The Relationship between Propofol Concentration and Bicoherence, normalized parameter of Bispectrum, of Electroencephalogram

Kotoe Kamata, MD *, Osamu Nagata, MD, PhD *, Satoshi Hagihira, MD, PhD #, Yukako Hotta, MD *, Makoto Ozaki, MD, PhD *, Takahiko Mori, MD, PhD §, and Masaki Takashina, MD, PhD ¶

*Department of Anesthesiology, Tokyo Women’s Medical University, Tokyo Japan,

# Department of Anesthesia, Habikino Hospital, Osaka, Japan, § Department of Anesthesia, Osaka Prefectual General Hospital, Osaka, Japan, ¶Surgical Center, Osaka University Hospital, Osaka, Japan

Introduction:

We previously reported that bicoherence, normalized parameter of bispectrum, of electroencephalogram (EEG) would become a good indicator for ‘depth of anesthesia’ under general anesthesia with isoflurane¹⁾. Here we investigated the relationship between propofol concentration and EEG bicoherence.

Materials and Methods:

After approval of institutional ethical committee and informed consent, we included 8 patients who underwent elective abdominal surgery. EEG signal as well as Bispectral Index (BIS) was continuously monitored by A-1050 EEG monitor with BIS Sensor Pulse (Aspect Medical Systems, USA), and recorded on an IBM-PC compatible computer. Diprifusor (AstraZeneca, UK) syringe pumps for target-controlled infusion (TCI) were used for propofol delivery, and both the predicted blood concentration and the effect-site concentration of propofol were continuously calculated by this system. Anesthesia was induced with 2 µg/kg of fentanyl and propofol at the target concentration of 4.0 µg/ml, and maintained with propofol, fentanyl, and epidural anesthesia (1-2 % of lidocaine was continuously administered 80-110 mg/hr). EEG bicoherence values and spectral edge frequency 90% (SEF90) were calculated by our hand-made software BSA¹⁾ every one minute. As our previous study revealed that bicoherence showed two peaks in the frequency versus frequency space around diagonal line (f₁=f₂) during isoflurane anesthesia¹⁾, we defined these peaks as pBIClow (around 3-6Hz) and pBIChigh (around 8-13 Hz). Then we compared these pBIC values at the propofol effect-site concentrations of 2.0, 2.5, 3.0 and 4.0 µg/ml during surgery (at first 4.0 µg/ml and then stepping down to 2.0 µg/ml). Each concentration was kept more than 15 minutes.

Results:

All bicoherence values before propofol delivery and after emergence from propofol anesthesia were small, while at higher concentrations both peak values increased. pBIChigh became higher in proportion to propofol effect-site concentration, and highest (30.2±11.0%) at 3.0 µg/ml. On the other hand, pBIClow reached to plateau (33.0±7.4%) at 2.5 µg/ml. The correlation between BIS and pBIChigh was very good (r=-0.938, p<0.01). In 4 cases, when we tried to decrease propofol concentration to 2.0 µg/ml, BIS increased beyond 65, which indicated the inadequate level of sedation.

Discussion: Our result revealed that peaks of bicoherence emerged in acceptable level of propofol anesthesia as well as isoflurane anesthesia, and that the peaks around 8-13 Hz showed good correlation to BIS values at the effect-site concentration from 2.0 to 4.0 µg/ml. These finding suggested that EEG bicoherence would become a good indicator of adequate level of hypnosis under propofol anesthesia as well as isoflurane anesthesia.

Reference:

1. Hagihira S, et al. Anesth Analg, 93(4) : 966-70, 2001.