A Retrospective Study of Intraoperative Awareness Withpdf

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Patient Safety Section Editor: Sorin J. Brull

A Retrospective Study of Intraoperative Awareness withMethodological Implications

George A. Mashour, MD, PhD

Luke Y.-J. Wang, MD

Christopher R. Turner, MD, PhD,MBA

John C. Vandervest, BS

Amy Shanks, MS

Kevin K. Tremper, PhD, MD

BACKGROUND: Awareness during general anesthesia is a problem receiving in-creased attention from physicians and patients. Large multicentered studies haveestablished an accepted incidence of awareness during general anesthesia asapproximately 1–2 per 1000 cases or 0.15%. More recent retrospective data,however, suggest that the actual incidence may be as low as 0.0068%.METHODS: To assess the incidence of awareness at our institution, we conducted areview of adult patients undergoing surgical procedures over a 3-year period.Information on awareness came from entries of “Intraoperative Awareness”captured during our standard evaluations on postoperative day one in ourperioperative information system. Patients were not questioned specifically aboutawareness.RESULTS: We reviewed 116,478 charts; 65,061 patients received general anesthesiaand 51,417 received other types of anesthesia. Of the patients receiving generalanesthesia, 44,006 had complete postoperative documentation. The reported inci-dence of undesired intraoperative awareness in this population was 10/44,006(1/4401 or 0.023%). Of the patients who received other anesthetic modalities, 22,885had complete postoperative documentation. Undesired intraoperative awarenesswas reported in 7/22,885 patients who did not receive general anesthesia (1/3269or 0.03%). The reported incidence of intraoperative awareness was not statisticallydifferent between the two groups (P 0.54). Relative risk of intraoperativeawareness during a general anesthetic compared with a nongeneral anesthetic was0.74, with 95% confidence interval [0.28, 2.0].CONCLUSION: Using a retrospective methodology, reports of intraoperative aware-ness are not statistically different in patients who received general anesthesiacompared with those who did not. These results suggest that, despite success withother rare perioperative events, the resolution of retrospective database analysesmay be too low to study intraoperative awareness.(Anesth Analg 2009;108:521–6)

Awareness during general anesthesia, which de-notes both awareness and subsequent explicit recall ofintraoperative events, is a problem receiving increasedattention by both patients and clinicians. A proportionof patients experiencing awareness may subsequentlydevelop serious psychological sequelae, includingposttraumatic stress disorder (PTSD).1,2 In 2004, the

Joint Commission on Accreditation of Hospital Orga-nizations issued a Sentinel Alert to promote greaterattention to the problem.3 Despite recent attention by

the medical community and the lay press, the inci-dence of intraoperative awareness—and hence themagnitude of the problem—remains uncertain. A

multicenter study in the United States by Sebel et al.estimated an incidence of awareness with explicitrecall of approximately 0.13%, a rate consistent withlarge European studies demonstrating awareness in1–2/1000 cases.4,5 In contrast, a recent study of aware-ness in a regional medical system by Pollard et al.6

reported a much lower incidence of 1 episode ofawareness/14,560 cases, or 0.0068%.

Establishing the validity of the retrospective studyof awareness is necessary if we are to adjudicate

between these disparate reports. Retrospective analy-ses of data derived from an electronic perioperativeinformation system have been successfully used at ourinstitution to study rare events such as impossiblemask ventilation7 and postoperative renal failure.8 Tocompare the incidence of undesired awareness at ourinstitution with the disparate rates in the literature, aswell as explore the use of electronic databases for thestudy of intraoperative awareness, we conducted a re-view of more than 100,000 cases over a 3-year period.

METHODS

With IRB approval, a retrospective electronic chartreview was conducted on adult patients receiving

From the Department of Anesthesiology, University of MichiganMedical School, Michigan.

Accepted for publication March 3, 2008.

Supported solely through institutional and departmental funds.The authors have no conflicts of interest to declare.

Address correspondence and reprint requests to 1H247 UH/Box0048, 1500 East Medical Center Drive, Ann Arbor, MI 48109-0048.Address e-mail to [email protected].

