Anesthesia awareness
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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 sometimes extremely traumatizing for the patient, who is unable to communicate his or her distress due to the paralysis. Waking of patients during anesthesia is an uncommon complication, even though it is terrifying to both patients and doctors.
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[edit] Background
Under general anesthesia it is commonly required for the patient's muscles to be paralysed (with a neuromuscular blocking drug such as vecuronium) 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. However, 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 extremely uncommon in patients who have not been paralysed. Patients who have regional anesthesia (such as spinal or epidural anesthesia), are awake and can tell the anesthetist if they feel pain during the operation.
Under very rare circumstances, the anesthetic may be inadequate to keep the patient unconscious during an operation. It may be inadequate from the beginning, or wear off during the operation. This is almost never intentional (but see below). In this situation, a patient may be aware of the pain of surgery, as well as other discomforts such as the endotracheal tube. They may also be aware of sounds or conversation in the operating room. The patient is unable to communicate any distress because their muscles are paralysed. The patient is unable to move, to speak, to blink the eyes, or otherwise respond to the pain. However, they may be able to recall in considerable detail what happened during the operation.
This experience is extremely traumatic for the patient.
Muscle paralysis does not typically interfere with the functioning of the autonomic nervous system. This will 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 although the patient may not be able to directly signal their distress, they may exhibit signs of awareness which are detectable by normal clinical vigilance.
[edit] Definition
An exact definition of awareness is important. The most serious type of awareness is full awareness during surgery with recall of pain or other noxious stimuli, and explicit recall of intraoperative events. This is also the least common.
In some situations, the awareness may be incomplete, so that patients may have only hazy recollections of events or pain during the operation. Patients may claim they can remember some events during the surgery, but without (necessarily) remembering pain. This is more common than explicit recall.
Finally, some patients may recall fragments of conversation or music which they heard during induction of, or recovery from, the general anesthetic. These patients do not recall pain or unpleasant stimuli, but may be frightened by the belief that they were "awake" during the operation.
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.
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.
[edit] Incidence
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). However, these include cases in which the patient has only a hazy memory lasting a few seconds.
[edit] Outcomes
Patients who experience full awareness with explicit recall have suffered an enormous trauma. Some victims experience posttraumatic stress disorder (PTSD), leading to long-lasting after-effects such as nightmares, flashbacks, and insomnia.
Patients may refuse to undergo another general anesthetic, and their experience may affect their relationship with doctors for the rest of their lives. A small number of cases are publicised by the media as medical "scare stories".
Patients who experience lasting distress should be offered psychological counselling and support.
It is important that a case of suspected awareness is scrutinised closely by senior anesthetic medical staff, and all of the patient's reports documented carefully. Inspection of the anesthetic record and other records of the case is also important. The patient must be treated sympathetically and with compassion.
[edit] Risk factors
Possible causes of awareness include 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, human errors include inadequate drug dose, inadequate monitoring, and failure to refill the anesthetic machine's vaporisers with volatile anesthetic. 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] and the Sociedad Madrid Centro de Anestesiología y Reanimación [2].
Anesthesiologists have been educated about this phenomenon for more than ten years, and most hospitals have plans in place to prevent awareness as well as to respond to those cases that are not preventable.
There are four main categories of causes for anesthesia awareness:
[edit] 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 anesthetic in the motionless patient. This is probably the most common cause of awareness, and represents an iatrogenic mishap (a doctor's error). Because anesthetic concentrations that eliminate awareness are less than those that prevent motor responses to pain, an inadequately anesthetized, but nonparalyzed, patient usually responds to pain by movement. Deepening the anesthetic at this stage usually prevents awareness.
[edit] Increased anesthetic requirement
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.
[edit] Equipment problems
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.
[edit] Human error
For example, forgetting to turn on anesthetic agents, 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 outwith the boundaries of "normal" practice.
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.
[edit] Prevention
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.
Anesthesiologists 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 caesarean 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 sometimes her baby).
Some newer monitoring techniques purport to be able to indicate the depth of anesthesia. 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 is 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.
[edit] 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.
[edit] See also
- American Association of Nurse Anesthetists
- American Society of Anesthesiologists
- Resource for Student & Working Nurse Anesthetists
- Anaesthetic machine
- Carol Weihrer, victim and activist
- Iatrogenic
- Awake, a 2006 film about anesthetic awareness
- Rapid Anesthesia Scan - Anesthesia website that describes a system where an anesthetist can systematically check the patient and thus not miss out that the patient is aware
- Anesthesia, an award-winning horror film about anesthesia awareness.
[edit] References
- Ghoneim, M. M. M.D. "Anesthesiology Full Text Volume 92" February 2000 92:2 [3]
- Myles PS, Leslie K, McNeil J, Forbes A, Chan MTV, for the B-Aware trial group. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet 2004;363:1757-1763. Fulltext. PMID 15172773.
- Joint Commission on Accreditation of Healthcare Organizations (JCAHO). Sentinel Event Alert 6 Oct 2004;32. Online version.
