Table 4. Descriptions of Awareness
Age (yr)	ASA status	Procedure type	Anesthetic	Description
77	II	Ophthalmic	Propofol, fentanyl, nmb	The patient reported waking up during the operation and felt the surgeons working on her eye and could hear them talking. She tried to move and talk but could not and felt helpless. There was no pain.
21	I	Ophthalmic	N2O, propofol infusion, fentanyl, nmb	The patient said that she heard the chief surgeon or a male voice saying “careful, careful” and “to the left.” The voices “faded in and out.” No other sensation or discomfort. The experience did not bother her at the time.
49	II	Abdominal	Isoflurane, fentanyl, nmb	The patient recalled “a great deal of conversation.” She recalled hearing conversations about her tattoos and what they found in her abdomen. She remembered being unable to move and “it was like being in a box. It was dark and I could not move at all.”
29	I	Lower abdominal	N2O, isoflurane	The patient reported an “out of body experience” at some point during the surgery with her floating out of her body and watching the surgery from above. She thought it was very “weird.” She thought frequently about it.
28	I	Orthopedic	Propofol, remifentanil	She remembered waking up and feeling the tube in her throat. She wanted to make sure that the anesthesiologist knew that, because she did not know whether the surgery was still going on or not.
43	II	Abdominal hysterectomy	Isoflurane, N2O, fentanyl, nmb	“Feeling tube going down throat and could not breathe” was last thing remembered. “I tried to open my eyes and couldn’t. I tried to move my fingers. I then tried to breathe and couldn’t.”
31	III	Orthopedic	Sevoflurane, propofol, N2O, fentanyl, nmb	Reported choking on tube. Worst thing was “felt like couldn’t breathe.”
66	III	Orthopedic	Sevoflurane, propofol, N2O, fentanyl, nmb	“During this surgery I became conscious. I was in total darkness; I was paralyzed. I felt as if I wanted to take a few breaths, but I couldn’t. It was a terrible experience. After a few minutes I lost consciousness.”
40	III	Abdominal	Sevoflurane, fentanyl, nmb	Reported “Yes, feeling pain, cutting, someone asking for scalpel, feeling of cutting.” Worst thing was “waking up in OR while paralyzed.” “I woke up during the procedure and could hear the doctors talking and I could feel the pain in my wound. I was not able to move or speak and it is one of the worst scares I’ve had in my long history of serious illness.”
63	III	Abdominal	Sevoflurane, propofol, N2O, nmb	Reports “not being able to breathe, trying to move my hand to let them know I felt the mask being forced on my face and no air, couldn’t breathe, finally said this is it, I’m going to die and thought to myself ‘Oh well, the hell with it’ and just gave up.”
53	II	Head and neck	Propofol, fentanyl, nmb	“I remember trying to talk to them and telling that I was awake.” “I woke up during surgery enough to know that I was in surgery and was trying to figure out a way to tell them I was awake. I knew my arms were tied and my eyes were taped shut. PANIC!!”
49	III	Abdominal	Sevoflurane, propofol, fentanyl, nmb	He tried to open his eyes but couldn’t; tried to move his arms, couldn’t. Heard conversations in OR.
63	IV	Redo coronary artery surgery	Thiopental, isoflurane, fentanyl	Experienced the sound of somebody asking about liquid on floor. Heard 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. All these ran together.
49	III	Abdominal hysterectomy	Propofol, desflurane, N2O, fentanyl, nmb	Recollections with lights, sounds, noises, lots of noises, pain, sound of somebody asking “Where are you going? What are you doing?” The patient was unable to talk; felt like she was in a hurricane and had a sensation of wanting to get out.
67	IV	Coronary artery surgery	Thiopental, isoflurane, fentanyl, nmb	People were talking to each other saying things were okay. He tried to talk to tell them that he couldn’t breathe. No one was paying attention. Arms felt to be fastened down, had severe chest pain.
55	III	Urology	Propofol, sevoflurane, N2O, fentanyl	He heard the doctor ask for a stent which was identified by a number. He heard conversations off in the distance. No pain, no sensation of paralysis.
46	II	Cervical stenosis	Propofol, desflurane, N2O, fentanyl, morphine	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.
70	II	Abdominal	Isoflurane, N2O, fentanyl, nmb	The patient said she felt the incision and characterized the awareness as having associations with pain, paralysis, or stress. Patient said she had had recurrent memories about the operation. Patient states she has awareness of “the cut” but was unable to tell anyone. Afraid the pain was going to get worse, but it didn’t and then she went to sleep.
59	III	Vascular	Isoflurane, fentanyl, morphine, ketorolac, nmb	Claimed to remember a tube being put down his nose and vomiting. Characterized the experience as associated with pain, paralysis, or stress.
44	II	Left video thoracoscopy, excision of paraspinal tumor	Induction: midazolam, propofol, fentanyl, isoflurane, fentanyl, nmb	Patient remembers waking up on her side, unable to move, left arm suspended, with a breathing tube in her mouth. Remembers feeling pain on incision and the surgeon’s voice saying “she’s moving.”
69	III	Removal of hepatic artery infusion pump	Induction: midazolam, propofol, fentanyl Maintenance: N2O, nmb	Patient told the anesthesiologist he felt pressure at the surgical site during the operation but had no pain. He also heard voices and the instruments clanging.
39	III	Intracranial procedure	Isoflurane, fentanyl, remifentanil, nmb	He reported hearing the sound of something being “screwed into my head.” He recognized and remembered the sound when he heard the ICP monitor being removed after the operation.
46	II	Orthopedic	Isoflurane, N2O, fentanyl, nmb	Reports remembering feeling pain in hip and having a dream that “was interrupted by the pain.”
71	III	Gynecologic	Sevoflurane, nmb	Reports remembering “being intubated.” Remembers the “tube in my mouth.”
61	III	Thoracic	Not recorded	Reports awareness of intubation, “tube going down throat,” as last memory before falling asleep.

Discussion

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 m20 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.

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.

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