Awareness is a rare but distressing anesthesia complication, with an incidence of 0.1%–0.2% (1,2). It is usually associated with subhypnotic levels of anesthesia necessitated by hemodynamic instability, often during cardiac, obstetric, emergency, or trauma anesthesia. The presence of neuromuscular, sympathetic, or β-adrenergic blockade can prevent detection because of the patient’s inability to respond. It has been hoped that the development of more sophisticated monitors of electroencephalographic activity might add an additional measure of depth of consciousness and avoid this complication.
A 60-yr-old man, height 69 in, weight 137 kg (body mass index, 44) presented electively for open gastric bypass and cholecystectomy. He had a history of myocardial infarction, coronary angioplasty, an ejection fraction of 45%, and insulin-dependent diabetes. Beta blockade had been initiated before surgery. He used continuous positive airway pressure for obstructive sleep apnea. Chronic back pain was managed with Oxycontin™ 40 mg tid, Neurontin, and cyclobenzaprine.
An epidural catheter was placed at T9–10 to a depth of 9 cm and threaded 5 cm cephalad. A 3-mL test dose of 1.5% lidocaine with 1:200,000 epinephrine was followed by 3 mL of 2% lidocaine, producing bilateral sensory blockade from T4 to T12. The catheter was reinjected with 3 mL and 2 mL of lidocaine at 60-min intervals during the 3-h case. With standard monitoring, general anesthesia was induced with thiopental 400 mg and succinylcholine 160 + 60 mg to facilitate tracheal intubation.
Anesthesia was maintained with sevoflurane, in the range of 0.45%–0.8% end-tidal. From skin incision to the commencement of skin closure, end-tidal sevoflurane concentration was always more than 0.45% as determined by an agent-specific self-calibrating infrared gas analyzer (M1026A Anesthetic Gas Module; Agilent Technologies, Andover, MA).
Cisatracurium provided muscle relaxation. Hypotension, despite fluids and vasopressors, limited the use of sevoflurane. A Bispectral index® (BIS) monitor was placed before incision. The initial reading was 37, the average BIS during the surgical procedure was 44 ± 5 (mean ± SD), and the highest recorded value was 51 (Table 1). Good signal quality was indicated throughout. His heart rates were in the 80s throughout surgery with systolic blood pressures ranging between 80 and 130 mm Hg (baseline, 122/55 mm Hg).
At the end of surgery, neuromuscular blockade was reversed and sevoflurane discontinued. The patient awoke and was tracheally extubated. When asked if he was in pain, he responded “Not now, but I was during surgery.” On further questioning, he described no recall of intubation but vivid, painful recall of his surgery, with “unimaginable” pain and the sensation that people were “tearing at me.” He wished he were dead and tried to communicate his distress. He heard voices in the operating room but was unable to recall the content of what was said, remembering only that it was “shop talk.”
He continued to be troubled by recall and nightmares. A staff psychiatrist offered supportive therapy, advising continuing treatment for resolution of his experience.
Awareness under anesthesia for elective surgical procedures is rare but can have serious psychological consequences (3). It can occur with unintended interruptions of anesthetic delivery or when tolerance of anesthetics is limited by patient factors. Neuromuscular blockade that prevents movement in response to pain increases the likelihood of awareness. It may be undetected in the presence of sympathetic or β-blockade that prevents tachycardia and hypertension. Avoidance of opioids and benzodiazepines to reduce potential respiratory depression also increases the risk. Our patient’s anesthetic course included several of these risk factors for awareness. Conversely, there is a known reduction of inhaled anesthetic requirements in the presence of epidural anesthesia (4).
The BIS monitor is a complex, processed electroencephalogram that uses a computer algorithm to assign a numerical value to the probability of consciousness. The algorithm is presumed to indicate an adequate depth of consciousness with a BIS number in the range 40–60. There are limitations to this technology (5). It does not consistently reflect anesthetic depth for different anesthetic drug combinations (6,7) and has not been studied extensively in the presence of concomitant medications. The BIS algorithm may be altered by preoperative opioid and anticonvulsant use. Recent reports describe a decreased probability of awareness in high-risk patients when BIS monitoring is used (8) and in a general population when the use of BIS monitoring is compared with historical controls (9). There were instances of recall in the monitored group in both studies. Other studies have shown that there is large variability in the BIS values at times of measured awareness (10), which limits the predictability of awareness based on the BIS number (11). A case report confirmed explicit recall with a BIS of 47 (12).
In this case, not only did the BIS monitor not reliably predict the absence of awareness but its use may also have contributed to the occurrence of awareness. In the absence of this monitor, an end-tidal concentration of 0.45%–0.8% sevoflurane, even with a working epidural, might have been regarded as an inadequate anesthetic and the level of sevoflurane would have been increased or benzodiazepines added to prevent recall (with additional vasopressor support).
In summary, we report a case of awareness in a high-risk patient despite an “adequate” BIS reading. Although maintenance of BIS values <60 may reduce the probability of awareness, this risk is not eliminated.
1. Liu WHD, Thorp TAS, Graham SG, Aitkenhead AR. Incidence of awareness with recall during general anesthesia. Anaesthesia 1991;46:435–7.
2. Sandin RH, Enlund G, Samuelsson P, Lennmarken C. Awareness during anaesthesia: a prospective case study. Lancet 2000;355:706–11.
3. Moerman N, Bonke B, Oosting J. Awareness and recall during general anesthesia: facts and feelings. Anesthesiology 1993;79:454–64.
4. Hodgson PS, Liu SS. Epidural lidocaine decreases sevoflurane requirement for adequate depth of anesthesia as measured by the bispectral index® monitor. Anesthesiology 2001;94:799–803.
5. Drummond JC. Monitoring depth of anesthesia. Anesthesiology 2000;93:876–82.
6. Vernon JM,. Lang E, Sebel PS, Manberg P. Prediction of movement using bispectral electroencephalographic analysis during propofol/alfentanil or isoflurane/alfentanil anesthesia. Anesth Analg 1995;80:780–5.
7. Mi WD, Sakai T, Singh H, et al. Hypnotic endpoints vs. the bispectral index, 95% spectral edge frequency and median frequency during propofol infusion with or without fentanyl. Eur J Anaesthesiol 1999;16:47–52.
8. Myles PS, Leslie K, et al. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet 2004;363:1757–63.
9. Ekman A, Lindholm ML, Lennmarken C, Sandin R. Reduction in the incidence of awareness using BIS monitoring. Acta Anaesthesiol Scand 2004;48:20–6.
10. Schneider G, Gelb AW, Schmeller B, et al. Detection of awareness in surgical patients with EEG-based indices—bispectral index and patient state index. Br J Anaesth 2003;91:329–35.
11. Schneider G, Wagner K, Reeker W, et al. Bispectral Index (BIS) may not predict awareness reaction to intubation in surgical patients. J Neurosurg Anesthesiol 2002;14:7–11.
12. Mychaskiw G, Horowitz M, Sachdev V, Heath BJ. Explicit intraoperative recall at a bispectral index of 47. Anesth Analg 2001;92:808–9.