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Anesthesia awareness, or "unintended intra-operative awareness" occurs during general anesthesia, when a patient has not had enough general anesthetic or analgesic to prevent consciousness and the recall of events. This experience may be extremely traumatic for the patient.


In about 0.15% of patients undergoing general anesthesia, the anesthetic is inadequate to keep the patient unconscious during an operation. It may be inadequate from the beginning, or wear off during the operation. In this situation, a patient may feel the pain of surgery, pressure, hear conversations, and feel as if they cannot breathe. The patient may be unable to communicate any distress because they have been given a paralytic/muscle relaxant. Some patients become aware but do not feel pain or other unpleasant sensations.

The experience of anesthesia awareness[]

The most traumatic case of anesthesia awareness is full consciousness during surgery with pain and explicit recall of intraoperative events.

In less severe cases, patients may have only poor recollection of conversations, events, pain, pressure or of difficulty in breathing.

The experiences of patients with anesthesia awareness vary widely, and patient responses and sequelae vary widely as well.

What anesthesia awareness is not[]

Patients who have conscious sedation and/or regional anesthesia (such as spinal or epidural anesthesia), are expected to have some recall, and are not considered to have experienced anesthesia awareness. These patients are awake enough to indicate to the anesthetist if they feel pain during the operation. Many patients remember fragments of conversation they heard as they were drifting off into general anesthesia, while they were waking up, or while they were recovering in PACU (post anesthesia care unit). These patients do not recall pain or unpleasant stimuli of the surgery, but may be frightened by the belief that they were "awake" during the operation. Some patients may remember dreams.


Research suggests that approximately twenty million general anesthetics are administered each year in the United States; the incidence of one case in 500 anesthetics corresponds to 40,000 cases of awareness annually. The incidence of anesthesia awareness in the United States is believed to be 20,000 to 40,000 cases per year, which represents 0.1 percent and 0.2 percent of all patients undergoing general anesthesia (JCAHO 2004).


Patients who experience full awareness with explicit recall may have suffered an enormous trauma. Some patients experience posttraumatic stress disorder (PTSD), leading to long-lasting after-effects such as nightmares, flashbacks, and insomnia. There is evidence that early psychological counselling and support can reduce the amount of harm and chances of developing PTSD. The patient must be treated sympathetically and with compassion.

Patients may refuse to undergo another general anesthetic, and their experience may affect their relationship with doctors for the rest of their lives.

Prompt inspection of the anesthesia equipment and record is important and may help prevent future occurrences. It is also important that a case of suspected awareness is communicated to the patient's healthcare team, and the event is scrutinised closely by senior anesthetic medical staff.

Risk factors/Causes[]

Paralytics/muscle relaxant use[]

The most common risk factor is the use of a paralytic/muscle relaxant. Under general anesthesia it is common for the patient's muscles to be paralysed (with a neuromuscular blocking drug) in order to allow the surgeon safe access to the body cavities (e.g. abdomen, thorax or cranium), or to ensure the patient tolerates mechanical ventilation, or to keep the patient absolutely still for microsurgery, e.g. on the eye. The paralytic agent does not affect consciousness, or the ability to feel pain, at all. A fully paralyzed patient is unable to move, to speak, to blink the eyes, or otherwise respond to the pain. Muscle paralysis does not typically interfere with the functioning of the autonomic nervous system. This may result in signs such as an increased heart rate (tachycardia) and blood pressure (hypertension), as well as dilation of the pupils (mydriasis), sweating (diaphoresis), and the formation of tears (lacrimation) in response to pain. Therefore, even though the patient may not be able to directly signal their distress, they may exhibit signs of awareness which may be detectable by clinical vigilance.

Many types of surgery do not require the patient to be paralysed. A patient who is anesthetised, but not paralysed, is likely to move in response to a painful stimulus if the anesthetic is inadequate for any reason. This can happen without conscious perception or memory of the painful stimulus. Therefore, anesthetic awareness is uncommon in patients who have not been paralysed.

Light anesthesia[]

For certain operations, such as Cesarean section, or in hypovolemic patients or patients with minimal cardiac reserve, the anesthesiologist may aim to provide "light anesthesia". During such circumstances, consciousness and recall may occur because judgments of depth of anesthesia are not precise. The anesthesiologist must weigh the need to keep the patient safe and stable with the goal of preventing awareness. Sometimes it is necessary to provide lighter anesthesia in order to preserve the life of the patient (or sometimes her baby).

Improper equipment maintenance/anesthetist error[]

Human errors include inadequate drug dose, inadequate monitoring, and failure to refill the anesthetic machine's vaporisers with volatile anesthetic. Other causes of awareness include unfamiliarity with techniques used, e.g. intravenous anesthetic regimes, or inexperience. Poor anesthetic technique is a combination of all of the above, but also includes techniques which could be described as outside the boundaries of "normal" practice. The American Society of Anesthesiologists recently released a Practice Advisory outlining the steps that anesthesia professionals and hospitals should take to minimize these risks. Other societies have released their own versions of these guidelines, including the Australian and New Zealand College of Anaesthetists [1].

