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How Likely Is Awareness During Anesthesia?

Bowdle, T Andrew MD, PhD; Sebel, Peter S. MB BS, PhD, MBA; Ghoneim, Mohamed M. MD; Rampil, Ira J. MD; Padilla, Roger E. MD; Gan, Tong Joo MB BS, FRCA, FFARCS; Domino, Karen B. MD, MPH

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

University of Washington Medical Center Seattle, WA (Bowdle)

Emory University School of Medicine Atlanta, GA; peter_sebel@emoryhealthcare.org (Sebel)

University of Iowa Iowa City, IA (Ghoneim)

SUNY Stony Brook, NY (Rampil)

Memorial Sloan-Kettering Cancer Center, New York, NY (Padilla)

Duke University Durham, NC (Gan)

Harborview Medical Center Seattle, WA (Domino)

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In Response:

Drs. Eger and Sonner have raised interesting and worthwhile questions about our study that also pertain to most studies of intraoperative awareness.

Their first point is that the patient’s remembrance of intraoperative awareness should be unequivocally linked to an intraoperative event that can be corroborated. We agree that this is the most convincing kind of evidence to have in a case of intraoperative awareness. However, if we were to adhere to this very high standard of evidence, we would almost certainly underestimate the incidence of intraoperative awareness. Since the patient’s internal experience is subjective, it is impossible to know the “true” incidence of intraoperative awareness. The most inclusive standard of evidence for intraoperative awareness would be simply the patient’s belief that he or she was awake during surgery. If the patient believes that he or she was awake during surgery, and there is no evidence to the contrary, how can we know that he or she was not?

Because of the difficulty in judging the likelihood of intraoperative awareness based on patients' descriptions, we divided the reports in our study into 2 categories, likely awareness and possible awareness, based on the investigators' assessments of the reports. Realizing the subjective nature of this data analysis, we deliberately published the descriptions of the likely awareness cases for the readers to judge on their own. We believe that the incidence of awareness that we have reported is a reasonable estimate and is unlikely to either greatly overestimate or underestimate the true incidence.

Eger and Sonner ask whether interviewing the patients in the PACU and then at a later time might cause patients to falsely report intraoperative awareness. This seems unlikely to us. There was certainly no benefit of any kind to be gained for the patients in doing so. The questions, which were repeated in the second interview, may “jog” the patients' memories by their repetition but they do not contain leading information that might cause “false” memories. The second interview was included specifically because of previous studies showing that patients may not report an episode of intraoperative awareness immediately following surgery (1,2).

Eger and Sonner ask whether recall is more likely during IV versus inhaled anesthesia. Unfortunately, because this study was not designed to examine the effects of different anesthetic drugs, the drug data collected by the study are incomplete and we are not able to answer this interesting question.

Eger and Sonner state that the “5 patients [with putative intraoperative awareness] who were not given neuromuscular blocking drugs…were presumably adequately anesthetized”. We presume they mean because the patients had not received neuromuscular blocking drugs, and were apparently not moving during anesthesia, the patients must have been adequately anesthetized. We do not agree with this concept. Reports of intraoperative awareness occur in the absence of obvious clues to awareness, including the absence of movement, tachycardia or hypertension, so the absence of movement does not guarantee that the patient is adequately anesthetized or will not have intraoperative awareness. In fact, in the very case report cited by Drs. Eger and Sonner, a patient not receiving neuromuscular blocking drugs had a well-corroborated episode of intraoperative awareness without any apparent movement noticed by the anesthesiologist (3).

Finally, Drs. Eger and Sonner state that patients receiving potent inhaled anesthetics at 0.5 MAC or greater have a “vanishingly small” incidence of awareness. We are not sure what “vanishingly small” means, but we believe that an incidence of intraoperative awareness of 0.1%, or 1 in a 1,000 should be improved upon, and there is little doubt that patients would agree with us. We are not aware of the evidence that 0.5 MAC end tidal agent results in a “vanishingly small” incidence of awareness during surgery. While isoflurane at 0.45 MAC prevented recall in volunteers given verbal stimuli, the authors noted that “whether the arousing effect of surgery may increase the anesthetic concentrations required to prevent conscious and unconscious memory is not known” (4). Since there are many cases of intraoperative awareness that have been reported in the presence of what appeared to the anesthesiologist to be adequate doses of potent volatile agent, and because there is such enormous biological variation in response to anesthetic agents, we would advise caution in relying on any particular “recipe” of drug or dose for avoiding intraoperative awareness.

T. Andrew Bowdle, MD, PhD

University of Washington Medical Center Seattle, WA

Peter S. Sebel, MB BS, PhD, MBA

Emory University School of Medicine Atlanta, GA; peter_sebel@emoryhealthcare.org

Mohamed M. Ghoneim, MD

University of Iowa Iowa City, IA

Ira J. Rampil, MD

SUNY Stony Brook, NY

Roger E. Padilla, MD

Memorial Sloan-Kettering Cancer Center; New York, NY

Tong Joo Gan, MB BS, FRCA, FFARCS

Duke University Durham, NC

Karen B. Domino, MD, MPH

Harborview Medical Center Seattle, WA

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References

1. Sandin RH, Enlund G, Samuelsson P, Lennmarken C. Awareness during anaesthesia: a prospective case study. Lancet 2000;355:707–11.
2. Nordstrom O, Englstrom AM, Persson S, Sandin R. Incidence of awareness in total iv anaesthesia based on propofol, alfentanil and neuromuscular blockade. Acta Anaesthesiol Scand 1997;41:978–84.
3. Saucier N, Walts LF, Moreland JR. Patient awareness during nitrous oxide, oxygen and halothane anesthesia. Anesth Analg 1983;62:239–40.
4. Dwyer R, Bennett HL, Eger EI, Heilbron D. Effects of isoflurane and nitrous oxide in subanesthetic concentrations on memory and responsiveness in volunteers. Anesthesiology 1992;77:888–92.
© 2005 International Anesthesia Research Society