BIS (Controversies)

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Ask the Experts

Discussion with Michael Avidan and George Mashour: intraoperative awareness, detection, prevention and consequences



Whether or not BIS is indicated and in what populations.


Intraoperative awareness is thought to occur in as many as 1:500 anesthetics (although Avidan’s study put the incidence of definite awareness at 0.2%), thus the BIS was developed as an additional tool – values from 40-60 are generally considered optimal.

Arguments in Favor of BIS:

a Cochrane review concluded that BIS within 40 to 60 may improve anaesthetic delivery and postoperative recovery from relatively deep anaesthesia, and more important that BIS-guided anaesthesia significantly reduces the incidence of intraoperative recall in surgical patients with high risk of awareness (ex. TIVA) [Punjasawadwong et al. Cochrane Database Syst Rev. 4: CD003843, 2007]. The largest study in Cochrane was Myles et al. (2463 patients), the second-largest was only 268 patients. Cochrane did not review Avidan (see below), which was published later.

Myles et al. conducted a randomized, double blind trial of 2463 “high risk” patients (caesarean section, high-risk cardiac surgery [EF <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 bronchoscopy, significant impairment of cardiovascular status and expected intraoperative hypotension requiring treatment, severe end-stage lung disease, past history of awareness, anticipated difficult intubation where an awake intubation technique was not planned, known or suspected heavy alcohol intake, chronic benzodiazepine or opioid use, or current protease inhibitor therapy, found two reports of awareness in the BIS-guided group (goal 40-60) and 11 reports in the routine care group (p=0.022), i.e., BIS-guided anaesthesia reduced the risk of awareness by 82% (95% CI 17-98%), although there was no overall difference in MAC [Myles et al.].

Arguments Against BIS:

Avidan’s study of 1941 patients showed no difference in awareness or use of volatile gas (note, however, that no patient had definite or probable awareness if BIS was kept under 50 and MAC > 1.0, suggesting that a combination of BIS < 50 and MAC > 1.0 may be a better threshold) [Avidan et al.]. Avidan’s study only included “high risk” patients (1 major or 2 minor criteria) – the major criteria were preoperative use of anticonvulsant agents, opiates, benzodiazepines, or cocaine, EF < 40%, history of awareness, history of difficult intubation or anticipated difficult intubation; ASA IV or V, aortic stenosis, end-stage lung disease, marginal exercise tolerance, pulmonary hypertension, planned open-heart surgery; and daily alcohol consumption, and the minor criteria were preoperative use of B-blockers, COPD, moderate exercise tolerance, smoking two or more packs of cigarettes per day, and BMI > 30. This study was grossly underpowered (assumed a 1% incidence, which was 5-fold higher than that found, as well as a reduction of 90%, which is very generous).

A follow-up, and even larger study by Avidan et al. showed nearly identical results – no difference in awareness between BIS-guided and ETAG guided anesthetic management in over 6000 high risk patients [Avidan MS et al. N Engl J Med 365: 591, 2011].