Preventing awareness during surgery: what’s the latest evidence?

In a blog for non-medical readers, anaesthetist Jennifer Kielty discusses a common fear about an uncommon problem – awareness during surgery, and looks at the latest Cochrane evidence on ways to reduce the risk of that happening.

“Keep me well asleep Doctor” and “I just want to be completely knocked out” are a few of the most common things I hear on meeting patients before an operation. The release of the film ‘Awake’ in 2007 did nothing to ease the fact that awareness while under anaesthetic (intraoperative awareness) is one the most common fears patients have before surgery. Unfortunately, in the rare scenario when it occurs, it can be highly distressing for the patient, and can lead to recurrent dreams, anxiety and depression.

We have only a few ways of monitoring for awareness under anaesthesia: clinical signs (such as the patient’s blood pressure and heart rate); end tidal volatile anaesthetic gas (ETAG) which is the amount of anaesthetic gas the patient exhales; and via processed electroencephalogram (EEG) (for example, Bispectral index or BIS) which involves placing electrodes on a patient’s head to monitor brain waves.

Awareness during surgery under general anaesthesia is a common fear.

An evidence gap

Current guidelines from the Association of Anaesthetists consider measuring the amount of exhaled anaesthetic part of the minimum monitoring required when someone has an anaesthetic. Brain wave monitoring, as mentioned above, is another method of monitoring depth of anaesthesia. However there is a lack of evidence comparing the efficacy of brain wave monitoring with the use of clinical signs or with monitoring of amount of exhaled anaesthetic gases.

New evidence from Cochrane

 An important Cochrane review (published in 2019) summarises the evidence on the effectiveness of brain wave monitoring to reduce the risk of the patient being aware while under general anaesthetic, and its effect on the time until a patient wakes up after the operation. It includes 52 research studies and over 41,000 adults having surgery under general anaesthetic. Brain wave monitoring (BIS) was compared with clinical signs as a guide to depth of the anaesthetic or with monitoring of amount of exhaled anaesthetic (ETAG).

Overall the findings of the review were:

  • BIS may improve the time it takes for a patient to wake up from an anaesthetic compared to use of clinical signs.
  • There may be little or no difference in the risk of intraoperative awareness between use of BIS or use of measurement of amount of exhaled anaesthetic gas.
  • Overall the incidence of intraoperative awareness was very small.

Limitations of the studies meant that the evidence was all assessed as being low-certainty, so the review authors can’t be confident about their results. They also note that most of the studies reported that none of the people taking part woke up during surgery as it’s a rare event!

Finally there were three studies which looked at people who were thought to be high risk of being aware while under anaesthetic and two studies that did not grade a patient’s risk.  They found that brain wave monitoring in these two groups may reduce the risk of intraoperative awareness compared to monitoring clinical signs.

Only one study comparing monitoring brain waves (BIS) with exhaled anaesthetic gases (ETAG) looked at recovery times, and this low-certainty evidence showed that discharge from the post anaesthetic care unit may be earlier in patients who had BIS guided anaesthesia compared to ETAG guided anaesthesia.

What does this mean?

This review supports what we know from the findings of the 5th National Audit Project (NAP5), that accidental awareness under anaesthetic is rare. This review also supports the recommendations in the 2015 guideline from the Association of Anaesthetists on the Standards of monitoring during anaesthesia and recovery, which advise: the continued presence of an anaesthetist; always using minimum monitoring devices, from before the patient receives an anaesthetic until after the patient completely wakes up; that these monitoring devices are checked and maintained appropriately; and the importance of audible alarms.

Talking to your anaesthetist

We can be reassured by knowing that awareness during anaesthesia is very rare. However if you are worried I would encourage you to meet with your anaesthetist before your operation. It is important to tell them about your past medical history, your regular medications and about any previous problems you may have had with anaesthesia. Ask about what kind of anaesthetic you will get and about possible risks and side effects. You should tell them about your concerns so that any specific worries can be addressed. By having this conversation and learning more we can help alleviate stress, and help you to have a more comfortable time while in hospital.

Take-home points

Take-home points: Awareness while having surgery under general anaesthesia is very rare. • Evidence from a Cochrane Review comparing ways of monitoring for signs of awareness during anaesthesia supports practice recommendations in UK guidelines. • Talking to your anaesthetist about your history and any concerns can help them to address your worries and help you have a less stressful experience.

Join in the conversation on Twitter with @CochraneUK or leave a comment on the blog. Please note, we cannot give medical advice and we will not publish comments that link to commercial sites or appear to endorse commercial products.

References (pdf).

Here you can read a Cochrane Clinical Answer: ‘How does the bispectral index (BIS) compare with clinical signs or end‐tidal anesthetic gas (ETAG) for guiding anesthesia?’

Preventing awareness during surgery: what’s the latest evidence? by Jennifer Kielty

is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International

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