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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.
For patients undergoing general anesthesia, about 1 in 700 (Sebel PS: The incidence of awareness during anesthesia: A multicenter United States study. Anesth Analg 2004; 99:833-9) have anesthesia that is inadequate to keep them unconscious during an operation. In this situation, a patient may feel the pain or pressure of surgery, hear conversations, or feel as if they cannot breathe. The patient may be unable to communicate any distress because they have been given a paralytic/muscle relaxant. If anesthesia awareness does occur about 40% feel the pain of the operation, 92% experience panic/anxiety and 70% experience lasting psychological symptoms (N. Moerman et al,Anesthesiology;79:454-464, 1993)
In some cases, post traumatic stress disorder (PTSD) may arise after intraoperative awareness, causing the patient to require counseling for an extended period.
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. This experience may be extremely traumatic for the patient.
Because the medical staff may not know if a patient is unconscious or not, it has been suggested that the staff maintain the professional conduct that would be appropriate for a conscious patient.
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 may be frightened by the belief that they were "awake" during the operation.
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.
The incidence of anesthesia awareness is higher when muscle relaxants are used.
A recent large-scale study reported a much lower rate (0.0068%). The modified patient interview used in this retrospective study did not specifically ask the patient if they had experienced awareness during the surgery.
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 be communicated to the patient's healthcare team, and that the event be scrutinised closely by senior anesthetic medical staff.
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, night terrors, flashbacks, insomnia, and in some cases even suicide. 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.
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.
For certain operations, such as Caesarean section, or in hypovolemic patients or patients with minimal cardiac reserve, the anesthesia provider may aim to provide "light anesthesia." During such circumstances, consciousness and recall may occur because judgments of depth of anesthesia are not precise. The anesthesia provider 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 patients.
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 any 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.
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 while still in training. Equipment which monitors depth of anaesthesia, such as bispectral index monitoring, should not be used in isolation.
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; avoidance of paralytics unless necessary; 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.
The most recent technological advancement in awareness monitors is the SNAP II. The SNAP II monitor is the only monitor to evaluate high and low frequency EEG in real time to create an objective metric SNAP Index, which helps measure the state of the brain and assess the level of consciousness. The SNAP monitor captures the most useful information in low-frequency EEG and utilizes high-frequency component in return to consciousness
Studies have shown that a low frequency band in the 1-15Hz range of 0.40 indicates declining alertness while high frequency range between 201-500Hz can be a marker of cognitive function and or capacity which the SNAP utilizes in return to consciousness. The SNAP monitor also minimizes the bands that are most heavily contaminated by Electromyography (EMG), which causes interference in other monitors.
A study done by the British Journal of Anesthesia in June 2006, stated, "the SNAP index 1 min before awakening had returned to or exceeded the baseline awake value in 64% of the subjects, and in 90% of the subjects at awakening. The corresponding percentages for the BIS were 8% at both times. Therefore the SNAP II may have an advantage compared with the BIS in predicting imminent awakening when compared with the awake baseline values. The faster return to baseline found with the SNAP device may be a result of the inclusion of the high-frequency EEG component in the Calculation of the index, as frequencies as high as 128Hz have shown a high prediction probability for separation of awareness and unresponsiveness."
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. A study from Sweden in 2002 attempted to follow up 18 patients approximately 2 years after previously diagnosed awareness under anesthesia. Four of the nine interviewed patients were still severely disabled due to psychiatric/psychological sequelae. All of these patients had experienced anxiety during the period of awareness, but only one had complained about pain. Another three patients had less severe, transient mental symptoms, although they could cope with these in daily life. Two patients denied any sequelae from their awareness episode.
New research has been carried out to test what people can remember after a general anesthetic in an effort to 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.
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 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Anesthesia_awareness". A list of authors is available in Wikipedia.|