Anesthesia awareness, or unintended intra-operative awareness occurs during general anesthesia, on the operating table, when the patient has not been given enough of the general anesthetic or analgesic to render the patient unconscious during general anesthesia (often when agents used to paralyze the patient have been administered). In brief, it is the post-operative recall of intra-operative events.
However, it can also occur in the post-anesthesia care unit (PACU) or in the intensive-care unit (ICU), where patients are kept sedated, tranquilized and sometimes paralyzed (and intubated) and are connected to life support systems, awaiting normalization of their physiology.
Awareness occurs in 20,000–40,000 patients out of the 20 million US surgeries performed each year (between 0.1% and 0.2%) [1] when patients have anesthesia that is inadequate to keep them unconscious during an operation. Large trials have demonstrated that around 1 to 2 per 1000 patients experience some form of awareness.[2][3] The majority of these do not feel pain although around one third did, be it a sore throat from an endotracheal tube or pain from the incision site. The incidence is halved in the absence of neuro-muscular blockade.[3] In this situation, the 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; without this, they can move and the anesthesiologists are alerted and provide more anesthetic drugs to render the patient unconscious again. If anesthesia awareness does occur, about 42% feel the pain of the operation, 94% experience panic/anxiety (sometimes because they cannot breathe), and 70% experience lasting psychological symptoms.[4] The quoted incidences are highly controversial as many cases of "awareness" are open to interpretation; only a small number are irrefutably so. These usually involve feeling severe pain, or clear recall of the conversations of the operating room staff. Some anesthetic drugs may cause vivid dreams in the period before going to sleep, which may be interpreted as "being awake". Some patients undergo sedation for smaller procedures such as biopsies and colonoscopies and are told they will be asleep, although in fact they are getting a sedation that may allow some level of awareness as opposed to a "general anesthetic"
In some cases, post traumatic stress disorder (PTSD) may arise after intraoperative awareness, causing the patient to require counseling for an extended period.[5]
There are two states of consciousness that may be present:
The incidence of a state with both responses in diverse degrees is also possible.[6] The drugs that induce paralysis would also prevent responding to commands.
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 difficulty in breathing. Some cases of difficulty in breathing are caused by intubation errors and/or problems with the ventilator and a patient might be suffocating.
The experiences of patients who have experienced anesthesia awareness vary widely depending on why they became aware, whether they were paralyzed and patient responses and sequelae vary widely as well. It is unusual for someone having experienced awareness without pain or suffocation to suffer bad sequelae. The experience may be extremely traumatic for the patient or not at all depending on whether they could breathe and what errors were made.
Because the medical staff may not know if a person is unconscious or not, it has been suggested that the staff maintain the professional conduct that would be appropriate for a conscious patient.[7]
There are various levels of consciousness. Wakefulness and general anesthesia are two extremes of the spectrum. Conscious sedation and monitored anesthesia care (MAC) refer to an awareness somewhere in the middle of the spectrum depending on the degree to which a patient is sedated. It is important to note that awareness/wakefulness is not necessarily correlated with pain or discomfort. The aim of conscious sedation or monitored anesthetic care is to provide a safe and comfortable anesthetic while maintaining the patient's ability to follow commands.
Under certain circumstances, a general anesthetic, whereby the patient is completely unconscious, may be unnecessary and/or undesirable. For instance, with a cesarean delivery, the goal is to provide comfort with neuraxial anesthetic yet maintain consciousness so that the mother can participate in the birth of her child. Other circumstances may include, but are not limited to, procedures that are minimally invasive or purely diagnostic (and thus not uncomfortable). Sometimes, the patient's health may not tolerate the stress of general anesthesia. The decision to provide monitored anesthesia care versus general anesthesia can be complex involving careful consideration of individual circumstances and after discussion with the patient as to their preferences.
Patients who undergo conscious sedation or monitored anesthesia care are never meant to be without recall.[8] Whether or not a patient remembers the procedure depends on the type of medications used, the dosages used, patient physiology, and other factors. Many patients undergoing monitored anesthesia care do not remember the experience.
The incidence of anesthesia awareness is higher when muscle relaxants are used.[9] This is because without relaxant the patient will move and the anesthesiologist will deepen the anesthesia.
Several studies have indicated this occurs in about 1 or 2 per 1000. Other studies show a much lower incidence, usually because the anesthesiologists know they are in a trial and are very experienced. Some studies suggest it is a rare phenomenon, with rates varying from 0.13% to as low as 0.0068%.[10][11]
Anesthesia equipment should always be checked prior to use but because of haste this does not always happen. Prompt inspection of the anesthesia equipment and record after a patient reports awareness must be done immediately and may help prevent future occurrences unless it was caused by human error (as it usually is). All drugs used and empty syringes must be carefully examined and tested because drug errors cause a high percentage of awareness cases. It is also important that a case of suspected awareness be communicated to the patient's healthcare team immediately, and that the event be scrutinized closely by senior anesthetic medical staff.
Patients who experience full awareness with explicit recall may have suffered an enormous trauma. Some patients experience post traumatic stress disorder (PTSD), leading to long-lasting after-effects such as nightmares, night terrors, flashbacks, insomnia, and in some cases even suicide.[12]
The biggest risk factor is the use of a medication that induces muscle paralysis, such as Suxamethonium. Under general anesthesia, the patient's muscles may be paralyzed in order to facilitate tracheal intubation, surgical exposure, or mechanical ventilation. The paralytic agent does not cause unconsciousness or take away the patient's ability to feel pain but it does prevent the patient from breathing so they must be ventilated correctly.
