title: Effect of progressive normobaric hypoxia on dynamic cerebral autoregulation.
abstract: What is the central question of this study? Acute hypoxia reduces dynamic cerebral autoregulation (dCA); however, it is unclear what level of hypoxia is necessary to exert this effect. We sought to investigate whether dCA would be reduced during progressive periods of normobaric hypoxia using a duplex Doppler ultrasound technique to evaluate the volumetric blood flow. What is the main finding and its importance? We showed that dCA decreased linearly as inspired O2decreased from 21 to 12%. Additionally, symptoms of acute mountain sickness were related to changes in dCA. Our results may provide a sensitive and clinically relevant test to evaluate the risk of acute mountain sickness. Cerebral blood flow is maintained at relatively constant levels over a wide range of perfusion pressures via cerebral autoregulation (CA). Although acute hypoxia reduces dynamic CA, it is unclear what level of hypoxia is necessary to exert this effect. We evaluated dynamic CA during progressive normobaric hypoxia (∼1 h at each of 21, 18, 15 and 12% O2) using duplex ultrasonography to measure volumetric changes in common carotid artery blood flow of 11 healthy young men. Dynamic CA was evaluated by the thigh-cuff method and represented as the rate of regulation of vascular conductance. On a separate occasion, symptoms of acute mountain sickness were evaluated during 6 h of prolonged hypoxia (fractional inspired O2of 14.1%) using the Lake Louise Questionnaire. Repeated-measures ANOVA with linear trend analysis indicated that dynamic CA decreased progressively as fractional inspired O2was reduced (P < 0.001). Spearman rank order analysis revealed that symptoms of acute mountain sickness were related to changes in the rate of regulation of vascular conductance from 21 to 15% (r = -0.869, P = 0.006) and from 21 to 12% O2(r = -0.648, P = 0.040), respectively. These results suggest that dynamic CA worsens with progressive hypoxia and that reductions in dynamic CA during moderate to severe hypoxia (<15% O2) may be related to the severity of acute mountain sickness.
authors.affiliation: Division of Human Environmental Science, Mount Fuji Research Institute, Japan. mhoriuchi@mfri.pref.yamanashi.jp.
authors.forename: Masahiro
authors.initials: M
authors.lastname: Horiuchi
authors.affiliation: Division of Human Environmental Science, Mount Fuji Research Institute, Japan.
authors.forename: Junko
authors.initials: J
authors.lastname: Endo
authors.affiliation: Integrated Graduate School Department of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, Japan.
authors.forename: Shohei
authors.initials: S
authors.lastname: Dobashi
authors.affiliation: Integrated Graduate School Department of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, Japan.
authors.forename: Masataka
authors.initials: M
authors.lastname: Kiuchi
authors.affiliation: Integrated Graduate School Department of Medicine, Engineering, and Agricultural Sciences, University of Yamanashi, Japan.
authors.forename: Katsuhiro
authors.initials: K
authors.lastname: Koyama
authors.affiliation: Department of Biology, University of Colorado, Colorado Springs, CO, USA.|Altitude Research Center, University of Colorado Anschutz Medical Campus, Aurora, CO, USA.
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Subudhi
mesh_terms: D000208:Acute Disease; D000328:Adult; D000532:Altitude Sickness / Q000378:metabolism / Q000503:physiopathology; D001921:Brain / Q000378:metabolism / Q000503:physiopathology*; D002339:Carotid Arteries / Q000378:metabolism / Q000503:physiopathology; D002560:Cerebrovascular Circulation / Q000502:physiology; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000378:metabolism / Q000503:physiopathology*; D010100:Oxygen / Q000378:metabolism; D055815:Young Adult
chemical_list: D010100:Oxygen
keywords: acute mountain sickness; duplex Doppler ultrasound; volumetric blood flow