Does Brain Monitoring Prevent Anesthesia Awareness?

Studies suggest that one or two people in every thousand who undergo general anesthesia experience awareness during the procedure, maybe more in children. The Joint Commission, which inspects hospitals in the US, has made awareness during general anesthesia a “sentinel event.” (If you’re a hospital, a practitioner, or a patient, you definitely want to avoid these). The movie “Awake” and various TV programs have dramatized the issue of awareness.

The victims of awareness tell us the experience can be excruciating. Fortunately, most individuals do not experience physical pain but there is the possibility of later developing debilitating post-traumatic stress disorder. (A study of seven children confirmed to have had awareness found none of them had PTSD or any other troubling psychological problems a year later. This does not establish that PTSD does not occur in children who have had awareness but does suggest that long term problems, at least in children, are far from inevitable).

Can awareness be prevented with brain monitoring technology? An important new NEJM study looks at this.

EEG image (Wikipedia commons)A brain monitor could be used to prevent awareness – the under-dosing problem of which we are so fearful. It could also be used to fine tune the amount of anesthesia so that relative “overdosing” is avoided. Overdosing is costly (increased anesthetic drug costs, increased recovery time) and might even be bad for the brain (postoperative cognitive dysfunction).

You can understand therefore why the quest for reliable brain monitoring of consciousness is “holy grail” material for anesthesiology. The quest is difficult because our understanding of where and how anesthetics act is still evolving.

The arrival of a commercial, reasonably affordable, easy-to-use processed EEG monitor about 10 years ago was therefore greeted with great enthusiasm in the anesthesia community. The BIS monitor, made by Aspect, instantly achieved popularity in hospitals around the US after its approval by the FDA. Aspect became a NASDAQ listed company quite soon after. (A quick look at the company’s website reveals fluctuating fortunes, if the stock price is anything to go by). The scientific validity of the BIS monitor has been addressed in many studies, but questions remain. Most important, “does BIS monitoring really prevent anesthesia awareness”. A secondary question: “does BIS monitoring save money?”

A major cause for irritation in the scientific community is the black-box nature of the algorithms in the BIS monitor which the company must protect, to avoid the emergence of competitive low-cost clones of the BIS monitor. (Aspect also developed ingenious, proprietary adhesive electrodes which must be used with the monitor).

A recent NEJM article asked these questions. Not many anesthesiology studies get in the NEJM so this one is automatically worth a look at. The study was independently funded (not by Aspect).

The Study

  • Half the patients were monitored with BIS.
  • In all patients the anesthetic gas concentrations were monitored. Anesthetic gas monitoring is a standard, and both BIS and non-BIS study groups had this available. All modern anesthesia monitors measure the concentration of gas delivered and expired. It is the expired gas that reflects the amount in the brain.
  • Anesthesiologists were instructed to attempt to keep the BIS level below 60 (out of 100) to avoid awareness. The non-BIS patients were to be kept within a specific monitored anesthetic gas range.

Results of the Study

  • The number of patients who had awareness was about the number expected (two per thousand).
    • The same number of people (two) had awareness in both groups of roughly 1,000 patients.
  • There was no difference in the amount of anesthetic agent used.
  • The recommended dose of anesthetic was maintained for 45% of patients in the BIS group but for only 26% of those in the vapor-monitoring group.


A central question in a scientific study of a rare phenomenon like awareness is whether there are enough study subjects to detect a “statistically significant” difference between the study groups. In this case, the study was powered to detect about a 50% difference in the incidence of awareness with 95% confidence. So a difference of, let’s say 30% either way in the incidence of awareness would not likely be detected. To do this you would need even more patients in the study, and at almost 2,000 subjects this study was already a large one.

The difference in the time spent out of the recommended range of anesthetic concentration is interesting. It tells us that anesthesiologists, even when asked explicitly to keep the anesthetic in a dose range, use other over-riding criteria to decide how much to give. The factors that affect those concentrations, and their effect on the brain, include age, state of health and organ function, genetic determinants and others. It is not clear from the study description whether the recommended anesthetic dose range was calculated to take any of these factors into account.

Intravenous anesthesia, not involving gas agents, has become more popular. Real-time measurements of intravenous drug concentrations are not available and an educated guess must be made based on the patient’s responses (movement, blood pressure, heart rate) professional intuition/experience and/or machine calculated algorithms. If your anesthesia is based on intravenous drugs, which may in fact be more likely to result in awareness, and which cannot be directly measured with current commercial monitors, a BIS monitor may be more useful. This has not been proven in a large randomised controlled trial like the one above.

There are other circumstances where BIS monitoring may be valid and useful. These include cases where the risk of awareness is much higher and/or the risk of overdose is greater, such as patients with poor heart function. In these cases, the gas concentrations may be low, creating a fear of possible awareness. If the BIS number is okay it becomes this assists the anesthesiologist in deciding whether to provide more anesthesia at the risk of further decreasing the function of the heart.


Despite these caveats, the conclusion of this study is a validation of the current standard clinical monitoring practices for general anesthesia using inhalational (gas) anesthetics. It suggests that BIS monitoring does not add value in routine practice and may be over-sold – literally. Routine use of BIS monitoring would add about $360 million annually in the US if used in every general anesthetic.

What The Professionals Say

The excellent accompanying NEJM editorial by Bev Orser in Toronto summarizes the professional conclusions:

Titrating doses to a single parameter such as the BIS value is dubious, particularly when we do not yet understand the underlying mechanisms of the drugs, the algorithms of the electroencephalogram analysis, or the validity of the monitor.

Professionalism demands that care plans be based on a critical evaluation of the best available data, not on pressure from external forces such as fear of litigation or public demand.

The delegation of critical elements of patient care to a “black box” approach, in which decisive factors are under proprietary control, must be avoided.

The signals detected by monitors may or may not represent physiological processes of interest, such as learning and consciousness.

This study will be used by the anti-BIS camp. Among their many criticisms is that BIS monitoring could actually increase awareness if used primarily for economic reasons.

What You Can Do

  • Do not insist on BIS or other monitoring, despite what you may have seen in the movies or read in a magazine. Monitoring may add cost without benefit.
  • If you or a family member have previously experienced awareness under anesthesia, do tell your anesthesiologist. You may fall in a high risk category, in which BIS monitoring may be of value.
  • Do tell your anesthesiologist about any and all drugs you may be taking, including over-the-counter drugs, as these can affect the amount of anesthetic you need and justify additional monitoring.
  • If you’ve had a general anesthetic and remember things, you might have experienced awareness, and should tell your doctors. Keep in mind though that a lot of anesthetics these days are not general anesthetics but moderate or deep sedation in which you can expect to be aware, at least part of the time.

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