Copyright © 2009 International Anesthesia Research Society

DOI: 10.1213/ane.0b013e3181732b0c

Vol. 108, No. 2, February 2009 521



anesthesia at the University of Michigan Health Sys-

tem between January 1, 2004 and February 20, 2007.Information regarding awareness was obtained frompatient interview on postoperative day 1. Inpatientswere interviewed directly by residents. Nurses calledoutpatients by phone. All patients were asked if theyexperienced any problems related to anesthesia. Ifthey discussed intraoperative awareness, these datawere entered into a perioperative clinical informationsystem (Centricity™ from General Electric Healthcare,Waukesha, WI) by selecting the category of “Intraop-erative Awareness” in the postoperative documenta-tion window. Patients were not interviewed using a

Brice et al9

or modified Brice interview, i.e., they werenot asked specifically about awareness.The electronic charts were queried for postopera-

tive documentation of “Intraoperative Awareness.”After the initial query, all charts with reports of“Intraoperative Awareness” were reviewed and cor-related with existing Quality Assurance data regardingawareness. The intraoperative record was analyzed foranesthetic technique and anesthetic drugs, as well as theuse of benzodiazepines, opioids, and neuromuscular

blocking drugs. No electroencephalographic deviceswere used for the detection of intraoperative awarenessduring this time period examined. Statistical comparisonwas performed using a

2 test, as well as assessingrelative odds ratios. A P value of 0.05 was consideredsignificant.

RESULTS

We reviewed 116,478 charts between January 1,2004 and February 20, 2007; 65,061 patients receivedgeneral anesthesia and 51,417 received other anes-thetic modalities. Of the patients receiving generalanesthesia, 44,006 had complete postoperative docu-mentation, for a compliance rate of 67%. Of the 44,006

patients who received general anesthesia and hadcomplete postoperative documentation during the

time period of study, 10 complained of some degree ofawareness (Table 1), an incidence of 1/4401 or 0.023%.Demographic data for this patient cohort are shown in

Table 2. Analysis of the general anesthetics for thetime period of study indicates that 90% of cases wereperformed using inhaled anesthetics, whereas 10%used total IV anesthesia (TIVA).

Of the 10 patients who complained of awareness inthis group, five were men and five were women.There were no consistent findings regarding anes-thetic choice, use of benzodiazepines, or use of opi-oids. All 10 patients had received neuromuscular

blocking drugs at some time. Of the 10 cases ofawareness, two patients were in the high-risk cat-egory: one was undergoing an emergent cesarean

delivery, and the other was undergoing a heart trans-plant. One patient had a confirmed awareness eventduring TIVA after the discontinuation of nitrous oxide(patient 10). Figures 1A and B depict the anestheticregimen for each case. Several cases documentedinsufficient anesthesia on the electronic record thatcorrelated with complaints of awareness. For patient1, awareness at the end of the procedure was likelydue to low levels of isoflurane documented at minute210 of the case. Patient 2 had low sevoflurane concen-trations and insufficient IV anesthesia at the start ofsurgery (“S”), when he reported awareness. Patient 4

reported awareness at the beginning of the case,around the time a vaporizer leak was noted in the

Table 1. Perioperative Data of Patients Experiencing Awareness During General Anesthesia

Patient Age Sex ASA Surgery Awareness report

1 73 F 4 Abdominal abscess drainage Aware for approximately 15 min at the endof the case with 10/10 pain

2 82 M 3 Nissen Aware of pain on incision3 68 F 3 Cardiac ablation Vague recall of intubation4 49 M 2 Tibial plateau fracture repair Aware of leg manipulation and paralysis at

beginning of surgery5 33 F 2E Emergent cesarean delivery Experience of paralysis and pain

6 47 F 3 Incisional hernia repair Aware at some point during surgery, butunconcerned7 60 M 4 Heart transplantation Heard intraoperative conversations, but

unconcerned8 65 M 3 Carotid endarterectomy Heard voices, felt endotracheal tube and

unable to move9 84 M 3 Colectomy Heard voices, thought he was dead

10 49 F 3 Diskectomy Aware and paralyzed for a portion of thecase after intravenous line becamedisconnected during TIVA

Pt. patient; ASA American Society of Anesthesiologists Classification; TIVA total intravenous anesthesia.

