The subject of awareness is quite controversial. For instance, the American Society of Anesthesiologists (ASA) practice advisory that is discussed at the end of this chapter fails to acknowledge the subtleties involved and presented here. Part of the controversy stems from the notion that awareness refers to consciousness. Indeed, loss of consciousness is a common clinical term and important intraoperative endpoint. But what exactly is lost when patients lose consciousness? The biologic underpinnings of this mental state, or its counterpart—“to be conscious”—have yet to be determined despite persistent scientific interest and pursuit.¹ For reasons of practicality, one of several definitions may be adopted. Not uncommonly, awareness denotes a conscious subjective experience (“I heard a man talk”).² In clinical anesthesia, however, the term has a distinctly different meaning and refers to patients with memories or recall of the surgical procedure (“I heard the surgeon talk”). Therefore, when discussing awareness during anesthesia, we not only refer to subjective experiences, but also to memory. The significance of this distinction—between the literal meaning of awareness and its use in clinical anesthesia—will become apparent as we discuss monitoring techniques (see Section, “Are There Warning Signs During the Anesthetic That Tell Us Something Is Wrong?”). Adequate monitoring demands proper delineation of the subject matter at hand and raises the question: Is it intraoperative awareness that we are concerned with or postoperative memory? How the two are related but not the same will be elucidated in this chapter.

For now, let us distinguish between awareness without recall and awareness with recall. Although the consequences of awareness without recall are unknown, we have a considerable understanding of the occurrence and phenomenology of awareness with concurrent recall.

As for any complex phenomenon, a single statistic does not say it all. It is important to understand the context in which the data were generated. One context is the clinical setting under consideration because the incidence of awareness with recall depends on intraoperative variables, including cardiovascular responses. We will go into the clinical context more closely when we discuss risk factors (see Section, “Who Is at Risk?”). The second important context is that of standard of care, which constantly changes and improves. Consequently, the estimated incidence of awareness with recall has decreased. Compare, for instance, the incidence reported for trauma patients back in the 1980s (43%)³ and the one observed in a similar population 15 years later (1%).⁴ One explanation for this remarkable improvement relates to advancements in resuscitation in the field, which have aided in the anesthetic management of high-risk cases in the operating room. Hence, modern studies tend to produce lower incidence rates.

Besides improvements in the standard of care, research methods have also become more accurate over time. In earlier studies, for instance, any anecdotal evidence of recall after surgery was taken as evidence of intraoperative awareness.³ This is another explanation for the dramatic incidents that were sometimes reported in the past. Because sedatives tend to distort perception of time,⁵ patients may feel they were aware during the surgical procedure, although their memories were actually formed at other times during the perioperative period. They may confuse hearing voices before or after extubation with having heard voices during surgery. Given the fallibility of subjective reports, in particular when sedatives are involved, we have come to understand that memory must be probed carefully. A proper assessment, therefore, specifically delineates the anesthetic period. In its simplest form, a brief structured interview (see Table 26.1) explores memories pertaining to the intraoperative period, and has been well accepted in both clinical and research communities. By asking patients these five simple, yet specific, questions after recovery, we found a 1% rather than 43% incidence of recall after trauma surgery.⁴

Some studies investigating memory function during anesthesia test for specific stimuli presented during the anesthetic, such as a list of words presented through headphones. After recovery, the same stimuli are presented again, together with a new set of stimuli not presented during anesthesia. By having the study patients respond to both old and new stimuli, various forms of memory are tested. This chapter focuses on “explicit memory,” that is, the conscious recollection of information. Implicit (unconscious) forms of memory are reviewed elsewhere.⁶,⁷

The brief structured interview that addresses conscious recall (Table 26.1) is helpful, as patients are often reluctant to report awareness. If a patient confirms remembering intraoperative events, evidenced by answering “YES” to the third question, different strategies may be adopted, as exemplified and discussed further in section “What To Do?” It is important to note that the five initial questions that assess intraoperative awareness do not lead the subject on, but probe memory in a simple, open, and unbiased manner.

Using this set of questions, the incidence of awareness with recall was recently found to be 0.13% in the United States in a prospective cohort study of approximately 20,000 adult patients in seven academic medical centers across the country.⁸ Patients in the study received general anesthesia, had normal mental status and ability to give informed consent, and could be interviewed after surgery. The anesthetic care was left to the discretion of the attending anesthesiologist who was usually unaware of a patient’s participation in the study. Patients were interviewed first in the postanesthesia care unit (PACU), and a follow-up interview was attempted 1 week later. The investigators recognized that recall of awareness may be delayed, and therefore also undertook the follow-up interview.

