The bispectral index predicts responsiveness to verbal commands in patients emerging from nitrous oxide anesthesia supplemented with a subhypnotic concentration of isoflurane.
OBJECTIVE
Nitrous oxide (N2O) administered alone has minimal effects on the bispectral index (BIS), an electroencephalogram-derived parameter of hypnosis. However, because this gas is commonly supplemented with a volatile anesthetic, we sought to determine how it would affect the BIS when coadministered with a low concentration of isoflurane.


METHODS
Twelve patients were anesthetized with 70% N2O + 0.2% isoflurane (all concentrations are end-tidal). Following the end of surgery, the concentration of N2O was decreased in decrements of 10% while isoflurane was continued at 0.2%, and each new concentration of N2O was maintained for 15 min. This procedure was repeated until the patients first opened their eyes or squeezed the investigator's hand on command.


RESULTS
N2O 70% + isoflurane 0.2% reduced the BIS to 68 +/- 9 (mean +/- SD). When the concentration of N2O was decreased toward awakening (which occurred at the N2O concentration of 36% +/- 8%), the BIS progressively increased until it reached 93 +/- 5 on awakening.


CONCLUSIONS
The BIS reflects the level of hypnosis during N2O anesthesia supplemented with a low concentration of isoflurane.
Goto
Takahisa
T
Department of Anesthesia and Critical Care, Teikyo University School of Medicine, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan.
Ishiguro
Yoshiki
Y
Nakata
Yoshinori
Y
Morita
Shigeho
S
D000328:Adult; D000769:Anesthesia, Inhalation; D004569:Electroencephalography; D006801:Humans; D007530:Isoflurane / Q000008:administration & dosage*; D008875:Middle Aged; D009609:Nitrous Oxide / Q000008:administration & dosage*
D007530:Isoflurane; D009609:Nitrous Oxide