title: [Acute mild hypoxia impairment of dynamic cerebral autoregulation assessed by spectral analysis and thigh-cuff deflation].
abstract: OBJECTIVE
  Acute hypoxia may impair dynamic cerebral autoregulation. However, previous studies have been controversial. The difference in methods of estimation of dynamic cerebral autoregulation is reported to be responsible for conflicting reports. We, therefore, conducted this study using two representative methods of estimation of dynamic cerebral autoregulation to test our hypothesis that dynamic cerebral autoregulation is impaired during acute exposure to mild hypoxia.
  
  
  METHODS
  Eleven healthy men were exposed to 15% oxygen concentration for two hours. They were examined under normoxia (21% O(2)) and hypoxia (15% O(2)). The mean arterial pressure (MAP) in the radial artery was measured by tonometry, and cerebral blood flow velocity (CBFv) in the middle cerebral artery was measured by transcranial Doppler ultrasonography. Dynamic cerebral autoregulation was assessed by spectral and transfer function analyses of beat-by-beat changes in MAP and CBFv. Moreover, the dynamic rate of regulation and percentage restoration of CBFv were estimated when a temporal decrease in arterial pressure was induced by thigh-cuff deflation.
  
  
  RESULTS
  Arterial oxygen saturation decreased significantly during hypoxia (97±0% to 88±1%), whereas respiratory rate was unchanged, as was steady-state CBFv. With 15% O(2), the very-low-frequency power of CBFv variability increased significantly. Transfer function coherence (0.40±0.02 to 0.53±0.05) and gain (0.51±0.07 cm/s/mmHg to 0.79±0.11 cm/s/mmHg) in the very-low-frequency range increased significantly. Moreover, the percentage restoration of CBF velocity determined by thigh-cuff deflation decreased significantly during hypoxia (125±25% to 65±8%).
  
  
  CONCLUSIONS
  Taken together, these results obtained using two representative methods consistently indicate that mild hypoxia impairs dynamic cerebral autoregulation.
authors.affiliation: Division of Hygiene, Department of Social Medicine, Nihon University School of Medicine, Tokyo, Japan.
authors.forename: Hajime
authors.initials: H
authors.lastname: Katsukawa
authors.forename: Yojiro
authors.initials: Y
authors.lastname: Ogawa
authors.forename: Ken
authors.initials: K
authors.lastname: Aoki
authors.forename: Ryo
authors.initials: R
authors.lastname: Yanagida
authors.forename: Kenichi
authors.initials: K
authors.lastname: Iwasaki
mesh_terms: D000208:Acute Disease; D000328:Adult; D001783:Blood Flow Velocity; D002560:Cerebrovascular Circulation; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D020768:Middle Cerebral Artery / Q000503:physiopathology*; D010100:Oxygen / Q000097:blood; D017534:Radial Artery / Q000503:physiopathology; D013848:Thigh / Q000098:blood supply*; D014655:Vascular Resistance; D055815:Young Adult
chemical_list: D010100:Oxygen