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NEUROSURGICAL ANESTHESIA: Brief Report

A Comparison of Frontal and Occipital Bispectral Index Values Obtained During Neurosurgical Procedures

Shiraishi, Toshie MD; Uchino, Hiroyuki MD; Sagara, Takeshi MD; Ishii, Nagao MD

Editor(s): WARNER, DAVID S.

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doi: 10.1213/01.ANE.0000121344.69058.09
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Bispectral index (BIS) monitoring is a noninvasive and convenient method to evaluate anesthetic depth on the basis of the electroencephalogram (EEG) (1). BIS sensors have also been developed as a single simplified strip composed of 3 or 4 electrodes placed on the forehead; this is different from the international standard 10–20 system used in conventional EEG monitoring (2). Despite the use of this nontraditional electrode placement, it has been demonstrated that the effectiveness and utility of this form of monitoring have resulted in its widespread use (2). In most surgical procedures, frontal recording with a BIS sensor is used. However, for certain neurosurgical operations the operating field does not allow frontal placement of a BIS sensor. We examined whether it is possible to place the BIS sensor on a different area, such as the occiput, to allow continuous monitoring. To confirm the validity of this alternative approach, we placed BIS sensors on both the frontal and occipital area in neurosurgical patients to compare the BIS values obtained during neurosurgical procedures.

Methods

After we obtained approval from the local ethics committee and written, informed consent, 25 patients scheduled for clipping of unruptured cerebral aneurysms were enrolled in this study. Each was classified as ASA physical status I. We used 2 A-1050 BIS monitors (Version 3.3; Aspect Medical Systems, Newton, MA) and 2 standard BIS sensors (Aspect Medical Systems Part 186-0100). Before the induction of anesthesia, we placed one sensor with Electrode 1 on the center of the forehead, approximately 1.5 in above the nose, Electrode 2 at 1.1 in lateral to Electrode 1, and Electrode 3 electrode on either temporal area between the corner of the eye and the hairline. A second electrode was placed on the occiput with Electrode 1 on the central occipital area 1 in above the occipital process, Electrode 2 at 1.1 in lateral to Electrode 1, and Electrode 3 on the posterior-temporal area (Fig. 1). The frontal and occipital electrodes were placed on the same side of the head, on either the left or right. We started recording of BIS and spectral edge frequency 95% (SEF95) scores in both areas by using 2 BIS monitors. We carefully placed 2 sensors with equal distances between the electrodes, because the distance between two electrodes determines the amplitude of the signal that is recorded (3). Electrode impedance was maintained <5000 Ω to ensure adequate signal quality.

F1-48
Figure 1.:
A bispectral index sensor was placed on both the frontal (A) and occipital (B) areas. The detailed locations are described in the text.

Anesthesia was induced with IV propofol and fentanyl (2 mg/kg). The propofol plasma concentration was first set at 6 μg/mL by using a target-controlled infusion (TCI) (STC-525X; Terumo). Anesthesia was maintained with an infusion of fentanyl at 2 μg · kg−1 · h−1 and with TCI of propofol at a concentration adjusted to ensure BIS was maintained between 40 and 60.

Data were collected before the induction of anesthesia, after induction, after incision (start of surgery), and every 15 min after incision to obtain the full Signal Quality Index score.

Patient data are presented as mean ± SD. We compared scores of BIS and SEF95 recorded from the frontal and occipital areas and used linear regression analysis. Pearson's correlation coefficient with Fisher's r to z conversion was used to evaluate the correlation between frontal and occipital values in the resulting simple linear correlations. Probability values <0.05 were regarded as indicating significant differences.

Results

Table 1 shows the demographic data of patients. Patient ages were from 39 to 71 yr. Figure 2A shows the correlation of BIS values recorded from the frontal and occipital areas (r2 = 0.9612; P = 0.003). Figure 2B shows the correlation of SEF95 values recorded from the frontal and occipital areas (r2 = 0.8763; P = 0.018). Thus, our results showed a strong correlation between frontal and occipital recordings of both BIS and SEF95.

T1-48
Table 1:
Demographic Data of 25 Patients Who Underwent Cerebral Aneurysm Clipping
F2-48
Figure 2.:
A, The correlation of bispectral index (BIS) values recorded from both the frontal and occipital areas (r 2 = 0.9612; P = 0.003). B, The correlation of spectral edge frequency 95% (SEF95) values recorded from both the frontal and occipital areas (r 2 = 0.8763; P = 0.018).

Discussion

After numerous developments, BIS has been established as a widely used, practical monitoring method (4). BIS is the combined information of a large number of EEG data and is transformed into a linear dimensionless scale from 0 to 100 (5). The BIS level corresponds to the clinical state of anesthesia and predominant EEG pattern: synchronized high-frequency activity reflects an awake state, normalized low-frequency activity reflects a moderate hypnotic level, and EEG suppression reflects a deep hypnotic level (2).

Michenfelder (6) demonstrated a strong relationship between EEG activity and cerebral metabolic rate during anesthesia. Alkire (7) further investigated the relationship between BIS and whole-brain cerebral metabolic reduction (CMR) by using positron emission tomography. BIS correlated in a linear manner with the anesthesia-induced CMR changes. Further, CMR was relatively homogeneous throughout the brain. On this basis, BIS also should be similar throughout the brain. However, the extent to which this can be extrapolated to the anesthetized patient is unknown, given the well known local heterogeneity of CMR effects caused by different anesthetics.

Glass et al. (8) demonstrated that BIS recorded from several different montages provided similar results. They used a frontal (Fp1 and Fp2) to CZ montage, as well as a montage (FPz–At) that approximates the BIS sensor montage.

We recommend placement of the sensor on the standard frontal area except in the few cases in which the frontal location cannot be used because of the surgical approach. However, one limitation of this study was that most of the patients were maintained within a narrow BIS range. We also do not know whether similar correlations will be found if different anesthetics are tested. It is also possible that patients with different intracranial pathology may respond differently.

In conclusion, BIS showed a strong correlation between frontal and occipital montage during propofol/fentanyl anesthesia. It may be valid to measure BIS with the sensor placed on the occipital area in cases in which it is difficult to place it on the frontal area, such as in patients undergoing neurosurgical operations.

We are grateful to Dr. Paul Manberg at Aspect Medical Systems, Inc., for his advice and instruction in executing this study.

References

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8. 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.
© 2004 International Anesthesia Research Society