trial), designed and independently conducted by us.

1. Dowd M, Cheng D, Karski J, et al. Intraoperative awareness in fast-track cardiac anaesthesia. Anesthesiology 1998; 89: 1068-1073.
2. Liu W, Thorp T, Graham S, et al. Incidence of awareness with recall during general anaesthesia. Anaesthesia 1991; 46: 435-437.
3. Lyons G, Macdonald R. Awareness during Caesarean section. Anaesthesia 1991; 46: 62-64.
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15. Thornton C, Konieczko K, Jones JG, et al. Effect of surgical stimulation on the auditory evoked response. Br J Anaesth 1988; 60: 372-378.
16. Sleigh J, Donovan J. Comparison of bispectral index, 95% spectral edge frequency and approximate entropy of the EEG, with changes in heart rate variability during induction of general anaesthesia. Br J Anaesth 1999; 82: 666-671.
17. Sigl JC, Chamoun NG. An introduction to bispectral analysis for the electroencephalogram. J Clin Monit 1994; 10: 392-404.
18. Gan T, Glass P, Windsor A, et al. Bispectral Index monitoring allows faster emergence and improved recovery from propofol, alfentanil, and nitrous oxide anesthesia. Anesthesiology 1997; 87: 808-815.
19. Leslie K, Sessler DI, Schroeder M, et al. Propofol blood concentration and the Bispectral Index predict suppression of learning during propofol/epidural anesthesia in volunteers. Anesth Analg 1995; 81: 1269-1274.
20. Myles P. Why we need large randomized studies in anaesthesia. Br J Anaesth 1999; 83: 833-834.
21. Simini B. Awareness of awareness during general anaesthesia. Lancet 2000; 355: 672-674.

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