Awareness under anaesthesia


Being awake during a general anaesthetic is extremely rare but the fear of awareness is not uncommon. British anaesthesia leads the world in undertaking national audit programmes (NAPs) and in 2014 NAP5 reported on awareness after analysing 2,800,000 anaesthetics. This report has significantly increased our understanding and allows us to reduce the risk and provide reassurance to our patients.


The commonest reports of awareness are in patients who are receiving sedation as opposed to general anaesthesia. You will find another blog on this site about sedation but the primary difference is that sedation is the removal of anxiety in the conscious patient. Patients undergoing sedation should always be awake although they may have retrograde amnesia and so these complaints reflect poor communication and confusion.


True awareness under general anaesthesia is extremely rare. It occurs in 1:19600 anaesthetics. If the patient is able to move or talk and tell the anaesthetist they are aware it appears to have very few consequences. The problem arises when a patient is paralysed due to neuromuscular blocking drugs and is awake but cannot move or talk. This makes awareness difficult to detect, reflected by the fact that awareness occurs in 1:135,900 cases without the use of neuromuscular blocking drugs but in 1:8200 cases where they are used. These drugs are now used far less frequently but there are a number of operations where they are essential


Awareness is more common in obstetric anaesthesia, cardiac anaesthesia and paediatric anaesthesia although the latter may reflect general anaesthetics given by anaesthetists who are not paediatric specialists. It is also more common in women, young adults, the obese, and emergency operations as well as with more junior anaesthetists.


Armed with knowledge of these facts we can modify our anaesthetic technique, avoiding neuromuscular blocking drugs wherever possible and being particularly vigilant in high risk cases. There are two specific methods to guarantee there is no awareness: first is the isolated forearm technique where a tourniquet is applying to an arm before drugs are administered so that if the patient is aware they can continue to move that hand. Unfortunately the technique is time-consuming, requires expertise and is not practical on a day-to-day basis


The other method is to monitor the brain. NAP5 did not recommend routine EEG monitoring because very few anaesthetists are trained to read the EEG and the monitors that are normally used merely provide a computerised number to represent the depth of anaesthesia. This numerical display is very inaccurate and there are concerns that it may actually increase as opposed to decrease awareness by lulling anaesthetists into the belief that the patient is not aware. I am fortunate to have used EEG monitoring in the US and I worked with Pamela Pryor one of the foremost neurophysiologists at Barts for almost 20 years. I now work with the American company Masimo and use a monitor that actually provides raw EEG data that allows me to determine the depth of anaesthesia and to significantly reduce the risk of awareness. We have these monitors available at the Weymouth Street Hospital.


Finally a few comments on why awareness must be taken seriously. Awareness is extremely distressing in the presence of neuromuscular blockers. Some patients suffer from excruciating pain but even in the absence of pain awareness can result in post-traumatic stress disorder. If a patient complains of awareness they should always be believed and they should be referred as early as possible for psychological support and cognitive behavioural therapy.