title: Alterations in cerebral autoregulation and cerebral blood flow velocity during acute hypoxia: rest and exercise.
abstract: We examined the relationship between changes in cardiorespiratory and cerebrovascular function in 14 healthy volunteers with and without hypoxia [arterial O(2) saturation (Sa(O(2))) approximately 80%] at rest and during 60-70% maximal oxygen uptake steady-state cycling exercise. During all procedures, ventilation, end-tidal gases, heart rate (HR), arterial blood pressure (BP; Finometer) cardiac output (Modelflow), muscle and cerebral oxygenation (near-infrared spectroscopy), and middle cerebral artery blood flow velocity (MCAV; transcranial Doppler ultrasound) were measured continuously. The effect of hypoxia on dynamic cerebral autoregulation was assessed with transfer function gain and phase shift in mean BP and MCAV. At rest, hypoxia resulted in increases in ventilation, progressive hypocapnia, and general sympathoexcitation (i.e., elevated HR and cardiac output); these responses were more marked during hypoxic exercise (P < 0.05 vs. rest) and were also reflected in elevation of the slopes of the linear regressions of ventilation, HR, and cardiac output with Sa(O(2)) (P < 0.05 vs. rest). MCAV was maintained during hypoxic exercise, despite marked hypocapnia (44.1 +/- 2.9 to 36.3 +/- 4.2 Torr; P < 0.05). Conversely, hypoxia both at rest and during exercise decreased cerebral oxygenation compared with muscle. The low-frequency phase between MCAV and mean BP was lowered during hypoxic exercise, indicating impairment in cerebral autoregulation. These data indicate that increases in cerebral neurogenic activity and/or sympathoexcitation during hypoxic exercise can potentially outbalance the hypocapnia-induced lowering of MCAV. Despite maintaining MCAV, such hypoxic exercise can potentially compromise cerebral autoregulation and oxygenation.
authors.affiliation: Department of Physiology, University of Otago, Dunedin, New Zealand. philip.ainslie@stonebow.otago.ac.nz
authors.forename: Philip N
authors.initials: PN
authors.lastname: Ainslie
authors.forename: Alice
authors.initials: A
authors.lastname: Barach
authors.forename: Carissa
authors.initials: C
authors.lastname: Murrell
authors.forename: Mike
authors.initials: M
authors.lastname: Hamlin
authors.forename: John
authors.initials: J
authors.lastname: Hellemans
authors.forename: Shigehiko
authors.initials: S
authors.lastname: Ogoh
mesh_terms: D000208:Acute Disease; D000328:Adult; D001783:Blood Flow Velocity; D001794:Blood Pressure; D001921:Brain / Q000098:blood supply* / Q000378:metabolism; D002302:Cardiac Output; D002560:Cerebrovascular Circulation; D015444:Exercise; D005260:Female; D005583:Fourier Analysis; D006339:Heart Rate; D006706:Homeostasis; D006801:Humans; D016857:Hypocapnia / Q000209:etiology / Q000503:physiopathology; D000860:Hypoxia / Q000150:complications / Q000503:physiopathology*; D016014:Linear Models; D008297:Male; D008955:Models, Cardiovascular; D018482:Muscle, Skeletal / Q000098:blood supply / Q000378:metabolism; D010101:Oxygen Consumption; D012123:Pulmonary Ventilation; D012146:Rest; D013564:Sympathetic Nervous System / Q000503:physiopathology*; D017585:Ultrasonography, Doppler, Transcranial