Machine malfunction or misuse may result in an inadequate delivery of anesthetic. This may be caused by an empty vaporizer (or nitrous oxide cylinder) or a malfunctioning intravenous pump or disconnection of its delivery tubing. Problems with flowmeters or monitors may also contribute to risk of awareness.

To reduce the likelihood of awareness, anesthetists must be adequately trained and supervised whilst still in training. Equipment which monitors depth of anaesthesia, such as bispectral index monitoring, should not be used in isolation.

Patient physiology[]

Possible causes of awareness include drug tolerance, or a tolerance induced by the interaction of other drugs. Some patients may be more resistant to the effects of anesthetics than others. Younger age, tobacco smoking or long-term use of certain drugs (alcohol, opiates, or amphetamines) may increase the anesthetic dose needed to produce unconsciousness. There may be genetic variations that cause differences in how quickly patients clear anesthetics, and there may be differences in how the sexes react to anesthetics as well. Anxiety prior to the surgery can increase the amount of anesthesia required to prevent recall.


The risk of awareness is reduced by simple steps and good clinical practice: well-trained personnel; careful checking of drugs, doses and equipment; good monitoring, and careful vigilance during the case.

Recent advances have led to the manufacture of monitors of awareness. Typically these monitor the EEG, which represents the electrical activity of the cerebral cortex, which is active when awake but quiescent when anaesthetised (or in natural sleep). The monitors usually process the EEG signal down to a single number, where 100 corresponds to a patient who is fully alert, and zero corresponds to electrical silence. General anaesthesia is usually signified by a number between 60 and 40 (this varies with the specific system used). These newer technologies include the bispectral index (BIS), EEG entropy monitoring, auditory evoked potentials, and several other systems. Of these, only BIS has been shown to reduce the incidence of awareness.

None of these systems are perfect. For example, they are unreliable at extremes of age (e.g. neonates, infants or the very elderly). Secondly, certain agents, such as nitrous oxide, ketamine or xenon, may produce anesthesia without reducing the value of the depth monitor. This is because the molecular action of these agents (NMDA receptor antagonists) differs from that of more conventional agents, and they suppress cortical EEG activity less. Thirdly, they are prone to interference from other biological potentials (such as EMG), or external electrical signals (such as diathermy). This means that the technology does not yet exist which will reliably monitor depth of anaesthesia for every patient and every anaesthetic.


Currently, the anesthesia provider community accepts that anesthesia awareness occurs, however there is not much of a consensus on the incidence or on how often patients experience long term mental distress.


New research has been carried out to test what people can remember after a general anesthetic in an effort to help doctors more clearly understand anesthesia awareness and help to protect patients from experiencing it. A memory is not one simple entity; it is a system of many intricate details and networks. Memory is currently classified under two main subsections. First there is explicit or conscious memory, which refers to the conscious recollection of previous experiences. An example of explicit memory is remembering what you did last weekend. When it comes to an anesthetized patient, a doctor may ask the patient after undergoing general anesthesia if he or she could remember hearing any distinct sounds or words while under anesthesia. This approach is called a "recall test" because patients are asked to recall any memories they had during surgery. The second main type of memory is implicit memory or unconscious memory, which refers to the changes in performance or behavior that are produced by previous experiences but without any conscious recollection of those experiences. An example of this is a recognition test, where patients are asked which of the following words were played to you during your surgery. As a further example please note the following scenario. Patients were exposed during anesthesia to a list of words containing the word "pension". Postoperatively, when they were presented with the three-letter word stem PEN___ and were asked to supply the first word that came to their minds beginning with those letters, they gave the word "pension" more often than "pencil" or "peninsula" or others.

Some researchers are now formally interviewing patients postoperatively to calculate the incidence of anesthesia awareness. Most patients who were not unduly disturbed by their experiences do not necessarily report cases of awareness unless being directly asked. It has been found that some patients may not recall experiencing awareness until one to two weeks after undergoing surgery. It was also found that some patients require a more detailed interview to jog their memories for intraoperative experiences.

Awake craniotomy[]

Under very unusual circumstances, neurosurgeons may wish to wake a patient during an operation in order to test the function of specific parts of their brain while they are awake. This procedure is called an awake craniotomy.

Normally the patient is counselled extensively before such a procedure is contemplated. The patient is anesthetised and kept under general anesthesia while the skull is opened and the brain exposed. Then the anesthetist deliberately lightens the anesthetic. It is normal for opioid drugs (such as remifentanil) to be used, so that the patient does not experience pain. A depth of anesthesia monitor such as BIS may help to guide the process. Typically the anesthetic is lightened to the point where the patient can obey simple commands. When the surgeon has identified the appropriate parts of the brain, the anesthetic is deepened again until the end of the operation.

See also[]


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