The incidence of dreaming varied markedly by site (1.1%–10.7%; P < 0.001). Dreaming was associated with the following patient characteristics: younger age (OR, 2.43; 95% CI, 2.03–2.91 for <40 yr compared with ≥60 yr), lower ASA physical status (OR, 1.48; 95% CI, 1.29–1.70 for ASA status I–II compared with ASA status III–V), undergoing elective surgery (OR, 2.53; 95% CI, 1.04–6.15 compared with emergency surgery), and undergoing surgery on an ambulatory basis (OR, 1.40; 95% CI, 1.02–1.91 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.
This study demonstrates that the incidence of awareness with recall after general anesthesia (0.13%) in the US is comparable to that described in other countries (2,3,6,14). Awareness during anesthesia therefore appears to be a ubiquitous phenomenon that occurs at an incidence of 1 to 2 cases per 1000, irrespective of geographic location and potential differences in anesthetics and techniques. Assuming that approximately 20 million general anesthetics are administered in the US annually, we can expect, on the basis of our data, approximately 26,000 cases of awareness to occur each year, or approximately 100 per workday.
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 (3–6). 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 (3–6). 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 III–V) 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 1–2 per 1000 patients interviewed at each site.
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.
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© 2004 International Anesthesia Research Society
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