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*Emory University School of Medicine, Atlanta, Georgia;
University of Washington Medical Center, Seattle, Washington;
University of Iowa, Iowa City, Iowa;
State University of New York, Stony Brook, New York;
||Memorial Sloan-Kettering Cancer Center, New York, New York;
¶Duke University, Durham, North Carolina; and
#Harborview Medical Center, Seattle, Washington
Address correspondence to Peter Sebel, MB BS, PhD, MBA, Department of Anesthesiology, Emory University School of Medicine, 49 Jesse Hill Jr. Dr., S.E., Atlanta, GA 30303. Address e-mail to peter_sebel{at}emoryhealthcare.org Reprints will not be available from the authors.
| Abstract |
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IMPLICATIONS: Data from 19,575 patients indicate that the incidence of awareness with recall after surgery under general anesthesia is 0.13%. This means that the minimum incidence of awareness is 1.3 patients per 1000.
| Introduction |
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All data on the incidence of awareness during anesthesia come from outside the United States (US). Sandin et al. (3) reported an overall incidence of 0.16% in 11,785 patients treated at 2 hospitals in Sweden; the rate was 0.18% when neuromuscular blocking drugs were used and was 0.11% in their absence. Long-term follow-up of the patients who reported awareness showed a frequent incidence (approaching 50%) of PTSD 2 yr after the incident, even though patients initially did not report much distress (8). In a study from Australia, Myles et al. (2) reported an incidence of awareness of 0.10%; it was the highest risk factor for patient dissatisfaction after anesthesia.
Traditional clinical monitoring modalities during anesthesia are ineffective in preventing awareness. For instance, hypertension and tachycardia are generally not associated with reports of awareness (5,7), and end-tidal anesthetic concentration monitoring is also ineffective (3). Recently, a monitor that uses a processed electroencephalogram (EEG) derivative, the Bispectral Index® (BIS®; Aspect Medical Systems, Newton, MA) has been introduced into clinical practice for monitoring anesthetic effects on the brain (9,10). It may have the ability to measure the hypnotic component of the anesthetic state (10). The effectiveness of BIS monitoring to prevent awareness is unknown (11), and it has even been suggested that guiding anesthetic administration by using BIS monitoring may be associated with an increased incidence because of deliberate reductions in anesthetic dose on the basis of BIS data (12).
Although the incidence of awareness is thought to be infrequent, many patients remain concerned about this potential adverse experience (1,2), and this issue has attracted considerable public media attention. The incidence of awareness may vary between countries or institutions depending on their respective anesthetic practices and patient populations. This multicenter prospective cohort study was therefore undertaken to establish the incidence of awareness with recall during routine general anesthetic practice in the US and to determine (where possible) the BIS values associated with intraoperative awareness events.
| Methods |
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18 yr, apparently normal mental status (excluding obviously impaired patients), and ability to provide informed consent. Patients were excluded if they were not expected to survive, were transferred directly to the intensive care unit (ICU) and were not tracheally extubated within 1 wk, could not speak English, or had abnormal mental status that precluded answering the required questions. A sample size of 20,000 patients was estimated on the basis of previous incidence studies outside the US (3). Anesthetic care, including anesthetic drugs and use (or otherwise) of the BIS monitor (A2000 or A1050; Aspect Medical Systems) during the time of this study was entirely at the discretion of the attending anesthesiologist and was not influenced by participation in this study. In general, the attending anesthesiologist was not aware of patient participation in the study. Each patient was interviewed by research staff with the same structured interview, modified from Brice et al. (13), that was used in prior incidence studies (3,13,14) (Table 1). Patients were interviewed first in the postanesthesia care unit (PACU) (if they were sent to the PACU and not directly to the ICU). At one site, the IRB required the interview to be conducted after the patient had left the PACU. A follow-up interview was attempted at least 1 wk after anesthesia.
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2 tests with Yates correction. Logistic regression models (SPSS; SPSS Inc., Chicago, IL) were used to determine associations of patient demographics with awareness and dreaming. Variables found to be significant on univariate analysis were entered into the forward-selection multivariate model. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. P < 0.05 was accepted as significant. | Results |
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60 yr), lower ASA physical status (OR, 1.48; 95% CI, 1.291.70 for ASA status III compared with ASA status IIIV), undergoing elective surgery (OR, 2.53; 95% CI, 1.046.15 compared with emergency surgery), and undergoing surgery on an ambulatory basis (OR, 1.40; 95% CI, 1.021.91 compared with disposition to ICU). Thirty-eight percent of all cases in the study were monitored for portions of each case by using BIS. However, the use of BIS monitoring varied widely between study sites from 0% to 74% (P < 0.001), and not all cases that were BIS-monitored were monitored from induction to emergence. Figure 2 illustrates a case of awareness with BIS monitoring. BIS was consistently >60. There was no significant association between the use (or otherwise) of BIS and the incidence of awareness.
