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Values of the bispectral index do not parallel the hemodynamic response to the rapid increase in isoflurane concentration

[Les valeurs de l'index bispectral ne correspondent pas à la réponse hémodynamique à la suite de l'augmentation rapide de la concentration d'isoflurane]

Masayasu Nakayama, MD^*, Michiko Hayashi, MD^*, Hiromichi Ichinose, MD^*, Shuji Yamamoto, MD, Noriaki Kanaya, MD and Akiyoshi Namiki, MD PhD

^* From the Division of Anesthesia, Obihiro Kosei Hospital, and
the Sapporo Medical University School of Medicine, Sapporo, Japan.

Dr. Masayasu Nakayama, Department of Anesthesiology, Sapporo Medical University School of Medicine, South 1, West 16, Sapporo 060-8543, Japan. Phone: 81-11-611-2111; Fax: 81-11-631-9683; E-mail: miyabi{at}zc4.so-net.ne.jp

Purpose: To investigate the changes in hemodynamic variables and bispectral index (BIS) in response to a rapid increase in isoflurane or sevoflurane concentration.

Method: Thirty adult patients were anesthetized with either isoflurane (isoflurane group) or sevoflurane (sevoflurane group). Two minutes after induction of anesthesia with thiamylal, the inspired concentrations of isoflurane and sevoflurane were rapidly increased from 0.5 minimum alveolar anaesthetic concentration (MAC) to 3 MAC and maintained for five minutes. Heart rate (HR), mean arterial pressure (MAP), and BIS were measured every minute.

Results: An increase in the anesthetic concentration caused increases in HR and MAP in the isoflurane group and a decrease in MAP in the sevoflurane group. Consequently, HR and MAP in the isoflurane group were significantly higher than those in the sevoflurane group. After inhalation of high concentrations, BIS significantly and progressively decreased in both groups.

Conclusion: BIS values decrease after a step increase in volatile agent concentration, whether or not a hyperdynamic action occurs.

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