Awareness Under Anesthesia

Abstract

You begin to hear distant rock music playing, people conversing about their weekend. Then the scalpel is requested—incision. Pain sends you reeling and you attempt to muster a scream. No one seems to hear you and you are unable to lift a finger. The scenario is so rare that numerous movies have been made about awareness under anesthesia. Awareness under anesthesia is a rare event, however, it is a complication that no one ever desires to occur. We will explore how frequent awareness is during surgery, what risk factors are involved, and what tools anesthesia providers utilize to ensure everyone undergoing surgery is adequately anesthetized.

History of Anesthesia

In what may have been the first recorded use of an anesthetic, the ancient Greek god Apollo demonstrated successful use of herbal medicines to achieve hypnosis, anesthesia, and amnesia.¹ Providing sleep was a divine process entrusted to only a few mortals that aimed to shepherd ancient Greeks through their most troubling ailments.² The modern anesthesiologist now takes on a similar responsibility of ferrying patients across this perioperative journey—from their preoperative optimization to their postoperative recovery.

Figure 1.

Oksana Tevis, A. A. with BIS monitor. St. Louis, MO, Feb 2022. Used with permission.

Despite the truly amazing achievements that have made undergoing anesthesia in the United States exceptionally safe, adverse events continue to plague clinical care as a whole.³ One of the most common concerns patients report when undergoing general anesthesia is the fear of being conscious while paralyzed during surgery—unable to move or notify their anesthesiologist of their awareness.

The risk for such an occurrence is thankfully rare. One study reviewed three million cases of anesthesia delivery in the United Kingdom and found that one in 19,000 had some degree of accidental awareness, ranging from relatively benign memories of fleeting ambient sounds to recalling full physical sensations.⁴ More commonly, aggregate studies project the risk to be about one to two in 1,000.⁵ The gravity of such a horrifying event, as rare as it may be, is enough to percolate through news media, popular culture, and social media, and in turn, terrify prospective patients.

Difference Between Anesthesia and Sedation

Several different types of anesthesia exist and may be utilized to assist a patient through their procedure as delineated in the American Society of Anesthesiology Continuum of Depth of Sedation table. It is important that the patient thoroughly understands the type of anesthesia they will receive and what exactly that means. Anesthesia and sedation are a spectrum. Many procedures do not require general anesthesia and can be done with sedation. Sedation implies that a patient is not completely “asleep” (even though sleep is not exactly the proper term). There are several depths of sedation, and the goal level depends upon several factors, especially the discomfort involved. During propofol sedation, such as for a colonoscopy, a patient could wake up, look around, and remember things from the procedure. All of this is to be expected because the patient is only receiving sedation. It is imperative patients are informed beforehand of when such instances can be expected.⁶ On the other hand, anesthesia awareness refers to when the patient is undergoing general anesthesia with a breathing device in place such as an endotracheal tube or a laryngeal mask airway (LMA). Accidental awareness intraoperatively during general anesthesia has serious long-term psychological sequalae and has resulted in post-traumatic stress disorder and other deep psychological trauma for patients and their families.⁷

Risk Factors

Patient and procedural risk factors that heighten the risk for accidental awareness are fortunately well-known. Risk factors include agents that desensitize the brain to sedatives, including chronic alcohol abuse, benzodiazepine use, as well as a history of long-term opioid dependence (Table 1). The method of delivery of general anesthesia may also increase risk. General anesthesia can be achieved by administration of volatile inhaled agents or by intravenous infusion. The former allows for monitoring of exhaled gas concentrations to ensure the patient is adequately asleep. Each inhaled agent has a minimum alveolar concentration (MAC) for different levels of anesthesia, such as MAC awake, MAC for movement with painful stimulus, MAC-BAR (blocking the autonomic response to pain). The intravenous method poses an increased risk of unintended awareness because of the inability to easily monitor sedative concentrations in the blood. Certain surgeries also increase the incidence of awareness, such as cardiovascular bypass surgeries and emergency procedures like trauma and general anesthesia for Cesarean sections, where the hemodynamics dictate a lighter anesthesia to maintain circulation.⁸ It is not uncommon for all these risk factors to be compounded in a single patient.