Table 2. Demographic Data of Patient Cohort Analyzed for

Awareness During General Anesthesia

Number of patients 65,061

ASA 2.27Age 49 (18)Men 30,774Women 34,287

ASA American Society of Anesthesiologists Classification.

522 Intraoperative Awareness ANESTHESIA & ANALGESIA



chart. Finally, patient 10 experienced awareness afternitrous oxide was discontinued and an IV line infil-tration was recorded in the record. It was not possibleto identify the temporal location of awareness in theremaining cases.

An additional 51,427 patients underwent proce-dures during the time period of study, but did notreceive general anesthesia. Of the patients who re-ceived other anesthetic modalities, 22,885 had com-plete postoperative documentation for a compliance of45%. Of this cohort, seven patients complained of beingaware during the case, an incidence of 1/3269 or 0.03%.These patients had been managed with a variety ofanesthetic modalities (Table 3). Of the seven patientscomplaining of awareness in this population, six werewomen. In the case of the one male patient, the report ofawareness was given by his daughter. This was the onlypatient in the study who did not report intraoperativeawareness independently.

There was no statistically significant difference be-tween the incidence of awareness in the general anes-thesia (0.023%) and nongeneral anesthesia (0.03%)populations at our institution (P 0.54). Relative riskof undesired intraoperative awareness during a gen-eral anesthetic compared with a nongeneral anestheticwas 0.74, with 95% confidence interval [0.28, 2.0].

DISCUSSION

Awareness during general anesthesia is a problemthat has captured the attention of clinicians, patients,and the general public. Although awareness is a

significant source of fear for many patients undergo-ing surgery, the actual incidence and sequelae ofawareness remain a matter of controversy. This ishighlighted by recent studies reporting rates of aware-ness and subsequent PTSD that were lower thanpreviously thought.2,6

In this retrospective study, we found the incidence ofcomplaints of intraoperative awareness during generalanesthesia to be 1/4401 or 0.023%. We acknowledge thatthese data likely represent an under-estimate of theactual incidence of awareness in the population studied.As Sebel et al.4 noted: “A single short postoperative visit

by an anesthesiologist without use of a structured inter-view is unlikelyto elicit many cases of awareness,” an effectalso noted by Moerman et al.10 Sandin et al. and Sebel et al.found considerably increased reports of awareness duringthe second interview at 1 wk postoperatively.4,5 Indeed, ourdata were obtained retrospectively, our patients did notreceive a Brice interview or other technique of specificallyassessing awareness, and our patients were interviewed onpostoperative day 1, all of which might result in an under-estimation of the true incidence.

Even with suboptimal conditions for detection ofawareness, our rate of undesired awareness was still

more than three times that of 0.0068% reported byPollard et al.,6 in which a structured interview was used.

It is important to note that the structured interview in thePollard et al. study omitted a question specifically assess-ing recall that is present in the standard Brice interview.Our demographic data appeared comparable, with anaverage ASA classification of 2.27 (vs 2.37 in. Pollard et al.),an average age of 49 18 years (vs 46 16), and amale:female ratio of 1:1.1 (vs 1:1.3). Although data reportedin the Pollard et al. study were gathered, in part, at anacademic medical center,no resident or nurse trainees wereidentified as being involved in patient care. Since residentphysicians are routinely involved in patient care at ourinstitution, this may account forsome disparity in outcome.

Another possible difference relates to the use ofTIVA at our institution. The centers in the Pollard et al.study “rarely used” IV drugs as the sole anesthetic; inour study, 1 of the 10 patients who complained ofawareness during general anesthesia experienced theevent during a failed TIVA. Given the large number ofcases analyzed, it is difficult to establish the precisenumber of anesthetics in which TIVA was used at somepoint. We have established, however, that approximately9/10 cases used an inhaled anesthetic. Thus, the rate ofawareness during known TIVA (1/10) is comparable to theoverall use of TIVA in the study population (1/10).

