ArticlesBispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial
Introduction
Anaesthesia can be defined as a state of drug-induced unconsciousness in which the patient neither perceives nor recalls noxious stimulation.1 Awareness is the postoperative recollection of events occurring during general anaesthesia. The incidence of awareness is 0·1–0·2% in the general surgical population, 2, 3 but is greater during cardiac surgery, caesarean section, and trauma surgery.4, 6
Awareness is a distressing complication of anaesthesia.7, 8, 9, 10 Affected patients report perception of paralysis, conversations, and surgical manipulations, accompanied by feelings of helplessness, fear, and pain. Some patients have rated it as their worst hospital experience;6 post-traumatic stress disorder can develop in those who are severely affected.8, 9, 10 However, despite numerous attempts over more than 150 years, the definitive monitor for predicting awareness has not been established.11
Clinical signs, such as blood pressure and heart rate, are routinely used by anaesthetists to monitor anaesthetic depth, but such methods are unreliable.5 Early attempts to monitor anaesthetic depth using the spontaneous11, 12, 13, 14 or evoked 15, 16 electroencephalograph were also unsuccessful. The bispectral index (BIS) is a monitor of anaesthetic depth approved by the Food and Drug Administration in the USA. BIS incorporates time-domain, frequency-domain, and bispectral analysis of the electroencephalograph, and is displayed as a dimensionless number between 0 (deep anaesthesia) and 100 (awake),17 with 40–60 being suitable for surgical anaesthesia.18 BIS correlates well with hypnotic state and anaesthetic drug concentration,19, 20, 21 and use of BIS can shorten recovery times.18, 22 However, the predictive value of BIS as a monitor for awareness has not previously been assessed in a randomised trial.11, 23, 24 We therefore did a large trial to assess whether BIS monitoring decreases the incidence of awareness during relaxant general anaesthesia in routine surgical patients at high risk of awareness.
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Study population
Surgical patients undergoing relaxant general anaesthesia at one of the participating centres (listed at the end of the paper) were eligible if they were aged 18 years or older, and had at least one of these risk factors for awareness: caesarean section, high-risk cardiac surgery (eg, ejection fraction <30%, cardiac index <2·1 L/min per m2, severe aortic stenosis, pulmonary hypertension, or undergoing off-pump coronary artery bypass graft surgery), acute trauma with hypovolaemia, rigid
Results
2503 patients were recruited into the trial, but 40 patients were withdrawn because of cancellation of surgery (BIS group 13, routine care group 13), withdrawal of consent (six, two), surgery done without general anaesthesia (four, none), or the patient was under-age (none, two), leaving 2463 patients in the final data set: 1225 patients (49·7%) randomised to the BIS group and 1238 patients (50·3%) to the routine care group (figure 1). Six patients in the routine care group received BIS
Discussion
In this study, BIS monitoring reduced the risk of awareness by 82% in at-risk adults undergoing relaxant general anaesthesia. BIS monitoring had little effect on the time needed to recover from general anaesthesia, as measured by eye opening, and no measurable effect on the risks of postoperative complications. Our findings confirm previous observational data suggesting that awareness during BIS monitoring is less common than during routine care.29 Some patients could not be interviewed at each
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Participatin members listed at end of paper