Table 1.

Risk Factors for Anesthesia Awareness

Chronic Drug Use (opioids, benzodiazepines, alcohol)

Cardiopulmonary Bypass

Emergency Surgery

General Anesthesia for Cesarean Section

Trauma Patients

Despite being completely unconscious and asleep, patients undergoing general anesthesia maintain some degree of muscular tone, and their reflexes to pain or intense stimulation remain intact. Without a means to inhibit these reflexes, the patient—unbeknownst to them—may move with stimulation. Therefore, for many procedures, paralyzing agents are requested for surgical exposure. One of the most influential—and modifiable—risk factors for awareness is the use of neuromuscular paralytic agents. The incidence of accidental awareness increases significantly when these drugs are administered.⁴ However, up to 40% of surgical cases require paralytic. Evidence shows that the risk of awareness is minimized with extra vigilance in intraoperative monitoring. The American Society of Anesthesiology elucidated a minimum standard for monitoring patients under any anesthetic (Figure 1). These include a patient’s oxygenation, ventilation, circulation, and temperature.⁹ Basic physiologic parameters, like heart rate and blood pressure, are helpful factors to determine unconsciousness. However, it is not uncommon that sympathetic stimulation can cause tachycardia and hypertension even with adequate anesthetic depth.¹⁰ This scenario can mimic a patient having awareness, when they are in fact adequately anesthetized.

Figure 1.

Continuum of Depth of Sedation: Definition of General Anesthesia and Levels of Sedation Analgesia

Minimal Sedation (Anxiolysis) is a drug-induced state during which patients respond normally to verbal commands. Although cognitive function and physical coordination may be impaired, airway reflexes, and ventilatory and cardiovascular functions are unaffected.

Moderate Sedation/Analgesia (“Conscious Sedation”) is a drug-induced depression of consciousness during which patients respond purposefully** to verbal commands, either alone or accompanied by light tactile stimulation. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate. Cardiovascular function is usually maintained.

* Monitored Anesthesia Care (“MAC”) does not describe the continuum of depth of sedation, rather it describes “a specific anesthesia service performed by a qualified anesthesia provider, for a diagnostic or therapeutic procedure.” Indications for monitored anesthesia care include “the need for deeper levels of analgesia and sedation than can be provided by moderate sedation (including potential conversion to a general or regional anesthetic.”1

** Reflex withdrawal from a painful stimulus is NOT considered a purposeful response.

Deep Sedation/Analgesia is a drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully** following repeated or painful stimulation. The ability to independently maintain ventilatory function may be impaired. Patients may require assistance in maintaining a patent airway, and spontaneous ventilation may be inadequate. Cardiovascular function is usually maintained.

General Anesthesia is a drug-induced loss of consciousness during which patients are not arousable, even by painful stimulation. The ability to independently maintain ventilatory function is often impaired. Patients often require assistance in maintaining a patent airway, and positive pressure ventilation may be required because of depressed spontaneous ventilation or drug-induced depression of neuromuscular function. Cardiovascular function may be impaired.

Because sedation is a continuum, it is not always possible to predict how an individual patient will respond. Hence, practitioners intending to produce a given level of sedation should be able to rescue*** patients whose level of sedation becomes deeper than initially intended. Individuals administering Moderate Sedation/Analgesia (“Conscious Sedation”) should be able to rescue*** patients who enter a state of Deep Sedation/Analgesia, while those administering Deep Sedation/Analgesia should be able to rescue*** patients who enter a state of General Anesthesia.

** Reflex withdrawal from a painful stimulus is NOT considered a purposeful response.

*** Rescue of a patient from a deeper level of sedation than intended is an intervention by a practitioner proficient in airway management and advanced life support. The qualified practitioner corrects adverse physiologic consequences of the deeper-than-intended level of sedation (such as hypoventilation, hypoxia and hypotension) and returns the patient to the originally intended level of sedation. It is not appropriate to continue the procedure at an unintended level of sedation.

