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TECHNOLOGY, COMPUTING, AND SIMULATION

The Relationship Between Bispectral Index and Electroencephalographic Parameters During Isoflurane Anesthesia

Yasuhiro Morimoto, MD^*, Satoshi Hagihira, MD, Yumika Koizumi, MD^*, Kazuyoshi Ishida, MD^*, Mishiya Matsumoto, MD^*, and Takefumi Sakabe, MD^*

*Department of Anesthesiology-Resuscitology, Yamaguchi University School of Medicine, Yamaguchi, Japan; and Department of Anesthesiology, Osaka Prefectural Habikino Hospital, Osaka, Japan

Address correspondence and reprint requests to Yasuhiro Morimoto, MD, Department of Anesthesiology-Resuscitology, Yamaguchi University School of Medicine, 1-1-1 Minami-Kogushi Ube, Yamaguchi, 755-8505, Japan. Address e-mail to naa01346{at}nifty.ne.jp

Bispectral index (BIS) integrates various electroencephalographic (EEG) parameters into a single variable. However, the exact algorithm used to synthesize the parameters to BIS values is not known. The relationship between BIS and EEG parameters was evaluated during nitrous oxide/isoflurane anesthesia. Twenty patients scheduled for elective ophthalmic surgery were enrolled in the study. After EEG recording with a BIS monitor (A-1050) was begun, general anesthesia was induced and maintained with 0.5%–2% isoflurane and 66% nitrous oxide. Using software we developed, we continuously recorded BIS, spectral edge frequency 95% (SEF95), and EEG parameters such as relative beta ratio (BetaRatio), relative synchrony of fast and slow wave (SynchFastSlow), and burst suppression ratio. BetaRatio was linearly correlated with BIS (r = 0.90; P < 0.01; n = 253) at BIS more than 60. At a BIS range of 30 to 80, SynchFastSlow (r = 0.60; P < 0.01; n = 3314) and SEF95 (r = 0.75; P < 0.01; n = 3339) were linearly correlated with BIS. The correlation between BIS and SEF95 was significantly better than the correlation between BIS and SynchFastSlow (P < 0.01). At BIS less than 30, the burst suppression ratio was inversely linearly correlated with BIS (r = 0.76; P < 0.01; n = 65). At BIS less than 80, burst-compensated SEF95 was linearly correlated with BIS (r = 0.78; P < 0.01; n = 3404). In the range of BIS from 60 to 100, BIS can be calculated from BetaRatio. At surgical levels of anesthesia, BIS and SynchFastSlow (a parameter derived from bispectral analysis) or burst-compensated SEF95 (derived from power spectral analysis) are well correlated. However, our results show that SynchFastSlow has no advantage over SEF95 in calculation of BIS.

IMPLICATIONS: The relationship between bispectral index (BIS) and electroencephalographic parameters was evaluated during nitrous oxide/isoflurane anesthesia. At surgical levels of anesthesia, BIS and the relative synchrony of fast and slow wave (a parameter derived from bispectral analysis) or burst-compensated spectral edge frequency 95% (a parameter derived from power spectral analysis) are well correlated.

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