Awareness during Anesthesia

Abstract

MEMORY is not a single entity. Current classification distinguishes between two types: explicit, or conscious memory, and implicit, or unconscious memory. Explicit memory refers to the conscious recollection of previous experiences. Implicit memory , by contrast, refers to changes in performance or behavior that are produced by previous experiences but without any conscious recollection of those experiences. 1Explicit memory is equivalent to “remembering” (e.g. , can you remember what you did last Tuesday evening?). In the case of an anesthetized patient, one might ask the patient in the postoperative period “can you remember hearing any words or sounds during your operation?” (i.e. , a recall test) or “which of the following words were played to you during surgery?” (a recognition test). As an example of implicit memory, consider the following scenario. Patients were exposed during anesthesia to a list of words containing the word “pension.” Postoperatively, when they were presented with the three-letter word stem “PEN___” and were asked to supply the first word that came to their minds beginning with those letters, they gave the word “pension” more often than “pencil” or “peninsula” or others. The term “awareness” will be used to describe explicit memory during anesthesia.The incidence of awareness during anesthesia and surgery is best estimated by formally interviewing patients postoperatively. Patients, particularly those who were not unduly disturbed by their experiences, may not voluntarily report them without being asked directly. 2Some patients also may not recall events shortly after surgery but may recall them 1–2 weeks after surgery. 3It is also possible that some patients may need a detailed and extensive interview to “jog” their memories for the intraoperative experience. 4When a structured interview was used in a large series of patients, the incidence of awareness in nonobstetric and noncardiac surgical cases was 0.2%. 2The incidence is greater, however, when light anesthesia is used: the incidence in cardiac surgery ranges from 1.1–1.5%. A higher incidence also has been reported for obstetric cases (0.4%) and major trauma cases (11–43%), and this incidence varies according to the dose of anesthetic administered. 5The incidence of intraoperative recall is 2% when 70% nitrous oxide (N2O) alone is used for anesthetic maintenance. 6This incidence is not reduced by intermittent boluses of opioids. 4Nevertheless, Jones 7estimates that only 0.01% of patients report pain while being aware. It is probable that opioids, together with N2O, provide sufficient analgesia to decrease complaints from pain. Some or all of the quoted estimates about the incidence of awareness may need to be updated.Patients also may recall dreams, some of which are disturbing. Dreams may be recalled more often than actual events. For example, in Utting’s 6series of 500 patients anesthetized with N2O, the incidence of dreams that the patients considered to be the worst features of their perioperative experiences was 7%versus 2% for recall of intraoperative events. Utting 6proposed a continuum, with adequate anesthesia resulting in complete amnesia, lighter anesthesia resulting in dream recall, and still lighter anesthesia resulting in explicit recall.Awareness usually is associated with one of three situations (fig. 1):The pain was like that of a tooth drilled without local anesthetic—when the drill hits a nerve. Multiply this pain so that the area involved would equal a thumb-print, then pour a steady stream of molten lead into it.This quotation is from a physician describing her almost unbearable pain when she was aware during a cesarean section. 11There is no doubt that pain during surgery is the most distressing feature of awareness. Other frequent complaints are the ability to hear events during surgery, sensations of weakness or paralysis, and feelings of helplessness, anxiety, panic, and impending death. The patients also may be tormented, if they were in an obtunded state, by doubts about whether what they experienced really happened or whether there is something wrong with their minds 12(fig. 1).For some patients, awareness may cause temporary after effects, including sleep disturbances, nightmares, and daytime anxiety, which subside eventually. Nevertheless, there may remain a fear that awareness may happen again if they require anesthesia in the future. In some patients, post-traumatic stress disorder develops, marked by repetitive nightmares, anxiety, and irritability; a preoccupation with death; and a concern with sanity that make the patients reluctant to discuss their symptoms. 12It is not apparent why post-traumatic stress disorder develops in some patients and not in others. Patient personality, predisposition to mental disease, emotional response to illness, and reason for surgery may be factors.There are also medicolegal consequences to awareness. Domino et al. 13recently analyzed claims from the American So