In lieu of other large trials,⁹ the investigators classified each individual into one of the following four groups:

In the recovery room, 0.3% of interviewed patients reported remembering something between going to sleep and waking up (YES to Question 3, Table 26.1). During follow-up 1 week later, that number increased (0.6% reported intraoperative memories). Dreaming, by contrast, was reported more frequently in the recovery room (6% YES to Question 4) and decreased at follow-up (3.4%). On the basis of interviews, 25 (0.13%) awareness cases were identified. In all of these cases, the recalled event was confirmed or deemed very likely to have happened.

The incidence of recall after general anesthesia in the United States is comparable to that observed in other countries worldwide.⁹,¹⁰,¹¹,¹² Therefore, awareness with recall appears to be a ubiquitous phenomenon that occurs at an
incidence of 1 to 2 cases per 1,000, irrespective of geographic location and potential differences in anesthetics and techniques. With approximately 20 million general anesthetics administered in the United States annually, approximately 26,000 cases of awareness with recall can be expected to occur each year, or 100 per work day.

In contrast to those of us who consider the number presented above sufficiently high to warrant continued efforts to prevent awareness, others feel that postoperative recall is relatively rare and not worth the trouble. If you feel the same way, keep in mind that many patients are concerned about this potentially adverse outcome. Before their anesthetic, up to 54% worry about the possibility of pain, paralysis, and mental distress during surgery.¹³ Furthermore, when suffered, awareness is a major source of patient dissatisfaction.¹⁰ This is not surprising given the subjective feelings and experiences of individuals who were aware during their surgery. Although not all accounts of intraoperative awareness are necessarily horrific, the following review of patient recollections represents a typical frame of reference.

Patients with recall of awareness are most likely to remember sounds and conversations (30% to 90%), as opposed to seeing, feeling, and smelling things.¹⁴ A significant number (up to 40%) may remember being in pain, an experience that mediates adverse aftereffects. Interviews suggest that it is not necessarily the awakening itself that distresses patients most, but rather the inability to move or communicate (“awake paralysis”). Even when pain is not experienced, the complete lack of control gives rise to feelings that worse is yet to come. Most patients who wake up paralyzed (70% to 90%) panic and experience anxiety, whereas half feel helpless and powerless. This situation is compared to being buried alive, with mental trauma mounting when pain is added. A minor proportion (15%) may experience suffocation, impending death, or believe they are in a coma and will not emerge from the anesthetic. Approximately two thirds of patients report a change in their attitude toward anesthesia after having experienced awareness.

Up to 70% of patients with recall of awareness experience unpleasant sequelae, including sleep disturbances, recurrent nightmares, flashbacks, daytime anxiety, and distress. This group is also most likely to change its opinion about anesthesia and become more afraid and apprehensive about anesthetic procedures. Patients describe staying away from hospitals and doctors to avoid being reminded of the traumatic event, a situation that compromises treatment compliance and success. When symptoms persist 1 month and profoundly affect the way in which people feel, behave, and function, a posttraumatic stress disorder (PTSD) may have developed (see Table 26.2). Depending on the awareness experience and reaction of hospital staff, 15% of those with recall require some form of psychotherapy, whereas 10% develop PTSD.¹⁴,¹⁵ Even years after the event, 50% of those affected may still suffer from the experience and be severely disabled because of psychiatric sequelae.¹⁶ Therefore, in spite of its relatively infrequent occurrence, recall of awareness can be a life-changing event for the patient. In the section, “What To Do?” suggestions are offered for what you can do, intraoperatively and postoperatively, when you suspect awareness.

It is important to note that adverse sequelae are primarily associated with the use of neuromuscular blocking drugs. Of the 14 patients in a Swedish study who experienced awareness and had been given a muscle relaxant, 11 reported unpleasant aftereffects.⁹ In comparison, none of the four nonparalyzed patients that reported awareness experienced postoperative sequelae.