A fully paralyzed patient is unable to move, speak, blink the eyes, or otherwise respond to the pain. It is incorrect to think that physiological signs such as increased heart rate (tachycardia), blood pressure (hypertension), dilation of the pupils (mydriasis), sweating (diaphoresis), and the formation of tears (lacrimation) will continue to occur normally in response to pain in the anesthetized state. If neuromuscular blocking drugs are used this causes skeletal muscle paralysis and often interfere with the functioning of the autonomic nervous system. The patient cannot signal their distress and they may not exhibit the signs of awareness that would be expected to be detectable by clinical vigilance.
Many types of surgery do not require the patient to be paralyzed. A patient who is anesthetized but not paralyzed can move in response to a painful stimulus if the analgesia is inadequate. This may serve as a warning sign that the anesthetic depth is inadequate. Movement under general anesthesia does not correlate with full awareness but is a sign that the anesthesia is light. Even without the use of paralytics the absence of movement does not necessarily correlate with amnesia.
For certain operations, such as Cesarean section, or in hypovolemic patients or patients with minimal cardiac reserve, the anesthesia provider may aim to provide "light anesthesia" and should discuss this with the patient to warn them. 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 patient. 'Light' anesthesia means less drugs by the intravenous route or via inhalational means, leading to less cardiovascular depression (hypotension) but, causing 'awareness' in the anesthetized subject.[13]
Human errors include oesophageal intubation, inadequate drug dose, drug given by the wrong route or wrong drug given, drugs given in the wrong sequence, inadequate monitoring, disconnections and kinks in tubes from the ventilator, and failure to refill the anesthetic machine's vaporizers with volatile anesthetic. Other causes of awareness include unfamiliarity with techniques used, e.g. intravenous anesthetic regimes, or inexperience. Most cases of awareness are caused by inexperience and poor anesthetic technique, which can be any of the above, but also includes techniques that 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.[14]
Machine malfunction or misuse may result in an inadequate delivery of anesthetic. Many Boyle's machines used in many hospitals have the oxygen regulator serving as a slave to the pressure in the nitrous oxide regulator, to enable the nitrous oxide cut-off safety feature. If nitrous oxide delivery suffers due to a leak in its regulator or tubing, an 'inadequate' mixture can be delivered to the patient, causing awareness. Many World War II vintage Boyle 'F' models are still functional and used in UK hospitals. Their emergency oxygen flush valves have a tendency to release oxygen into the breathing system, which when added to the mixture set by the anesthesiologist, can lead to awareness. This may also be caused by an empty vaporizer (or nitrous oxide cylinder) or a malfunctioning intravenous pump or disconnection of its delivery tubing. Patient abandonment, when the anesthesiologist leaves, causes some cases of awareness and death.
To reduce the likelihood of awareness, anesthetists must be adequately trained and supervised while still in training. Equipment that monitors depth of anesthesia, such as bispectral index monitoring, should not be used in isolation.
Rare 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, women need more anesthetic than men, there are several possible reasons for this, e.g. less alcohol consumption, female hormones, different fat distribution in the body, but nobody has researched this. Younger age, obesity, tobacco smoking, or long-term use of certain drugs (alcohol, opiates, or amphetamines) may increase the anesthetic dose needed to produce unconsciousness but often this is used as an excuse for poor technique. 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. In addition, anesthetic requirement is increased in persons with naturally red hair.[15] Marked 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 anesthetized (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 anesthesia is usually signified by a number between 60 and 40 (this varies with the specific system used). There are several monitors now commercially available. These newer technologies include the bispectral index (BIS),[16] EEG entropy monitoring, auditory evoked potentials, and several other systems such as the SNAP monitor and the Narcotrend monitor.
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 electrosurgery). This means that the technology that will reliably monitor depth of anesthesia for every patient and every anesthetic does not yet exist.
A study from Sweden in 2002 attempted to follow up 18 patients for approximately 2 years after having been previously diagnosed with awareness under anesthesia.[17] 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 stated feeling 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. Many who are greatly disturbed report their awareness but anesthesiologists deny it. 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 but these are only untraumatic cases. Some researchers have found that anesthesia awareness does not commonly occur in minor surgeries. It occurs more frequently in more serious surgeries.
A number of mainstream cognitive psychologists have studied memory as a basic cognitive process under anesthesia.
Dr. John F. Kihlstrom of the University of California, Berkeley is one such cognitive psychologists and he has contributed to a book on consciousness with a chapter on anesthesia.[20]
Dr. Chantal Kerssens, Ph.D. of Emory University in Atlanta was trained as a cognitive psychologist (MS, PhD). Her research interests have been in memory function during anesthesia, in particular its relation to depth of sedation.1 She can be observed in a video lecture whose topic is: Neuroimaging Anesthetic Effect on Brain Networks. 2
Professor Jackie Andrade of the Applied Psychology Group at University of Plymouth, U.K. has a special interest in “Priming and awareness during anaesthesia”. 3 An abstract of an article she has written with a title “Unconscious memory formation during anaesthesia” can be assessed from the footnote link. 4
Dr. Phil Merikle, Distinguished Professor Emeritus of the Department of Psychology, University of Waterloo, Ontario, Canada has written and published on the subject of “Memory and Anaesthesia”.5 He coauthored an influential meta-analysis article in this area in 1996 in a journal entitled Consciousness and Cognition.6
Dr. Daniel L. Schacter former Chair, Department of Psychology, Harvard University 7 has been actively concerned with this topic since at least 1990, when he published a book chapter entitled: “Anesthesia, amnesia, and the cognitive unconscious”. In the same year he published a journal article entitled “Implicit and Explicit Memory Following Surgical Anesthesia”. 8
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