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A Case of Awareness Despite an “Adequate Depth of Anesthesia” as Indicated by a Bispectral Index® Monitor

Rampersad, Sally E. MB, FRCA; Mulroy, Michael F. MD

doi: 10.1213/01.ANE.0000148121.84560.8D
Technology, Computing, and Simulation: Case Report

We report a case of awareness that occurred despite the presence of an “adequate” depth of anesthesia as measured by Bispectral Index®. Our patient was at high risk for this complication. Hypotension limited the use of sevoflurane, and neuromuscular, sympathetic, and β-adrenergic blockade prevented the patient from responding to his awareness. Preoperative use of opioids and gabapentin for chronic pain may have modified his response to anesthesia or affected his Bispectral Index reading. Our attempt to measure depth of anesthesia may have resulted in false reassurance concerning adequacy of anesthesia and contributed to awareness.

IMPLICATIONS: Although the use of processed brain wave monitoring may help reduce the chance of patient recall resulting from inadequate depth of anesthesia during surgery, our patient illustrates that this cannot universally be predicted or avoided.

Department of Anesthesiology, Virginia Mason Medical Center, Seattle, Washington

Accepted for publication October 1, 2004.

Address correspondence and reprint requests to M. F. Mulroy, MD, Department of Anesthesiology, Virginia Mason Medical Center, 1100 Ninth Ave., B2-AN, PO Box 900, Seattle, WA, 98111. Address e-mail to anemfm@vmmc.org.

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.

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Case Report

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).

Table 1

Table 1

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.

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Discussion

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.

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References

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