Awareness Under Anesthesia

During interventional/therapeutic procedures, patients may receive drugs that affect the central nervous system (CNS) along a continuum from the awake state to general anesthesia. Dependent upon the nature of the procedure and patient wishes, anesthetic drugs may be given with the intent to produce different levels of sedation or general anesthesia. Patients consenting to receive drugs with the intent to produce only sedation should do so with the understanding that they may have recall of intraoperative events. Conversely, a fundamental component of general anesthesia is unconsciousness and subsequent amnesia. Patients consenting for general anesthesia do so with the expectation that they will not see, hear, feel, or remember intraoperative events.

Amnesia has been a fundamental tenet of training and continuing medical education in anesthesia. Yet, many patients who undergo general anesthesia report preoperative fears of intraoperative awareness, and awareness has been the most important cause of patient dissatisfaction with anesthesia.


Memory consists of explicit, or conscious, memory and implicit, or unconscious, memory. Explicit memory refers to the conscious recollection of previous experiences and is equivalent to remembering. Awareness during anesthesia describes conscious recall (explicit memory) of intraoperative events. However, many more anesthetized patients may respond to commands, yet lack conscious recall of intraoperative events (implicit memory). The anesthetic depth required to block implicit memory is greater than that required to block explicit memory (intraoperative recall).

Intraoperative awareness is best estimated by formally interviewing patients postoperatively, well after discharge from the postanesthesia recovery room. Moreover, memory formation for intraoperative awareness may be delayed beyond the immediate recovery period. Sandin and associates reported that only one third of cases of awareness were identified before the patient left the postanesthesia care unit. Often, patients will not voluntarily report awareness if they were not disturbed by it, if embarrassed to do so, or if coping with pain and recovery after major surgery. Therefore, a structured interview (i.e., modified Brice interview) is recommended to evaluate the incidence of awareness :

  • 1.

    What was the last thing you remember before you went to sleep?

  • 2.

    What is the first thing you remember after your operation?

  • 3.

    Can you remember anything in between?

  • 4.

    Can you remember if you had any dreams during your procedure?

  • 5.

    What was the worst thing about your procedure?

The methodologies used to assess the incidence of intraoperative awareness are inconsistent, and the results have predictable variation ( Table 47.1 ). In prospective studies when the Brice interview was used, intraoperative awareness occurred with surprising frequency (1 to 2 per 1000 or greater). The first prospective evaluation of awareness in nearly 12,000 patients undergoing general anesthesia was conducted in Sweden and revealed an incidence of awareness of 0.18% in cases in which neuromuscular blocking drugs were used and 0.10% in the absence of such drugs, for an overall incidence of 0.13% (see Table 47.1 ). A similar incidence (1 per 1000 patients) was observed in the United States in tertiary care centers. Patients with coexisting morbid conditions tend to have a greater incidence of awareness. Intraoperative awareness and subsequent recall are more likely with a light level of anesthesia, such as occurs during obstetric and cardiac anesthesia. The incidence of awareness is underestimated when assessed using quality improvement and patient self-reporting (see Table 47.1 ). In 2013, Mashour and associates compared the incidence of awareness in patients who received a standard postanesthesia evaluation to those who received a single modified Brice survey. They found 19 cases of intraoperative awareness out of 19,000 patients were detected by the Brice survey, whereas only 3 were detected by spontaneous report. Significantly, the 3 reported spontaneously were also detected by the Brice survey. It is likely that many patients may have intraoperative awareness that is not detected.

Table 47.1

Reported Incidence of Intraoperative Awareness

Incidence N a Prospective Design Reference b
0.007% 384,786 No 10
0.1% 10,811 No 2
0.13% 18,575 Yes 8
0.15% 11,785 Yes 3
0.2% 1000 Yes 7
0.23% 44,006 No 9
0.41% 11,101 Yes 6
0.6% 4001 Yes 5

a Numbers of patients in reported series.

b Numbers correspond to references listed at the end of the chapter.