Source: American Society of Anesthesiologists. Position on Monitored Anesthesia Care. Last amended on October 17, 2019. Used with permission.

Monitoring General Anesthetics

No one standard monitor measures the target organ for all general anesthetics that aim to induce hypnosis and amnesia - the brain. With the advent of more advanced intraoperative electroencephalographic (EEG) monitoring in the past two decades, anesthesiologists can attempt to titrate depth of anesthesia by analyzing EEG data intraoperatively. The article in Missouri Medicine last year entitled Awareness with Recall: Review and Guide to Electroencephalographic Depth-of-Anesthesia Monitoring so nicely outlined the mechanism of the Bispectral Index (BIS) device and its utility in EEG monitoring and assisting in shedding light on the anesthetic depth continuum.¹¹ This monitor processes the patient’s raw EEG signals, and through statistical modeling, computes a scaled 0–100 BIS score, with higher values denoting “awake” states and lower values defined as “asleep.”

Large randomized controlled trials, nevertheless, have not shown use of the BIS monitor to be superior in preventing accidental awareness compared to heightened vigilance in monitoring exhaled concentrations of anesthetics.^12,13,14 These studies demonstrate how vigilance and setting appropriate alarms in the operating room can be an effective means to minimize the incidence of intraoperative awareness. Yet, the utility for monitors like the BIS may still lie in helping minimize other neurological sequalae from surgery such as postoperative cognitive dysfunction. There are numerous studies that suggest some benefit to utilizing EEG monitors such as BIS to target the anesthetic depth to a lighter plane of anesthesia, with the hope of decreasing postoperative neurologic complications. One study showed a reduction in postoperative delirium and decreased cognitive impairment at one year.¹⁵ However, a randomized clinical trial failed to show any reduction in postoperative delirium, which is seen more acutely following surgery. They did not look at postoperative cognitive dysfunction however, which is seen from one week up to one year following surgery.¹⁶ Another interesting study utilized the BIS preoperatively in addition to other tools such as the Mini-Mental State Exam, to help establish risk of postoperative delirium and cognitive decline after surgery.¹⁷ Further studies are needed to assist in guiding clinical practice for depth of anesthesia and reducing effects on cognitive decline in our aging population.

Brain Monitoring

Monitoring the brain is strongly advocated when using total intravenous type of anesthesia, to ensure patients are adequately anesthetized. In this scenario, EEG data provides vital information about depth of anesthesia, because again, there is no end tidal gas concentration available. The utility of the BIS monitor has also been questioned in pediatric patients. It has been shown that repeated exposure to anesthetics in children under three years of age could negatively affect development.¹⁸ This knowledge adds motivation to try to spare the amount of anesthetic the pediatric patients receive whenever possible. There have been a few studies showing some benefit in using BIS for pediatric patients over age three,¹⁹ while other studies have not shown any difference in anesthesia consumption.²⁰ Other investigators suggest that the BIS monitor is not effective at all in the younger pediatric population due to poor correlation with the BIS reading and end tidal sevoflurane levels, and that a different monitor all together may be more effective for younger patients.²¹ Larger scale studies are needed to further delineate the role of BIS in children. Regardless, interpreting basic EEG waveform analysis, and recognizing sleep versus awake EEG waveforms will only further enhance the provider’s ability to quickly and effectively determine when a patient may be approaching an unintended conscious state.

Conclusion

In conclusion, anesthesia has never been safer than the present day. Protecting the patient’s mental well-being is just as important as ensuring their hemodynamic stability. By maintaining adequate depth of anesthesia and being diligent to ensure patients are in fact anesthetized, patients can rest assured that they will be well cared for during such a critical time. Medicine is an ever-changing landscape, and the pursuit of improved monitors for our most complex organ, the brain, is paramount.