Although our incidence of 0.023% was considerablyless than that reported by Myles et al.11 and Sebel etal.4 (which ranged from approximately 0.10% to0.20%), this disparity is likely mitigated by our lackof a structured interview and a 1 wk postoperativeinterview. Although not in perfect agreement witheither study, it is easier to reconcile our data with thatof Sebel et al.3 than Pollard et al.6

All patients who have postoperative complaints re-ceive follow-up phone calls. Those reporting intraopera-tive awareness are offered psychiatric counseling. Of the17 patients reporting undesired intraoperative aware-ness, only one requested psychiatric care (patient 10receiving general anesthesia, described in Table 1). Al-though formal postawareness psychiatric evaluationwas not systematically performed on all patients, itwould appear that the occurrence of sustained psychiat-ric sequelae in our population was likely closer to thatreported by Samuelsson et al.2 rather than Osterman etal.1 It must be noted, of course, that patients afflictedwith PTSD often avoid health care professionals andclinical settings because they can serve as triggers thatevoke traumatic memories.12

The most surprising finding of the present study isthat the incidence of intraoperative awareness in pa-tients who did not undergo general anesthesia (0.03%,n 22,885) was not statistically different comparedwith those who did (0.023%, n 44,006) (P 0.54).There are several possible interpretations of this find-ing. We could postulate that, since sedation alsosuppresses consciousness and memory, perhaps theincidence was truly the same in anesthetized and

sedated patients. This is, however, an absurd conclu-sion. Sedated patients are often very aware of their

Vol. 108, No. 2, February 2009 © 2009 International Anesthesia Research Society 523



surroundings, as well as talking with the anesthesiolo-

gist and surgical team during the procedure. Wetherefore reject this interpretation. The more likely

interpretation is that the resolution of this retrospec-

tive study of more than 100,000 anesthetics at a singleinstitution was too low to capture the incidence with

Figure 1. Graphical representation of the anesthetic regimen for the 10 cases of intraoperative awareness during general anesthesiaCases 1–5 (A), Cases 6–10 (B). Data from cases 1–2, 4–9 were taken directly from the electronic record and were based on end-tidalgas concentrations; data for case 3 were taken from a paper record and were based on vaporizer gas concentrations.




accuracy. Thus, the current study suggests criticalmethodological limitations to retrospective analyses,despite large sample sizes, and supports prospective

approaches to assessing intraoperative awareness.The finding of awareness complaints in patients notreceiving general anesthesia is provocative. It is im-portant to note that it was not simply awareness ofpain, but awareness itself that was a source of distress.Several patients reported that they heard conversa-tions during their procedure, indicating that this levelof consciousness was inconsistent with their expecta-tions. Furthermore, although 5/7 patients in thisgroup reported pain, it was not the sole complaint. Forexample, patient 3 (Table 3) had a functioning spinalanesthetic but was distraught at hearing conversa-

tions, seeing bright lights, and feeling as if she haddied. Although the significance is unclear, complaints

of intraoperative awareness in patients receiving gen-eral anesthesia had a 1:1 male:female ratio, whereasthis ratio was 1:6 in patients who did not have general

anesthesia. Undesired intraoperative awareness in pa-tients not receiving general anesthesia indicates thepotential for disparity between the expectations of thepatient and those of the anesthesiologist. We must alsorecognize that prior patient conversations with oursurgical colleagues may establish expectations (e.g.,complete unconsciousness) that are not met during theprocedure itself. Unmet expectations, rather than eventsin themselves, may contribute to patient distress.

In conclusion, the incidence of undesired aware-ness during general anesthesia at our institutionwas more than three times as high as that recently

reported by Pollard et al.,

6

despite the fact that noformal awareness interview was used. The self-

Figure 1. Continued.

Vol. 108, No. 2, February 2009 © 2009 International Anesthesia Research Society 525



reported incidence of intraoperative awareness wasnot statistically different in patients receiving gen-eral anesthesia and those who did not. These resultssuggest that large retrospective analyses are prob-

ably inadequate to study intraoperative awareness.Furthermore, the dissatisfaction with awarenessduring nongeneral anesthetics suggests that pro-spective studies should evaluate the relationship

between patient’s pre-procedure expectations andpost-procedure perceptions of anesthetic adequacy.

