title: The impact of nitrous oxide on electroencephalographic bicoherence during isoflurane anesthesia.
abstract: BACKGROUND
  We previously reported that electroencephalographic (EEG) bicoherence, the degree of phase coupling among the frequency components of a signal, showed 2 peaks during isoflurane anesthesia. Hayashi et al. (Br J Anaesth 2007;99:389-95) also revealed that the peak frequency of bicoherence around 10 Hz increased when ketamine was added. Because nitrous oxide (N(2)O) and ketamine share several common features, they are often treated as the same category of anesthetic. Here, we investigated the effect of N(2)O on EEG bicoherence and other EEG derivatives during isoflurane anesthesia.
  
  
  METHODS
  Twenty patients (aged 34-72 years, ASA physical status I and II) of either gender who underwent elective laparoscopic surgery were included. Raw EEG data, along with EEG-derived parameters, were recorded using an A-1050 Bispectral Index (BIS) monitor and our self-authored Bispectral Analyzer for BIS software. We compared 2 peaks of EEG bicoherence (pBIC-low, around 4 Hz; and pBIC-high, around 10 Hz), as well as BIS and spectral edge frequency 95% (SEF95). Anesthesia was induced with 3 mg · kg(-1) thiopental and 3 μg · kg(-1) fentanyl. After tracheal intubation, anesthesia was maintained with isoflurane (expired concentration at 1.0%), oxygen, and nitrogen. Fentanyl was added and maintained at an estimated effect-site concentration of >1.5 ng · mL(-1). We obtained baseline data 1 hour after induction of anesthesia, then 70% N(2)O was added for 30 minutes.
  
  
  RESULTS
  Before N(2)O, pBIC-low and pBIC-high were 49.3% ± 8.3% and 42.4% ± 11.0%. Ten minutes after starting N(2)O, pBIC-high decreased to 14.9% ± 5.9% (P < 0.001), and it was statistically significantly lower throughout the N(2)O period. Meanwhile, pBIC-low transiently decreased to 37.2% ± 12.8% (P = 0.01) during the early phase of N(2)O administration. Before N(2)O, BIS and SEF95 were 43.2 ± 4.9 and 13.1 ± 2.0 Hz, respectively. Both BIS and SEF95 slightly but statistically significantly decreased during N(2)O administration. Fifteen minutes after starting N(2)O, BIS and SEF95 were 35.7 ± 6.2 (P < 0.001) and 8.6 ± 1.8 Hz (P < 0.001) and they decreased more when large δ waves emerged. Fifteen minutes after stopping N(2)O, BIS, SEF95, as well as pBIC-low and pBIC-high returned to pre-N(2)O values.
  
  
  CONCLUSIONS
  Dissimilar to the effect of ketamine, N(2)O significantly decreases pBIC-high during isoflurane anesthesia.
authors.affiliation: Department of Anesthesiology, Osaka Prefectural Habikino Hospital, Osaka, Japan. hagihira@anes.med.osaka-u.ac.jp
authors.forename: Satoshi
authors.initials: S
authors.lastname: Hagihira
authors.forename: Masaki
authors.initials: M
authors.lastname: Takashina
authors.forename: Takahiko
authors.initials: T
authors.lastname: Mori
authors.forename: Takashi
authors.initials: T
authors.lastname: Mashimo
mesh_terms: D000328:Adult; D000368:Aged; D000769:Anesthesia, Inhalation; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D004569:Electroencephalography / Q000187:drug effects*; D005260:Female; D006801:Humans; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D008875:Middle Aged; D009609:Nitrous Oxide / Q000494:pharmacology*
chemical_list: D018685:Anesthetics, Inhalation; D007530:Isoflurane; D009609:Nitrous Oxide