Anesth Analg 2004;98:1036-1038
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000103261.86889.DE
TECHNOLOGY, COMPUTING, AND SIMULATION
Unusually Low Bispectral Index Values During Emergence from Anesthesia
Satoshi Hagihira, MD, PhD*,
Kenta Okitsu, MD
, and
Maki Kawaguchi, MD
*Department of Anesthesiology, Osaka University Graduate School of Medicine, and the
Department of Anesthesiology, Osaka Prefectural Habikino Hospital, Osaka, Japan
Address correspondence and reprint requests to Satoshi Hagihira, Department of Anesthesiology, Osaka University Graduate School of Medicine, 2–2 Yamadaoka, Suita City, Osaka, Japan 565–0871. Address email to hagihira{at}masui.med.osaka-u.ac.jp
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Abstract
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We observed unusually low BIS values during emergence from anesthesia apparently caused by misanalysis (as "suppression") of low voltage electroencephalogram.
IMPLICATIONS: When BIS values do not adequately correspond with clinical status, it is necessary to check raw electroencephalogram waveforms to more clearly characterize patient status.
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Introduction
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It is widely accepted that bispectral index (BIS) information well characterizes "depth of anesthesia" or "level of hypnosis" (1,2). Some reports, however, state that in some specific situations BIS does not properly indicate "level of hypnosis" (3–5).
We report two cases in which, just before emergence, electroencephalogram (EEG) voltage greatly diminished (
3–4 µV) and BIS values decreased to less than 30.
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Case Reports
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Case 1
A 16-yr-old female was scheduled for right middle and lower lobectomy for right lung tumor. Ten minutes before admission to the operating room, morphine (2 mg) was administered IV as premedication. EEG was monitored with an A-1050 EEG monitor (software version 3.4; Aspect Medical Systems, Natick, MA). After mild ablation of the skin, four self-sticking frontal surface electrodes (At1 and At2 with Fpz as reference and Fp1 as the ground) were placed on both sides of the outer malar bone. All impedance values were kept below 5 k
. Along with BIS and other processed data, raw EEG signals were also acquired from the A-1050 and recorded on an IBM PC using our software BSA for BIS (6). The expired concentration of sevoflurane and carbon dioxide were monitored by CAPNOMACTM (Datex, Helsinki, Finland) and similarly recorded on the computer. Anesthesia was induced with 3 mg/kg of thiopental followed by 4 µg/kg of fentanyl and 0.15 mg/kg of vecuronium. The patient’s trachea was intubated with a 35F left-side Bronchocath® (Mallinckrodt, St. Louis, MO), and an epidural catheter was placed in the intervertebral space at T5-6. Anesthesia was maintained with 1.5%–1.6% of sevoflurane in oxygen. Fentanyl 1 µg/kg was intermittently administered, eventually totaling 1200 µg. Lidocaine 1% (70–80 mg) was also intermittently injected via the epidural catheter. During the operation, BIS values were kept between 40 and 50 and EEG waveform displayed the spindle-wave-dominant pattern shown in Figure 1A. Operating time was 6 h. After surgery, sevoflurane was discontinued and, for a while, BIS values gradually increased; however, BIS then decreased without any abnormal physiological signs. BIS values bottomed at 27 when the expired concentration of sevoflurane was 0.5%. The BIS monitor SQI (signal quality index) was 100%, and no artifacts were observed. At this point, the spindle wave pattern disappeared completely and, as Figure 1B shows, a very low-voltage fast wave appeared. Although no burst and suppression pattern was observed, the suppression ratio (SR) value was 48. At this time, systolic blood pressure was 122 mm Hg, heart rate was 63 bpm, and SpO2 was 100%. Computer simulation indicated an estimated effect site concentration of fentanyl of about 3.0 ng/mL. The SR value then decreased while BIS increased. After that, the patient opened her eyes, reacted to her name, and was soon tracheally extubated. No neurological deficit was detected. Figure 2 shows the changes in BIS values and expired sevoflurane concentration.
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Figure 1. Case 1. A, electroencephalogram (EEG) waveform obtained from bispectral index (BIS) monitor and power spectrum during surgery. Expired concentration of sevoflurane was 1.6%. B, EEG waveform and power spectrum just before emergence, when BIS value decreased to 26.
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Figure 2. Changes in bispectral index (BIS) values and expired concentration of sevoflurane during emergence in Case 1. X-axis indicates the local clock time; Y-axis at left indicates BIS values; and Y-axis at right indicates expired concentration of sevoflurane. Open circles indicate BIS and open diamonds indicate expired concentration of sevoflurane. a, end of surgery; b, chest radiograph examination; c, response to calling of name; and d, extubation.
