Bilateral bispectral index monitoring during suppression of unilateral hemispheric function

Abstract

Bispectral Index (BIS) has been used to monitor level of "sedation" based on the electroencephalogram (EEG). Patients evaluated for surgery to control a seizure disorder undergo Wada testing, during which one hemisphere is rendered functionally inactive after injecting a short-acting barbiturate. We surmised that the BIS values would reflect these functional changes. Eight epileptic patients were enrolled. A full array of 21 EEG electrodes and 2 BIS XP (Quatro) strips over each frontal region of the scalp were applied. The EEG was continuously recorded. BIS values from each hemisphere were recorded every minute. Angiography was performed by advancing a catheter into each internal carotid artery. Amobarbital or methohexital was injected until the patient developed a hemiparesis. The EEG confirmed a significant lateralized cortical effect of the barbiturate. Repeated measures analysis of variance was used to analyze the differences between the BIS values from monitor electrode strips placed on the left (left BIS) and the right (right BIS) sides of the head as well as the differences in the left and right BIS values before and after each injection of the barbiturate. Injection of barbiturate into either the left or right internal carotid artery produced a significant change on the 21-electrode EEG. However, there was no difference between left BIS to right BIS values (P = 0.84). With repeated injections of barbiturates, some patients became sedated. At these times, both left BIS and right BIS values decreased together before and after injection of barbiturate. The BIS monitor was unable to distinguish significant hemispheric EEG and clinical functional changes except when the patient became sedated.

Figure 1

Placement of Bispectral Index (BIS) XP electrodes. Two strips of BIS XP electrodes were placed over each side of the forehead. They were labeled left and right. We show both electrode strips in this figure. The electrodes were numbered 1 through 4. Electrodes 1 and 3 are active and record the electroencephalogram. Electrode 2 is the reference electrode and electrode 4 is used to measure eye movement artifact.

Figure 2

Electroencephalogram (EEG) recorded with 10-20 system of electrode placement. The filter settings were 1–70 Hz. Only 8 midline electrodes from right (even numbers) and left (odd numbers) are shown for illustrative purposes. Two different epochs of EEG are shown: baseline before and just after injection of methohexital into the right internal carotid artery. After an injection of methohexital, there is significantly more slowing of the EEG on the right with more δ-θ activity of large amplitude and increased muscle artifact compared with changes on the left. Associated with these EEG changes is the development of a left hemiplegia (strength 0/5) with normal strength on the right (motor strength on the left is shown below the EEG). The scales for time and voltage amplitudes are shown on the figure.

Figure 3

All bilateral Bispectral Index (BIS) values for right and left injections. This is a continuous recording of BIS values from both hemispheres for the same patient as illustrated in Figure 2: the right BIS values are represented by the dashed line with the open diamonds, and the left, by the continuous line with the solid squares. The solid bars are the times of injection of methohexital and are so labeled. BIS values are unitless. The open stars are the approximate times of the cerebral angiograms first from the right internal carotid artery, second from the left internal carotid artery. Figure 2 is taken from the beginning of the right injection. Time is denoted in minutes along the x axis.