Awareness occurs in 20,000-40,000 patients out of every 20 million US surgeries every year when patients have anesthesia that is inadequate to keep them unconscious during an operation. 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. If anesthesia awareness does occur about 42% feel the pain of the operation, 94% experience panic/anxiety and 70% experience lasting psychological symptoms.
In some cases, post traumatic stress disorder (PTSD) may arise after intraoperative awareness, causing the patient to require counseling for an extended period.
There are two states of consciousness which may be present:
The incidence of a state with both responses in diverse degrees is also possible.
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 or not at all.
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.
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 minimal 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 neuroaxial 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.
Patients who undergo conscious sedation or monitored anesthesia care are never meant to be without recall. Whether or not a patient remembers the procedure depends on the type of medications uses, the dosages used, patient physiology, and other factors. Many patients undergoing monitored anesthesia care do not remember the experience.
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 to 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 he/she 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.
A common risk factor is the use of a medication that induces muscle paralysis. Under general anesthesia, the patient's muscles may be paralysed 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.
A fully paralyzed patient is unable to move, speak, blink the eyes, or otherwise respond to the pain except through physiological signs such as increased heart rate (tachycardia), blood pressure (hypertension), dilation of the pupils (mydriasis), sweating (diaphoresis), and the formation of tears (lacrimation) in response to pain. This is because these paralytic drugs cause skeletal muscle paralysis but do not typically interfere with the functioning of the autonomic nervous system. Even though the patient cannot directly signal their distress, they may exhibit signs of awareness detectable by clinical vigilance.
Many types of surgery do not require the patient to be paralysed. A patient who is anesthetised but not paralysed can move in response to a painful stimulus if the analgesia is inadequate. This can serve as a warning sign that the anesthetic depth is inadequate. However, "moving" under general anesthesia does not necessarily correlate with awareness, nor does "not moving" under general anesthesia necessarily correlate with amnesia.
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 patient. 'Light' anaesthesia means less drugs by the intravenous route or via inhalational means, leading to less cardiovascular depression (read hypotension) but, unfortunately, causing 'awareness' in the anesthetized subject.
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.
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.
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). There are several monitors now commercially available. These newer technologies include the bispectral index (BIS), 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 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. Anesthesia awareness is often discussed at anesthesiology meetings.
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. Some researchers have found that anastesia awareness does not commonly occur in minor surgeries. It occurs more frequently in more serious surgeries.