In the American incidence study, awareness with recall was associated with a surgical procedure (abdominal/thoracic, cardiac, and ophthalmology vs. others) and sicker patients (American Society for Anesthesiologists’ Physical Status Classification III to V [ASA III-V]).⁸ The latter finding probably reflects the use of lower anesthetic doses in such patients, a common cause of awareness with recall. A higher incidence of awareness has been reported for cesarean section, and cardiac and trauma surgery, ranging from 1% to 4%.¹⁷,¹⁸,¹⁹

The use of neuromuscular blocking agents clearly increases the occurrence of recall. In the Swedish incidence study, twice as many patients developed memories of awareness episodes when paralysis was maintained throughout the surgical procedure, as opposed to general anesthesia without relaxants.⁹ When no neuromuscular blocking agents are used, patients can signal awareness through movement, and the anesthetic may in turn be deepened. On the basis of research findings such as these,
vigilance and the judicious use of muscle relaxants is advised. These relaxants should be administered only when absolutely necessary, for instance with endotracheal intubation and during ventilation. However, avoiding neuromuscular blockade does not necessarily prevent awareness. For instance, some patients will not attempt to move despite their awareness of being awake.

Whereas the use of neuromuscular blocking drugs affects the incidence of awareness with recall, benzodiazepines do not. Because of their sedative properties, these drugs are valuable for relieving preoperative anxiety, and therefore are commonly used. Because of their amnestic properties, they are also often used to prevent patients from having unpleasant memories. Benzodiazepines have a strictly anterograde effect, meaning that the drugs specifically interfere with the creation of new memories rather than disrupting old ones.²⁰ An individual on benzodiazepines may accurately perceive and process information, and may respond appropriately to commands or questions, but very little if anything of the experience will be stored in memory. This anterograde amnesia for events after the drug is given may be desired when an adverse event (e.g., pain) is anticipated. Indeed, the prophylactic use of benzodiazepines is not uncommon, and only few ponder about the ethical issue it raises: Where did the adverse event go if not into memory?²¹ Contrary to its established effect, however, benzodiazepines are often given after an adverse event to ameliorate unpleasant memories. Retrograde targeting of memory is useless given the drugs’ pharmacokinetics and may explain why there is no evidence in the awareness literature to suggest that benzodiazepines or scopolamine are any more successful than ordinary anesthetics in inducing amnesia. Similar to ordinary anesthetics, benzodiazepines may impair memory by inducing sedation, but no studies suggest that either class of drugs directly interferes with memory that is already formed. Even when given before or upon induction of anesthesia, benzodiazepines do not reduce the incidence of awareness with recall.⁹,²²

The ASA evaluated more than 4,000 nondental claims from a number of American insurance companies for adverse anesthetic outcomes that occurred between 1961 and 1995.¹⁵ Seventy percent of those claims were filed between 1980 and 1990. Awareness accounted for a small proportion (2%) of claims, which were split into the following two categories:

Most claims for awake paralysis were found to be related to intravenous infusion errors (56%) or syringe swaps (44%): Bags or syringes containing neuromuscular blocking drugs were unlabeled, mislabeled, or properly labeled but not checked before administration. The periods of highest vulnerability were the preinduction and induction periods when a muscle relaxant was given instead of a hypnotic or sedative agent. The analysis confirmed that some practitioners injected a benzodiazepine after the muscle relaxant in an unsuccessful attempt to achieve retrograde amnesia. Expert reviewers judged most of the awake paralysis cases (94%) to represent substandard care and, accordingly, settlement payments and jury award were often granted (78%). The report did not address sequelae of awake paralysis.

Recall in comparison was mostly likely to involve the maintenance phase of anesthesia (80% to 85%), and a number of contributing factors were identified (see Table 26.3). Here, the anesthetic care was judged substandard in fewer cases (43%), and payments were granted approximately half the time. Eighty-four percent of recall plaintiffs sustained temporary emotional distress, whereas 10% had developed PTSD.

The closed-claims project further showed that, independent of standard of care, anesthetic techniques that rely on intraoperative opioids, muscle relaxants, and no or low volatile anesthetic concentrations substantially increase the frequency of (legal) claims for recall after general anesthesia.¹⁵ Other variables such as age, ASA status, anesthesia personnel, use of benzodiazepines, different induction technique, and inhalational agents or intravenous anesthesia, generally do not affect the occurrence of recall. Female gender was also associated with an increased frequency of recall claims, although it is unclear whether this reflects an intrinsic higher risk that is possibly associated with sex differences in drug metabolism²³ or a greater propensity for women to file suit. Failure to demonstrate gender differences in large prospective awareness trials⁸,⁹ supports the latter.