Etiology and Risk Factors for Intraoperative Awareness

The three major causes of intraoperative awareness of anesthesia are light anesthesia, increased patient anesthetic requirements, and anesthetic delivery problems. Inadequate anesthesia due to reduced anesthetic doses generally occurs because of hemodynamic intolerance of anesthetic drugs or during procedures in which the anesthetic dose is kept deliberately small, such as in cesarean delivery or open heart surgery. Reduced anesthetic doses may be necessary for optimal physiology and safety in patients who are hypovolemic or have limited cardiac reserve. Patients with American Society of Anesthesiologists (ASA) physical status 3 to 5 undergoing major surgery are at increased risk for intraoperative awareness and indeed have a higher incidence of awareness. Patients who experienced intraoperative awareness are more likely to have impaired cardiovascular status, undergo emergency surgery, receive smaller doses of volatile anesthetics, and have experienced an anesthetic with technical difficulties. Children are also more likely to experience awareness.

Anesthetic technique is important in the pathogenesis of awareness during anesthesia. Intraoperative awareness is more likely to occur during induction of anesthesia with nitrous oxide and intravenously administered anesthetics and is less likely to occur when volatile anesthetics are used. Use of volatile anesthetics in concentrations at or above 0.7 MAC (minimum alveolar concentration) prevents conscious recall in anesthetized patients similar to that achieved by a brain function monitor of anesthetic depth. Unfortunately, neuromuscular blockade prevents an early sign of inadequate anesthesia, namely patient movement. Smaller anesthetic concentrations are needed more to prevent awareness than to render immobility; therefore, an inadequately anesthetized, nonparalyzed patient usually moves first and demonstrates clear evidence of inadequate anesthesia.

Some patients, such as those using alcohol, opioids, amphetamines, and cocaine, may require an increase in anesthetic dose. Moreover, although incompletely defined, genetic factors may influence anesthetic requirements. Patients with a past history of awareness are more likely to experience awareness compared to patients without a past history of awareness. Finally, equipment problems with the vaporizer or intravenous infusion devices may lead to awareness, although these are less common causes of awareness, especially with use of end-tidal anesthetic gas analysis.

Psychological Sequelae

Awareness under general anesthesia can be a traumatic experience, with approximately one third of patients experiencing long-term psychological sequelae. However, some patients do not develop long-term psychological sequelae after intraoperative awareness and many patients without awareness have psychological symptoms consistent with posttraumatic stress disorder (PTSD). Some of the most common recalled awareness experiences include auditory sounds, feelings of paralysis, seeing lights, and feelings of helplessness, fear, or anxiety. Pain is less common, although it does occur in some patients, particularly those with complete neuromuscular blockade who are unable to move. Psychological sequelae of recalled memories may include flashbacks, anxiety/nervousness, loneliness, nightmares, and fear/panic attacks that vary from bothersome to distressing. Some patients develop severe, persistent symptoms that profoundly interfere with interpersonal relationships and daily activities.

The risk factors for developing PTSD after awareness during general anesthesia are not completely known. An acute emotional reaction to the experience significantly predicted the development of long-term psychological sequelae. Dissociation related to surgery and perceiving that one’s life was threatened were associated with PTSD. Paralysis from neuromuscular blockade is particularly traumatic. The role of premorbid depression and other psychological conditions is unclear, but may contribute to risk of PTSD. Recurrence of trauma can trigger previous psychological symptoms.

More reporting from patients also increases the understanding of the experiences. In 2007, the ASA established the Anesthesia Awareness Registry to address the concerns of patients with regard to intraoperative awareness. The Registry collected patient self-reports of unintended awareness during general anesthesia to provide a patient perspective on their expectations and experiences of awareness. The Registry was designed to be consistent with “patient-centered” care and focused on patient preferences, needs, and values. Although the Registry relied on patients to volunteer to participate, and therefore had response bias, the results are valid to point out causes and possible solutions to patient dissatisfaction with unexpected recall during surgery.

One finding from the Registry is that patients may have different expectations than anesthesia providers concerning the lack of explicit recall during regional anesthesia or sedation. The Anesthesia Awareness Registry recruited patients who self-identified as having awareness during general anesthesia. However, upon review of the perioperative records, one third of patients had mistakenly believed that they received general anesthesia; instead, they actually received sedation or regional anesthesia. This result shows a disconnect between anesthesia providers’ and patients’ expectations concerning unconsciousness during surgery. This disconnection may be resolved by improved physician-patient communication concerning the possible recall of events during sedation as well as improved informed consent.