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Case 2
A 65-yr-old male with lung cancer was scheduled for left upper lobectomy. Initially, an epidural catheter was placed in the intervertebral space at T5-6. As in Case 1, EEG was monitored using an A-1050 EEG monitor. Anesthesia was induced with 3 mg/kg of thiopental followed by 3 µg/kg of fentanyl and 0.15 mg/kg of vecuronium. His trachea was intubated with a 39F right-side Bronchocath®. Anesthesia was maintained with 0.9% of isoflurane in oxygen. Lidocaine 1% (60 mg) was also intermittently administered (every 40–50 min) via epidural catheter. Fentanyl was not administered after induction. During surgery BIS was maintained in the 40–50 range. Operating time was 4 hours. After surgery, isoflurane was discontinued and BIS gradually increased. However, 7 min after cessation of isoflurane, the BIS value decreased to 28 and SR increased to 45. SQI was 100%. The EEG waveform was similar to that in Case 1 at the same stage of recovery. At this time, systolic blood pressure was 115 mm Hg, heart rate was 67 bpm, and SpO2 was 100%. Within 3 min, BIS rebounded and soon after that, the patient opened his eyes and responded to his name. He was then tracheally extubated and transferred back to ward. No neurological deficits were detected.
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Discussion
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In deep anesthesia, EEG waveforms alter and show a burst and suppression pattern characterized by alternating periods of normal to high voltage activity changing to low voltage or even isoelectricity. Here, we present two cases in which the EEG waveform pattern was misanalyzed by the BIS monitor as burst and suppression. Rampil (7) has described suppression as periods of longer than 0.5 seconds during which EEG voltage does not exceed approximately ±5.0 µV, and he explained that the SR algorithm used for BIS calculations uses a similar definition. EEG amplitude, however, varies for each patient because of different skull and scalp thickness, as well as individual variation in neuronal activities or degree of synchronization of cortical neurons. When criteria are based on the measurement variable of voltage, some misinterpretation is almost inevitable.
Schnider et al. (3) have reported a case in which, because the patient’s EEG amplitude was genetically very low, the BIS value was 40 during consciousness. This case also likely resulted from misinterpreted suppression data. In both our cases, EEG waveforms during anesthesia were typical and their amplitudes were large enough; therefore BIS values during anesthesia were considered to be adequate. Therefore, the phenomenon we describe here would not have been caused by genetics.
Muncaster et al. (5) have described similar paradoxical changes of BIS during recovery from sevoflurane and remifentanil anesthesia. They first withdrew sevoflurane while remifentanil continued to infuse at 0.4 µg · kg-1 · min-1. After cessation of remifentanil, for 6 of 21 patients, BIS values decreased and SR values increased. It is possible that these paradoxical changes in BIS values could have been caused by the use of the opioid remifentanil. In our Case 2, however, the analgesic was epidural lidocaine, which is not an opioid. Furthermore, in Case 2 we applied isoflurane, but not sevoflurane. The phenomenon that we are presenting in the current report does not appear to be drug specific.
Burst and suppression also occurs in states of severe cerebral damage, most characteristically in anoxic encephalopathy (8). Mérat et al. (9) have reported that BIS monitoring might be useful for detecting severe cerebral ischemia because it is likely that BIS would unexpectedly decrease in a progressive manner as brain ischemia advances. Even so, when anomalous BIS values are observed, an alternative diagnosis is easily made just by observing the EEG waveform. In the current study we observed an EEG pattern that resembled rippled waves, which are sometimes observed in the initial stage of natural sleep. When adequate analgesia is provided, doses of anesthetic may be reduced to levels in which the status resembles natural sleep.
In conclusion, if unusual or unexpected BIS values occur, we strongly recommend observation of EEG waveforms.
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References
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- Sebel PS, Lang E, Rampil IJ, et al. A multicenter study of Bispectral electroencephalogram analysis for monitoring anesthetic effect. Anesth Analg 1997; 84: 891–9.[Abstract]
- Glass PS, Bloom M, Kearse L, et al. Bispectral analysis measures sedation and memory effects of propofol, midazolam, isoflurane, and alfentanil in healthy volunteers. Anesthesiology 1997; 86: 836–47.[ISI][Medline]
- Schnider TW, Luginbuhl M, Petersen-Felix S, Mathis J. Unreasonably low bispectral index values in a volunteer with genetically determined low-voltage electroencephalographic signal. Anesthesiology 1998; 89: 1607–8.[ISI][Medline]
- Detsch O, Schneider G, Kochs E, et al. Increasing isoflurane concentration may cause paradoxical increases in the EEG bispectral index in surgical patients. Br J Anaesth 2000; 84: 33–7.[Abstract/Free Full Text]
- Muncaster ARG, Sleigh JW, Williams M. Changes in consciousness, conceptual memory, and quantitative electroencephalographical measures during recovery from sevoflurane- and remifentanil-based anesthesia. Anesth Analg 2003; 96: 720–5.[Abstract/Free Full Text]
- Hagihira S, Takashina M, Mori T, et al. Practical issues in bispectral analysis of electroencephalographic signals. Anesth Analg 2001; 93: 966–70.[Abstract/Free Full Text]
- Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology 1998; 89: 980–1002.[ISI][Medline]
- Kuroiwa Y, Celesia GG. Clinical significance of periodic EEG patterns. Arch Neurol 1980; 37: 15–20.[Abstract]
- Mérat S, Levecque JP, Le Gulluche Y, et al. BIS monitoring may allow the detection of severe cerebral ischemia. Can J Anaesth 2001; 48: 1066–9.[Abstract/Free Full Text]
Accepted for publication October 10, 2003.
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Abstract
Introduction
