title: MRI measures of middle cerebral artery diameter in conscious humans during simulated orthostasis.
abstract: OBJECTIVE
  The relationship between middle cerebral artery (MCA) flow velocity (CFV) and cerebral blood flow (CBF) is uncertain because of unknown vessel diameter response to physiological stimuli. The purpose of this study was to directly examine the effect of a simulated orthostatic stress (lower body negative pressure [LBNP]) as well as increased or decreased end-tidal carbon dioxide partial pressure (P(ET)CO(2)) on MCA diameter and CFV.
  
  
  METHODS
  Twelve subjects participated in a CO(2) manipulation protocol and/or an LBNP protocol. In the CO(2) manipulation protocol, subjects breathed room air (normocapnia) or 6% inspired CO(2) (hypercapnia), or they hyperventilated to approximately 25 mm Hg P(ET)CO(2) (hypocapnia). In the LBNP protocol, subjects experienced 10 minutes each of -20 and -40 mm Hg lower body suction. CFV and diameter of the MCA were measured by transcranial Doppler and MRI, respectively, during the experimental protocols.
  
  
  RESULTS
  Compared with normocapnia, hypercapnia produced increases in both P(ET)CO(2) (from 36+/-3 to 40+/-4 mm Hg, P<0.05) and CFV (from 63+/-4 to 80+/-6 cm/s, P<0.001) but did not change MCA diameters (from 2.9+/-0.3 to 2.8+/-0.3 mm). Hypocapnia produced decreases in both P(ET)CO(2) (24+/-2 mm Hg, P<0.005) and CFV (43+/-7 cm/s, P<0.001) compared with normocapnia, with no change in MCA diameters (from 2.9+/-0.3 to 2.9+/-0.4 mm). During -40 mm Hg LBNP, P(ET)CO(2) was not changed, but CFV (55+/-4 cm/s) was reduced from baseline (58+/-4 cm/s, P<0.05), with no change in MCA diameter.
  
  
  CONCLUSIONS
  Under the conditions of this study, changes in MCA diameter were not detected. Therefore, we conclude that relative changes in CFV were representative of changes in CBF during the physiological stimuli of moderate LBNP or changes in P(ET)CO(2).
authors.affiliation: Neurovascular Research Laboratory, School of Kinesiology, and the Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada.
authors.forename: J M
authors.initials: JM
authors.lastname: Serrador
authors.forename: P A
authors.initials: PA
authors.lastname: Picot
authors.forename: B K
authors.initials: BK
authors.lastname: Rutt
authors.forename: J K
authors.initials: JK
authors.lastname: Shoemaker
authors.forename: R L
authors.initials: RL
authors.lastname: Bondar
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D002245:Carbon Dioxide / Q000032:analysis; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003243:Consciousness; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000175:diagnosis / Q000503:physiopathology; D016857:Hypocapnia / Q000175:diagnosis / Q000503:physiopathology; D007024:Hypotension, Orthostatic / Q000175:diagnosis / Q000503:physiopathology*; D008279:Magnetic Resonance Imaging; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology*; D011311:Pressoreceptors / Q000502:physiology; D013564:Sympathetic Nervous System / Q000502:physiology; D017585:Ultrasonography, Doppler, Transcranial
chemical_list: D002245:Carbon Dioxide