Patients also complain of psychological sequelae after awareness during regional anesthesia or sedation. Some patients may experience psychological consequences because of explicit recall of events during regional anesthesia that were similar to consequences resulting from recall during general anesthesia. Approximately 40% of these patients had persistent psychological sequelae, similar to those with awareness during general anesthesia.

Early psychotherapeutic intervention may reduce the likelihood of acute and long-term psychological sequelae. An explanation or validation of the awareness incident may affect the presence and duration of the psychological consequences. However, if patients do not inform their anesthesia provider of their recall from general anesthesia, they are less likely to know that they should seek psychological therapy.

The Anesthesia Awareness Registry found that most (75%) of the patients with awareness during general anesthesia were dissatisfied with the manner in which their concerns were addressed by their health care providers. Half of the patients reported that neither their anesthesia provider nor the surgeon expressed concern about their awareness experience. Few were offered an apology (10%) or referral for counseling (35%), an explanation (28%), or discussion or follow-up to the awareness episode (26%). Several patients mentioned they were too ill to care about their awareness experience while acutely recovering from surgery or their memory became clearer after days and weeks following surgery. Some patients recommended that anesthesia providers give them a business card to facilitate contact after hospital discharge. Clearly, patients need more systematic responses and follow-up by health care providers.

Prevention of Awareness

Conventional monitoring of anesthetic depth has included rudimentary signs such as patient movement, autonomic changes, tearing, perspiration, and subjective clinical instinct. Autonomic changes, such as an increase in arterial blood pressure and heart rate, do not reliably predict intraoperative awareness. Indeed, intraoperative awareness can occur in the absence of tachycardia or hypertension. With the advent of anesthetic gas analyzers, anesthetic depth has also been assessed by surrogate data such as determining the dose of volatile anesthetic administered to the patient. In addition, considerable effort has been devoted to establishing a monitor that will reliably determine a patient’s depth of anesthesia and hence the risk for intraoperative awareness. Several different devices are commercially available, yet none is 100% effective. These monitors typically collect spontaneous or evoked brain electrical activity, and then process the raw data by a proprietary algorithm and display data to the clinician as a quantitative data point (e.g., number from 0 to 100).

At present there are at least three inherent obstacles to the development of a “foolproof” monitor of anesthetic depth based on electrical activity of the brain and its ability to detect intraoperative awareness. First, at present we have not comprehensively validated a unitary mechanism of general anesthesia, and thus various anesthetics are likely to produce unique electrical activity at a given anesthetic depth. Consequently, a unique algorithm to each specific anesthetic regimen would likely be required for optimal correlation between electrical signals in the brain and anesthetic depth. Second, general anesthesia occurs on a continuum without a quantitative dimension, and there is considerable interpatient pharmacodynamic variability to a specific anesthetic. Attempting to translate a conscious or unconscious state into a quantitative number can at best be limited to the art of probability with an expectation of false positive and false negative data ( Fig. 47.1 ). Finally, there is the likelihood of cortical electrical activity having sensitivity and specificity to a biochemical event that occurs at a distant subcortical structure (hippocampus) that forms memory ( Fig. 47.2 ).

Fig. 47.1

Probability of adequate hypnosis based upon brain function monitoring index. The straight line is the ideal probability curve with 100% sensitivity and specificity. The curved line is a more realistic expectation of monitoring in which a progressive decrease of the monitored index value correlates with increased probability of adequate hypnosis.

From Cole DJ, Domino KB. Depth of anesthesia: clinical applications, intraoperative awareness and beyond. In Schwartz AJ, ed. ASA Refresher Courses in Anesthesiology. Philadelphia: Lippincott, Williams & Wilkins; 2007.

Fig. 47.2

A brain function monitor typically records cortical electrical activity from a sensor placed on the forehead. Memory is a biochemical function that occurs in the hippocampus, which is some distance from the recording of brain electrical activity.

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Oct 21, 2019 | Posted by in ANESTHESIA | Comments Off on Awareness Under Anesthesia
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