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ANESTHETIC PHARMACOLOGY

Nitrous Oxide Induces Paradoxical Electroencephalographic Changes After Tracheal Intubation During Isoflurane and Sevoflurane Anesthesia

Department of Anesthesiology and Intensive Care Medicine, Graduate School of Medicine, Osaka City University, Osaka, Japan; Department of Anesthesiology, Hoshigaoka Koseinenkin Hospital, Hirakata, Japan

Address correspondence and reprint requests to Yutaka Oda, MD, PhD, Department of Anesthesiology and Intensive Care Medicine, Graduate School of Medicine, Osaka City University, 1-5-7 Asahimachi, Abeno-ku, Osaka 545-8586, Japan. Address e-mail to odayou{at}msic.med.osaka-cu.ac.jp.

In this randomized, double-blind, controlled study, we tested the hypothesis that nitrous oxide (N₂O) affects bispectral index (BIS) and 95% spectral edge frequency (SEF95) in response to tracheal intubation during anesthesia with isoflurane and sevoflurane. In protocol 1, we randomly allocated 90 ASA physical status I patients to 6 groups (n = 15 each). Anesthesia was induced with isoflurane or sevoflurane with 0%, 33%, or 66% N₂O. The concentration of isoflurane and sevoflurane was gradually increased and end-tidal concentrations were maintained at 1.1% and 1.7%, respectively. Tracheal intubation was performed 12 min after induction of anesthesia. BIS was significantly increased 1 min after tracheal intubation compared before laryngoscopy in patients receiving only isoflurane or sevoflurane (P = 0.001 and 0.007, respectively). In patients receiving 66% N₂O-isoflurane or 66% N₂O-sevoflurane, both BIS and SEF95 were significantly decreased after tracheal intubation and significantly lower than in those patients receiving only isoflurane or sevoflurane, respectively (P < 0.01 for both). In protocol 2, 3 µg/kg of IV fentanyl completely abolished the decrease of BIS and SEF95 after tracheal intubation during anesthesia with 66% N₂O-isoflurane and 66% N₂O-sevoflurane (n = 10). We conclude that 66% N₂O induced a paradoxical decrease of BIS in response to tracheal intubation during anesthesia with isoflurane and sevoflurane.

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