Anesth Analg 2007;104:857-859
© 2007 International Anesthesia Research Society
doi: 10.1213/01.ane.0000258764.37968.bb
TECHNOLOGY, COMPUTING, AND SIMULATION
The Validity of Bispectral Index Values from a Dislocated Sensor: A Comparison with Values from a Sensor Located in the Commercially Recommended Position
Toshinori Horiuchi, MD,
Masahiko Kawaguchi, MD,
Naoko Kurita, MD,
Satoki Inoue, MD, and
Hitoshi Furuya, MD
From the Department of Anesthesiology, Nara Medical University, Nara, Japan.
Address correspondence and reprint requests to Masahiko Kawaguchi, MD, Department of Anesthesiology, Nara Medical University, 840 Shijo-cho, Kashihara, Nara 634-8522, Japan. Address e-mail to drjkawa{at}naramed-u.ac.jp.
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Abstract
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BACKGROUND: The influence of sensor dislocation on bispectral index (BIS) values is not clear. We compared the BIS values obtained from dislocated sensors with those from the commercially recommended positions.
METHODS: We used two BIS sensors for each patient receiving propofol-based anesthesia; one in the recommended position and one positioned around the lateral corner of the right eye.
RESULTS: Bland and Altman analysis revealed better agreement of two BIS values when the values during induction of and emergence from anesthesia were excluded.
CONCLUSIONS: The results indicate that during induction of and emergence from general anesthesia, a dislocated BIS sensor may produce questionable BIS values.
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Introduction
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The bispectral index (BIS) is currently the most evaluated parameter to measure the level of consciousness (1). BIS monitoring may be indicated during procedures where it is difficult to locate the BIS sensor at the commercially recommended position, e.g., during procedures on the head and face (2,3). Even BIS values can be obtained from a sensor in another location; the validity of such values has not been studied. Thus, in the present study, we compared the BIS values obtained from a dislocated sensor with those produced by a sensor located at the commercially recommended position.
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METHODS
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After institutional approval at Nara Medical University, Nara, Japan, written informed consent was obtained from all subjects. Twenty-seven patients, ASA PS 1 or 2 scheduled for elective abdominal or orthopedic surgery, were enrolled in this study (Table 1). The patients had no history of cerebral diseases. We used two BIS monitors (Aspect-2000 monitor, P/N 185-0070, Host Rev. 3.12, Aspect Medical Systems, Newton, MA) and two BIS sensors (BIS sensor PLUS, part no. 186-0076), incorporating three circular areas consisting of three electrodes, i.e., circle 1, 2, and 3, for each patient. The commercially recommended location of the sensor is as follows: circle 1 at the center of the forehead, approximately 1.5 in. (4 cm) above the nose, circle 2 at 2.8 cm lateral right to circle 1, and circle 3 on the temple area between the corner of the eye and the hairline. As shown in Figure 1, one sensor (sensor-1) was located at the commercially recommended position on the right side of the patient’s forehead, and the other (sensor-2) was located as follows: circle 1 on the zygomatic process of the right temporal bone, circle 2 on the outside corner of the right eye, and circle 3 just below circle 2 of sensor-1. The specific placement of sensor-2 was selected because it is often an available location and is close to the standard location on the forehead.
Before inducing anesthesia, we placed two sensors on each patient and recorded simultaneous BIS values. Each electrode impedance was checked and maintained at <5 k . All patients were anesthetized with fentanyl and propofol by target-controlled infusion (TCI pump TE-371, Terumo, Japan). The BIS values from each sensor (BIS-1 and BIS-2, respectively) were recorded at the following anesthetic states: awake, loss of response to verbal command (LORVC), target-controlled infusion of 3, 4, and 5 µg/mL propofol of effect-site concentration, eye opening (EO), and after tracheal extubation. We recorded the BIS values when the signal quality index scores were full.
For statistical analysis, the extent of agreement of BIS values from dislocated sensors with those from a sensor located at the commercially recommended position was assessed by Bland and Altman analysis (4). If the limits of agreement distributed between –10 and 10, we considered the data to indicate good agreement.
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RESULTS
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Bland and Altman analysis revealed a bias of –2.2 with 2sd of 14.5 (limits of agreement were distributed –16.7 to 12.2) when compared BIS-1 with BIS-2 (Fig. 2). When the data at LORVC and EO were excluded, that is, under stable anesthetic state, Bland and Altman analysis revealed a bias of –1.2 with 2sd of 8.5. (Limits of agreement were distributed –9.7 to 7.4.)
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Figure 2. Difference against mean for bispectral index (BIS) values obtained from patients for Bland and Altman analysis. The open triangles and the open circles are showing the anesthetic states of LORVC and EO, respectively. The filled circles are showing anesthetic states except for LORVC and EO. BIS-1, BIS values from sensor-1; BIS-2, BIS values from sensor-2; LORVC, loss of response to verbal command; and EO, eye opening (EO).
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DISCUSSION
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Our results showed that the Bland and Altman analysis indicated unacceptable disagreement between these two values when ±10 BIS units are used as the threshold for acceptability. However, if the data during the induction of anesthesia (LORVC) and at the emergence of anesthesia (EO) were excluded, Bland and Altman analysis indicated better, more acceptable agreement of these two values. These results suggest that BIS values obtained from a dislocated sensor are reliable only when the anesthetic depth is stable, not during induction and emergence from anesthesia.
In the present study, around LORVC, BIS values from a sensor located at the commercially recommended position often decreased earlier than those from dislocated sensors, and around EO, increased later. The exact reasons for the discrepancies are unknown. One possible reason may be that orbicularis oculi muscle contraction caused electromyogram to appear as artifact, and then increased BIS values from a dislocated sensor, which were closer to the muscle than a sensor located at the commercially recommended position. A recent BIS system (XP platform) might provide better results obtained from a dislocated sensor, because the XP platform detects and filters interference from the electromyogram.
As an example of a dislocated sensor for BIS monitoring, Hemmerling et al. (5) and Shiraishi et al. (6) applied the BIS sensor on the occipital region. The former reported only one case but indicated good agreement of the values between frontal (commercially recommended position) and occipital BIS (a bias of 1 with limits of agreement of –7 to 9, which was similar to our data under stable anesthetic state), whereas the latter showed a significant correlation between frontal and occipital BIS values but did not show the degree of agreement. They suggested the possibility of another sensor position, although little information was available on the limitations of occipital sensor placement. At the present time, we have to be careful about the interpretation of BIS values when the sensor is not in the commercially recommended position.
The present study was conducted with only propofol and fentanyl anesthesia. Relationships between BIS, depth of sedation (anesthesia), and concentrations of anesthetics have been validated with midazolam, alfentanil, isoflurane as well as propofol (7). We cannot exclude the possibility that the results of the present study may not be identical with other anesthetic.
In summary, we evaluated the validity of BIS values obtained from a dislocated sensor compared with a sensor located at the commercially recommended position. The results indicate that a dislocated BIS sensor may give questionable BIS values during the induction of, and emergence from, anesthesia. We often encounter situations in which a BIS sensor cannot be applied at the commercially recommended position because of overlap of the surgical fields. In such cases, BIS values may not be consistently reliable.
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Footnotes
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Accepted for publication December 20, 2006.
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Abstract
Introduction
