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How Likely Is Awareness During Anesthesia?

Eger, Edmond I. II MD; Sonner, James M. MD

doi: 10.1213/01.ANE.0000151467.79077.08
Letters to the Editor: Letters & Announcements

Department of Anesthesia and Perioperative Care; University of California, San Francisco; San Francisco, CA;

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To the Editor:

The report by Sebel et al. (1) provides data on an issue of great concern to anesthetists. In a prospective study using a structured interview, the investigators identified 25 cases of awareness during anesthesia in 19,575 patients for an incidence of 0.13%. In support of their finding, this incidence parallels the incidence found in several other similar studies cited by the authors. They added that their estimate of the incidence was relatively conservative and suggested the possibility that the true incidence might be closer to 0.36%. We suggest that a contrary interpretation is possible.

The investigators appear to define awareness by both objective and subjective criteria: the “recalled event was confirmed by attending personnel (objective criteria), or the investigators were convinced that the memory was real, but no confirmation could be obtained (subjective criteria).” Awareness during anesthesia is difficult to certify objectively. In our view it requires remembrance of an event by the patient that uniquely connects that remembrance to an event unequivocally occurring during anesthesia/surgery. The report by Saucier et al. supplies such an example (2). In the study by Sebel et al., the patient who “heard the doctor ask for a stent which was identified by a number” meets this criterion, assuming that the surgeon corroborated the remembrance. However, the patient who said she had an out of body experience provides subjective evidence. It would be helpful to know how many of the experiences in Table 4 in their article were corroborated by the surgeons/operating room staff and which relied solely on convincing the investigators.

Awareness during anesthesia (especially during surgery) may be difficult to distinguish from awareness (remembrance) during awakening or in the Post Anesthesia Care Unit (PACU). Could the surgeons indicate the timing of the remembrance of the voice that said that “the doctor forgot to connect the catheter of the bag; the floor was full of urine. Other jumbled conversations, someone was angry and yelling about it.” (“Yelling”? In a hospital in North America?) Without objective confirmation of the time of this conversation, we can only conjecture whether this represents recall of an event during surgery, emergence from anesthesia, or in the PACU.

In the same vein, the authors observed that “we interviewed patients in the PACU and again after seven days because it has previously been demonstrated that approximately 35% of cases are detected only at a delayed postoperative interview (3). Approximately one half of the cases in our study were detected only at the second interview.” Is it possible that repeated interviews of patients who knew they were participating in a study of awareness falsely (albeit unconsciously) increased the incidence of patients' self-reports?

We do not know the number of patients in the present study only given anesthesia with an IV anesthetic and thus cannot assign an incidence of awareness during one form of anesthesia (IV) versus another (potent inhaled anesthetic) or, indeed, to one as opposed to another inhaled anesthetic. This would be useful to know. Is recall more likely during IV versus inhaled anesthesia? If so, is the study population representative of the surgical population as a whole, in its proportion of patients receiving IV versus inhaled anesthesia? Five of the 25 cases were found in patients who did not receive a potent inhaled anesthetic. In an additional 5 or 6 patients, remembrance appears to have taken place during induction of anesthesia, again in the probable absence of a potent inhaled anesthetic. Because anesthesia solely with IV anesthetics is less usual in North America, and assuming the study population reflects this practice, it appears that awareness is more likely in patients given general anesthesia with an IV anesthetic.

The authors note that others have found an incidence of awareness 64% greater (0.18% versus 0.11%) in the presence of neuromuscular blockade (3). Approximately 80% of the patients with awareness in the present study had neuromuscular blocking drugs, and half these patients had a sense of paralysis as part of their remembrance of awareness. Paralysis is not anesthesia, and 20 of the patients in the present study may not have been adequately anesthetized. Of the 5 patients who were not given neuromuscular blocking drugs, and thus were presumably adequately anesthetized, one had an out of body experience (see above), one felt a tube in her throat and did not know if surgery was ongoing (was this awakening during intubation?), one was concerned about spilt urine (see above), one heard the doctor ask for a stent (see above), and one having a procedure for cervical stenosis had a “sensation of two flat surfaces moving on each other leaving sharp, intense pain; felt sensation in the neck, sensation of choking and felt bone being cut away from the neck” (but despite the report of noxious sensations there is no mention of movement in this unparalyzed patient). Apart from the patient whose surgeon asked for a stent, it is not clear to us that the remaining unparalyzed patients remembered events during anesthesia.

The authors explored several risk factors for awareness. We would have benefited from an evaluation of two that were not tested: 1. Did the choice of anesthetic affect the incidence of awareness (IV versus inhaled)? 2. Did the use of neuromuscular blocking drugs increase the risk of awareness? These may be factors of particular importance because, unlike demographic factors, they are under the control of the anesthetist.

We suggest that the report by Sebel et al. (1) may overestimate the incidence of awareness during anesthesia. We suggest that patients adequately anesthetized with a potent inhaled anesthetic (i.e., desflurane, isoflurane, or sevoflurane at 0.5 MAC or greater) have an incidence of awareness that is vanishingly small.

Dr. Eger is a paid consultant to Baxter Healthcare Corp, the manufacturer of desflurane.

Edmond I Eger II, MD

James M. Sonner, MD

Department of Anesthesia and Perioperative Care; University of California, San Francisco; San Francisco, CA;

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1. Sebel PS, Bowdle TA, Ghoneim MM, et al. The incidence of awareness during anesthesia: a multicenter United States study. Anesth Analg 2004;99:833–9.
2. Saucier N, Walts LF, Moreland JR. Patient awareness during nitrous oxide, oxygen, and halothane anesthesia. Anesth Analg 1983;62:239–40.
3. Sandin RH, Enlund G, Samuelsson P, Lennmarken C. Awareness during anaesthesia: a prospective case study. Lancet 2000;355:707–11.
© 2005 International Anesthesia Research Society