title: Hyperventilation-induced hypocapnia changes the pattern of electroencephalographic bicoherence growth during sevoflurane anaesthesia.
abstract: BACKGROUND
  Hyperventilation, with the resulting hypocapnia, reduces cerebral blood flow and causes slowing of the EEG activity. However, neuronal oscillating properties including the thalamocortical network during hyperventilation have not been elucidated. To assess these features provoked by hyperventilation, the present study examined quadratic phase coupling features by means of bicoherence analysis.
  
  
  METHODS
  Twenty-two patients were anaesthetized using sevoflurane 1.5% combined with remifentanil or epidural anaesthesia. After a stable normocapnic period, hypocapnia was induced by hyperventilation, and the raw EEG signals were collected. Bispectral analysis (bicoherence) and power spectrum analysis were performed before and after hypocapnia.
  
  
  RESULTS
  Mean (sd) peak bicoherence in the delta- area increased from 35.6 (10.9)% during normocapnia to 43.8 (10.9)% during hypocapnia (P<0.05), whereas mean (sd) peak bicoherence in the alpha area decreased from 42.8 (14.4)% during normocapnia to 37.5 (12.3)% during hypocapnia (P<0.05). Normalized power in the delta- frequencies on the power spectrum increased from 60.2 (13.1)% to 72.5 (12.7)% (P<0.05). Bispectral index and spectral edge frequency changed from 45.9 (7.0) to 40.1 (5.6) (P<0.05) and from 15.0 (2.3) to 14.0 (2.5) Hz (P<0.05), respectively. No significant differences in these values were observed between the two types of anaesthesia.
  
  
  CONCLUSIONS
  Hypocapnia enlarged bicoherence growth in the delta- frequency range, suggesting the contribution of subcortical oscillating mechanisms in regulating EEG during hyperventilation.
authors.affiliation: Department of Anesthesiology, Kyoto Prefectural University of Medicine, Graduate School of Medical Science, Kamigyo-ku, Kyoto 602-0841, Japan. zukko@koto.kpu-m.ac.jp
authors.forename: K
authors.initials: K
authors.lastname: Hayashi
authors.forename: M
authors.initials: M
authors.lastname: Fujikawa
authors.forename: T
authors.initials: T
authors.lastname: Sawa
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D002245:Carbon Dioxide / Q000097:blood; D004569:Electroencephalography / Q000187:drug effects*; D006801:Humans; D006985:Hyperventilation / Q000503:physiopathology*; D016857:Hypocapnia / Q000097:blood / Q000503:physiopathology*; D008738:Methyl Ethers / Q000494:pharmacology*; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods; D010313:Partial Pressure; D000077149:Sevoflurane; D012815:Signal Processing, Computer-Assisted
chemical_list: D018685:Anesthetics, Inhalation; D008738:Methyl Ethers; D002245:Carbon Dioxide; D000077149:Sevoflurane