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| Discussion |
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Our estimate of the incidence of awareness is relatively conservative. If the cases of possible awareness are also considered, then the overall incidence of awareness increases to 0.36%. It is interesting to note that the incidence data from Sweden included several cases that were described as "possible" cases (3) on the basis of inability to confirm the reports. If we compare the incidence of awareness between this study and the data from Sweden including "possible" cases of awareness, our incidence of awareness is approximately twice that reported previously. It is also possible that knowing that they were participating in a study of awareness may have increased the incidence of patients self-reports. However, this would be true of all awareness incidence studies.
The detection of awareness depends on the interview technique, timing of the interview, and structure of the interview. A single short postoperative visit by an anesthesiologist without use of a structured interview is unlikely to elicit many cases of awareness. We used the same structured interview that has been used in prior investigations (6,13,14). 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. The loss of follow-up at the postoperative interview would therefore bias our results in the direction of underestimating the incidence of awareness during anesthesia.
The descriptions of the awareness cases identified in this study closely resemble those reported previously (36). As might be expected, a significant proportion of the awareness episodes occurred either during endotracheal intubation or at surgical incision, i.e., times when the level of patients stimulation is highest. Patients reported auditory recollections, sensations of not being able to breathe, paralysis, panic, and pain (Tables 4 and 5 ), consistent with previous reports (36). Our study did not assess the long-term psychological sequelae of intraoperative awareness.
Awareness is caused by the administration of general anesthesia that is inadequate to maintain unconsciousness and to prevent recall during surgical stimulation. Common causes include large anesthetic requirements, equipment misuse or failure, and smaller doses of anesthetic drugs (1). Our finding of an increased risk of awareness with sicker patients (ASA physical status IIIV) undergoing major surgery (Table 7) may reflect the use of smaller anesthetic doses and light anesthetic techniques in sicker patients. However, specific details of anesthetic doses and intraoperative hemodynamics in patients with awareness compared with those without awareness were not obtained in this incidence study. Although female sex and younger age have been suggested as risk factors for intraoperative awareness on the basis of analysis of closed malpractice claims (7), our study found no association between sex and age and awareness during anesthesia.
Dreaming during anesthesia was described by 6% of patients in our study, and this is consistent with the common occurrence of perioperative dreaming reported in several small European studies (13,15,16). Dreaming was more frequently reported in the recovery room than later after surgery; this is also consistent with earlier studies (15). Dreaming was associated with younger, healthier patients undergoing ambulatory surgery. The widely varying incidence in dreaming by study site (1.1%10.7%) may reflect differences in patients or anesthetic drugs, or, alternatively, it may reflect bias in patient selection or responses between the geographically dispersed centers (16). The significance of dreaming and its relationship to awareness during anesthesia is unclear.
In many cases, awareness during anesthesia is a potentially avoidable adverse anesthetic outcome. In light of follow-up studies suggesting that such "victims of awareness" (8) may exhibit significant psychological aftereffects, such as PTSD, attempts to further reduce its incidence are warranted. Because awareness occurred despite the usual clinical monitoring of anesthetic depth (e.g., blood pressure, heart rate, and end-tidal anesthetic monitoring) in this study and others (3,5,7), a monitor of cerebral function and depth of anesthesia may be of theoretical benefit.
One such monitor, the BIS monitor, is a complex processed EEG derivative that assigns a numerical value to the probability of consciousness. Recovery of consciousness during general anesthesia without any recall (in the absence of surgical stimulus) has generally been associated with BIS values >60 (17,18). Cases of awareness during surgical stimulation with high BIS values (>60) have also been reported (19,20). Although there is at least one case report of awareness with a BIS of apparently <50 (21), BIS was subsequently found to be >60 at the probable time of awareness (22). In the present study, a number of the cases of awareness in which BIS was used also had high BIS values (see, for example, Fig. 2). We were unable to positively identify any cases of awareness with BIS values <60, but no firm conclusions can be drawn from this observation. This study was not designed to test the efficacy of BIS monitoring because the population that received additional monitoring was not randomly selected or matched to those who did not, and no specific guidelines for BIS were used. Other emerging data suggest that BIS monitoring is effective in reducing the incidence of awareness. Ekman et al. (23) investigated the incidence of awareness when the anesthetic was guided with BIS and found a 77% reduction in the incidence of awareness (23) compared with historical data (3). Myles et al. (24) also found that, in a double-blind study of patients at high risk for awareness, BIS-guided anesthesia resulted in an 82% reduction in the incidence of awareness.
In summary, the incidence of awareness during general anesthesia in the US was 0.13%. It occurred at a rate of 12 per 1000 patients interviewed at each site.
| Acknowledgments |
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Statistical processing support was provided by Aspect Medical Systems, Inc. We thank Jeff Sigl, PhD, and Paul Manberg, PhD, from Aspect Medical Systems for providing statistical analysis and helpful suggestions during the preparation of this manuscript. We also appreciate the assistance of Scott Acker, RN, Antonio Adam, MD, Kerith Brandt, Catherine Dobres, CRNA, Samantha Goldstein, BA, Meghan Holmes, MA, Kui-Ran Jiao, MD, David Kramer, Yumi Lee, MD, Jason Leggio, CRNA, Tanya Lipto, RN, Lee McClurkin, RN, Rachel Pessah, BS, Jacqueline Sumanis, CRNA, Meghan Swardstrom, and Thu Tran, BS.
| Footnotes |
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| References |
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