BIS Off? No need for that…

Avidan MS, Jacobsohn E, Glick D et al. A. Prevention of intraoperative awareness in a high-risk surgical population. New England Journal of Medicine, 2011; 365: 591-600.

RHH Journal Club. July 4th, 2013. Dr Helen Findley

Full-text article (if available)

Is keeping BIS 40-60 better at preventing awareness than keeping end tidal at minimum 0.7 age adjusted MAC?

Study design:

  • Three centres (University Hospitals in St Louis, Chicago, Manitoba, USA)
  • Patient and evaluator blinded. Randomised Controlled Trial.
  • Over 6000 patients



  • Adults. Elective surgery. Must have at least one risk factor for awareness.
  • Inhalational anaesthesia with iso/sevo or desflurane. Not TIVA.


  • Block randomization to BIS 40-60 (BIS) or age-adjusted MAC 0.7-1.3 (ETAC)
  • Alarms if fell outside of BIS or MAC limits
  • End tidal anaesthetic and BIS monitor visible in BIS group, only ET anaesthetic in ETAC group
  • Awareness assessed using modified Brice questionnaire at 72 hours and 30 days
  • Likelihood of awareness determined independently by 3 expert reviewers (definite, possible or not)

Primary outcome:

Incidence of intraoperative awareness

Powered to detect a 0.4% reduction in incidence of DEFINITE awareness with BIS protocol compared with ETAC protocol (ie reduce incidence from 0.5% in ETAC to 0.1% in BIS group)

87% power at a significance level of 0.05 (one tailed test) with 6000 patients

Secondary outcomes:

Reduction in definite or possible awareness in BIS group

Lower incidence of awareness with distress


  • Did groups start with a similar prognosis?


  • Numbers involved: 2861 in BIS group, 2852 in ETAC group (after exclusions and losses to follow up)
  • Randomisation  – Yes, blocks of 100. Ensured equality in groups.
  • Blinded

Investigators/evaluators  – yes

Patients – yes

  • Intention to treat principle applied.
  • Confounding factors. Midazolam administered in 80% of patients in both groups. No important differences in sedative, hypnotic, opioid or neuromuscular blockers in each group, although no details given.
  • Follow up: 46 lost to follow up in BIS group, 50 lost in ETAC group


Primary outcome:

  • A BIS guided protocol was NOT associated with lower incidence of awareness in high risk patients than a MAC guided protocol. 7 patients in BIS group were aware compared to 2 in ETAC group.

Secondary outcomes:

  • The incidence of definite or possible awareness was also higher in the BIS group compared to ETAC group,  (19 patients vs 8 patients), as was the incidence of awareness with distress (8 patients vs 1 patient)
  • ETAC protocol not associated with increase in post-op mortality or amount of anaesthetic used.
  • Incidence of definite awareness was 0.16%, definite or possible awareness was 0.47%.

Take home message/application of this research:

  • For inhalational anaesthesia BIS is no better for preventing awareness than keeping age adjusted MAC between 0.7 and 1.3. Backs up B-Unaware trial (Avidan 2008)
  • Brief educational interventions and use of the protocols to guide anaesthetic depth may have reduced the incidence of awareness (as incidence was lower than anticipated).



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