Anesthesia awareness, or "intra-operative awareness" occurs during general anesthesia, when a patient is paralyzed with muscle relaxants but has not had enough general anesthetic or analgesic to prevent consciousness or, more importantly, the sensation of pain and the recall of events.

The experience is often traumatizing for the patient, who is unable to communicate his or her distress due to the muscle relaxants.

Anesthetists have many approaches available to minimize the risk of awareness; however, it is not always possible to completely avoid it. Some patients, such as those undergoing cardiac or trauma surgery or emergency cesarean sections, have a somewhat higher risk. 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 the baby.

Possible causes of awareness include a predisposed drug tolerance or a tolerance induced by the interaction of other drugs. Inability to reliably measure consciousness with current technology is another important factor. Less frequently, causal human errors include inadequate drug dose, inadequate monitoring, and failure to refill the anesthetic machine's vaporisers with volatile anesthetic. The American Society of Anesthesiologist recently released a Practice Advisory outlining the steps that anesthesia professionals and hospitals should take to minimize these risks.

The prevalence 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). However, these include cases in which the patient has only a hazy memory lasting a few seconds. Patients who experience lasting stress should be offered psychological counseling and support. Otherwise, in some cases, the effects can extend further than the event itself. Some victims experience posttraumatic stress disorder (PTSD), leading to long-lasting after-effects such as nightmares, flashbacks, and insomnia.

Anesthetists have been educated about this phenomenon for more than ten years, and most hospitals have had plans in place to prevent awareness as well as to respond to those cases that are not preventable.

New research has been done to test what people can remember after going under general anesthesia in an effort to help Anesthetists more clearly understand anesthesia awareness and help in the fight to protect patients from experiencing it. A memory is not one simple entity, it is more or less 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.

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.

There are four main causes for anesthesia awareness

1. 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 is not surprising because judgments of depth of anesthesia are not precise. Muscle relaxants also lead to the problem of the unintentionally too light anesthesia in the motionless patient. This is probably the most common cause of awareness, and represents, in a sense, an iatrogenic mishap. Because anesthetic concentrations that block awareness are less than those that prevent motor responses to pain, an inadequately anesthetized, but nonparalyzed, patient usually communicates awakeness by movement. Deepening the anesthetic at this stage usually prevents awareness.

2. Increased anesthetic requirement of some patients. Some patients may be more resistant to the effects of anesthetics than others. Younger age, tobacco smoking, long-term use of certain drugs (alcohol, opiates, or amphetamines) may increase the anesthetic dose needed to produce unconsciousness.

3. Machine malfunction or misuse resulting in an inadequate delivery of anesthetic. This may be caused by an empty vaporizer (or N2O cylinder) or a malfunctioning intravenous pump or disconnection of its delivery tubing.

4. Human error, inexperience, poor anaesthetic technique. For example forgetting to turn on anaesthetic agents (human error), unfamiliarity with techniques used eg Intravenous anaesthetic regimes (inexperience). Poor anaesthetic technique is a combination of all of the above, but also includes techniques which could be described as outwith the boundaries of "normal" practice.

Waking of patients after anesthesia is administered is usually an uncommon complication, even though it is terrifying to both patients and Anesthetists . 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. Researchers have deduced that awareness is the direct result of incorrect dosage, and the risk factor is only catalyzed when muscle relaxants come into play. The biggest consequence of awareness is patients suffering from post-traumatic stress disorder. Anesthetists need to be very careful and precise when administering anesthesia to a patient. Cases of awareness should be handled with precision, detail, documentation and compassion. In order to prevent awareness, Anesthetists should avoid overly light anesthesia, learn about anesthetic requirements for patients, and continue to develop methods to detect the consciousness of patients while under general anesthesia. In this regard, relatively newer technologies such as the bispectral index (BIS), state of entropy, and auditory evoked-potentials are being increasingly used in an attempt to monitor the state of awareness in patients undergoing general anesthesia.