REFERENCES

1. Osterman JE, Hopper J, Heran WJ, Keane TM, van der Kolk BA.Awareness under anesthesia and the development of posttrau-matic stress disorder. Gen Hosp Psychiatry 2001;23:198–204

2. Samuelsson P, Brudin L, Sandin RH. Late psychological symp-toms after awareness among consecutively included surgicalpatients. Anesthesiology 2007;106:26–32

3. JCAHO. Joint Commission on Accreditation of Hospital Orga-nizations Sentinel Event Alert, Report No. 32, 2004

4. Sebel PS, Bowdle TA, Ghoneim MM, Rampil IJ, Padilla RE, GanTJ, Domino KB. The incidence of awareness during anesthesia:A multicenter United States study. Anesth Analg 2004;99:833–9

5. Sandin RH, Enlund G, Samuelsson P, Lennmarken C. Aware-ness during anaesthesia: A prospective case study. Lancet2000;355:707–11

6. Pollard RJ, Coyle JP, Gilbert RL, Beck JE. Intraoperative aware-ness in a regional medical system: a review of 3 years’ data.

Anesthesiology 2007;106:269–747. Kheterpal S, Han R, Tremper KK, Shanks A, Tait AR, O’ReillyM, Ludwig TA. Incidence and predictors of difficult and impos-sible mask ventilation. Anesthesiology 2006;105:885–91

8. Kheterpal S, Tremper KK, Englesbe MJ, O’Reilly M, Shanks AM,Fetterman DM, Rosenberg AL, Swartz RD. Predictors of post-operative acute renal failure after noncardiac surgery in patientswith previously normal renal function. Anesthesiology 2007;107:89

9. Brice DD, Hetherington RR, Utting JE. A simple study ofawareness and dreaming during anaesthesia. Br J Anaesth1970;42:535–42

10. Moerman N, Bonke B, Oosting J. Awareness and recall duringgeneral anesthesia. Facts and feelings. Anesthesiology 1993;179:454–64

11. Myles PS, Williams DL, Hendrata M, Anderson H, Weeks AM.

Patient satisfaction after anaesthesia and surgery: results of aprospective survey of 10,811 patients. Br J Anaesth 2000;84:6–10

12. Mashour GA, Jiang YD, Osterman J. Perioperative treatment ofpatients with a history of intraoperative awareness and post-traumatic stress disorder. Anesthesiology 2006;104:893–4

Table 3. Complaints of Awareness from Patients not Receiving General Anesthesia

Patient Age Sex ASA SurgeryAnesthetictechnique Awareness report

1 54 F 3 Femoral-popliteal bypasss

Spinal Complained of awareness during thecase.

2 56 F 3 Incisional herniarepair

Awakefiberoptic

Complained of awareness duringintubation. After induction and failedairway management, the patient wasawakened for fiberoptic intubation.

She was informed of this possibilitypreoperatively.

3 32 F 2 cesarean delivery Spinal Complained of hearing conversations,seeing bright lights, feeling as thoughshe were underwater, feeling asthough she were dead.

4 33 F 2 Excisional breast biopsy

Monitoredanesthesiacare

Patient was very upset in the recoveryroom because she was aware duringthe case and sometimes felt pain.

5 66 F 3 Right medial rectusrecession

Retrobulbar block

Patient reported conversations, pain andrecall of the doctor trying to give hermore drugs.

6 36 F 2E Emergent cesareandelivery

Epidural Patient reported awareness of incision, but was unconscious for the rest of thecase. In this case, the epiduralanesthesia was not sufficient andgeneral anesthesia was then induced.

7 54 M 3 Resection of backmelanoma

Spinal Reported pain at the end of the case andreported hearing conversations between anesthesiologist and surgeon.Patient was then given intravenoussedative-hypnotic.

ASA American Society of Anesthesiologists Classification.


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A Retrospective Study of Intraoperative Awareness Withpdf