What is the chance of waking up during an anaesthetic (awareness)?

Awareness under Anaesthesia


The potential of being aware while having a surgical procedure under general anaesthesia is a common and legitimate fear for many patients. The objective of a general anaesthetic is to induce a state of unconsciousness/hynosis through the use of drugs during which time the patient will have no awareness of the operation and there will be no subsequent recall or memory of these events. Awareness under anaesthesia can be defined as a period during the operation where a patient is conscious and will have recall of the events after waking from the anaesthetic. It is a complex topic because consciousness and memory formation is incompletely understood by scientists who study this subject. In this section an explanation of the current understanding of this topic will be presented.


The first problem encountered when dealing with this question is being able to decide if and when a patient was aware during an anaesthetic. To help with this four questions can be asked:-

(1) What is the last thing you remember before you went to sleep?

(2) What is the first thing you remember when you woke up?

(3) Do you remember anything between these periods?

(4) Did you dream during your operation?

There are also different levels of clarity with which patients can recall an intraoperative period of awareness:-

(1) Explicit recall without prompting

(2) Explicit recall with prompting or asking

(3) A sense of having been aware but not being able to recall specific events

(4) Vague recall after indepth questioning or hypnosis

If a patient reports a dream during an operation that is not necessarily recall from a period of awareness. There are instances where patients have explicit and detailed recall after an operation and may even be able to recount specific introperative events, for example what was said in theatre. If this experience is associated with feeling pain from the operation it will obviously be a very distressing experience. However in most instances it will take a panel of experts to review the situation in detail to decide if it was an experience of awareness.

The first study of awareness was conducted in 1960 and it reported an incidence of 1.2%. Although there are problems with the scientific accuracy of this research the figure is distressingly high and would be entirely unacceptable in the modern practice of anaesthesia. More recent studies report an incidence of 0.1 to 0.2%. This is an improvement but the goal remains to have a zero incidence of awareness under general anaesthesia.

There are certain operations and situations that have been identified as increasing the risk of awareness and these are as follows:-

(1) Light anaesthesia where the patient is too sick to tolerate the normal doses of anaesthetic drugs required for a general anaesthesia. This may occur in situations such as after trauma where the patient has a low blood volume.

(2) Types of surgery such as cardiac, obstetric, trauma and some Ear Nose and Throat surgery where there are problems with control of the patients airway

(3) Conditions that are associate with increased anaesthetic requirements such as having a history of awareness, chronic use of alcohol and drugs, genetic resistance to anaesthetics

(4) Female, young and overweight patients appear to have a higher incidence of awareness

(5) Insufficient knowledge on the part of anaesthetists

In addition awareness can result from anaesthetic errors such as administering incorrect doses of drugs, failure to notice problems with the breathing circuit and not recognizing the information from the monitors.

There are certain operations and situations where awareness can be expected and these include:-

(1) Procedures performed under sedation such as gastroscopy, colonoscopy, eye procedures. Sedation is in essence a form of light anaesthetic and the aim is to reduce the anxiety a patient may feel while having the procedure. For this reason it is possible that a patient may have some recall of the procedure.

(2) Regional/local anaesthetic procedures. Where a surgical procedure is performed where the patient is having a spinal, epidural, regional or local anaesthetic, the patient will have complete recall of the procedure. If the procedure is supplemented with sedation there may be limitation but not complete ablation of the awareness and recall.

(3) Awareness can occur after the surgery or procedure is completed and the patient is going through the normal waking process. This can be an unpleasant experience as there may still be an airway device in the mouth and someone may recall having this removed.

Another experience that occurs under anaesthetic that may be confused with awareness is dreaming. If this is going to happen it usually occurs during a short, sedative procedure but can happen at any time. The dreams are diverse in nature and mostly pleasant. It can be a difficult process deciding whether a dream (unpleasant) was an experience of awareness.

What is important in these instances is that the procedures are either not painful or the pain has been blocked by the regional anaesthetic so awareness and recall will not be an unpleasant experience.


The BIS Monitor (Bispectral Index) is a relatively new addition to the monitoring that an anaesthetist may employ. It is based on an Electroencephalogram (EEG) and measures the electrical activity in the frontal portion of the brain. The signal is processed and the monitor produces a reading which reflects the depth of anaesthesia in a patient. The monitor has certain limitations but is an useful addition to the other monitors that are routinely used during an anaesthetic.

BIS Monitor – the reading on large letters indicates the depth of the anaesthetic with 100 being fully awake and below 60 a state of general anaesthesia.


Unintended awareness while under general anaesthetic is a deviation from the goals of an anaesthetic. When a patient has this experience, it must be reported to the anaesthetist who must then investigate what has happened. In certain instances a patient may require counselling to prevent the development of psychological symptoms as a reaction to the experience. It is also important to investigate the report to prevent it happening again.