title: Impaired dynamic cerebral autoregulation at extreme high altitude even after acclimatization.
abstract: Cerebral blood flow (CBF) increases and dynamic cerebral autoregulation is impaired by acute hypoxia. We hypothesized that progressive hypocapnia with restoration of arterial oxygen content after altitude acclimatization would normalize CBF and dynamic cerebral autoregulation. To test this hypothesis, dynamic cerebral autoregulation was examined by spectral and transfer function analyses between arterial pressure and CBF velocity variabilities in 11 healthy members of the Danish High-Altitude Research Expedition during normoxia and acute hypoxia (10.5% O(2)) at sea level, and after acclimatization (for over 1 month at 5,260 m at Chacaltaya, Bolivia). Arterial pressure and CBF velocity in the middle cerebral artery (transcranial Doppler), were recorded on a beat-by-beat basis. Steady-state CBF velocity increased during acute hypoxia, but normalized after acclimatization with partial restoration of SaO(2) (acute, 78% ± 2%; chronic, 89% ± 1%) and progression of hypocapnia (end-tidal carbon dioxide: acute, 34 ± 2 mm Hg; chronic, 21 ± 1 mm Hg). Coherence (0.40 ± 0.05 Units at normoxia) and transfer function gain (0.77 ± 0.13 cm/s per mm Hg at normoxia) increased, and phase (0.86 ± 0.15 radians at normoxia) decreased significantly in the very-low-frequency range during acute hypoxia (gain, 141% ± 24%; coherence, 136% ± 29%; phase, -25% ± 22%), which persisted after acclimatization (gain, 136% ± 36%; coherence, 131% ± 50%; phase, -42% ± 13%), together indicating impaired dynamic cerebral autoregulation in this frequency range. The similarity between both acute and chronic conditions suggests that dynamic cerebral autoregulation is impaired by hypoxia even after successful acclimatization to an extreme high altitude.
authors.affiliation: Institute for Exercise and Environmental Medicine at Texas Health Presbyterian Dallas and University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75231, USA.
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Iwasaki
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Julie H
authors.initials: JH
authors.lastname: Zuckerman
authors.forename: Yojiro
authors.initials: Y
authors.lastname: Ogawa
authors.forename: Lærke H
authors.initials: LH
authors.lastname: Hansen
authors.forename: Benjamin David
authors.initials: BD
authors.lastname: Levine
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000208:Acute Disease; D000328:Adult; D000465:Algorithms; D000531:Altitude; D000532:Altitude Sickness / Q000503:physiopathology*; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000097:blood; D002560:Cerebrovascular Circulation / Q000502:physiology*; D002908:Chronic Disease; D005260:Female; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology; D008297:Male; D015656:Respiratory Mechanics / Q000502:physiology; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide