title: Influence of posture on the regulation of cerebral perfusion.
abstract: BACKGROUND
  Posture has a major influence on cerebral blood flow (CBF). Unlike head-up tilt (HUT), less is known about how CBF is regulated during head-down tilt (HDT). We hypothesized that CBF would be elevated during HDT and decreased during HUT.
  
  
  METHODS
  In 21 healthy young adults, while controlling for end-tidal Pco2, we combined concurrent measurements of middle cerebral artery velocity and posterior cerebral artery velocity (MCAv and PCAv, respectively), blood pressure (BP), and heart rate (HR). Measures were made at rest and, in a randomized order, during -90 degrees HDT and +900 HUT. Dynamic cerebral autoregulation was quantified using transfer function analysis. In a subgroup, volumetric blood flow recordings were obtained in the common carotid artery (CCA; N=11), internal and external carotid arteries (ICA; N=8 and ECA; N=6), and vertebral artery (VA; N=4).
  
  
  RESULTS
  End-tidal Pco2, CCA, ICA, VA, MCAv(mean) and PCAv(mean) remained unchanged during -90 degrees HDT and +90 degrees HUT compared to supine. During -90 degrees HDT, mean BP (+22 mmHg) and cerebral vascular resistance (CVR) in both the MCA and PCA were elevated relative to supine, whereas HR remained unchanged. During +900 HUT, when compared to supine, HR increased (+18 bpm), and mean arterial pressure (MAP) total power and low frequency (LF) power in the MCA and PCA increased. In both the very low frequency (VLF) and LF ranges, coherence during +90 degrees HUT increased (P < 0.05 vs. supine) in both the MCA and PCA. In contrast, coherence was reduced during -90 degrees HDT.
  
  
  CONCLUSIONS
  Despite marked changes in perfusion pressure with HUT or HDT, our findings indicate that cerebral perfusion is well maintained during acute severe changes in posture.
authors.affiliation: School of Health and Exercises Sciences, Faculty of Health and Social Development, University of British Columbia, Okanagan Campus, BC, Canada.
authors.forename: Jinelle C
authors.initials: JC
authors.lastname: Geinas
authors.forename: Katelyn R
authors.initials: KR
authors.lastname: Marsden
authors.forename: Y C
authors.initials: YC
authors.lastname: Tzeng
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Smirl
authors.forename: Kurt J
authors.initials: KJ
authors.lastname: Smith
authors.forename: Chris K
authors.initials: CK
authors.lastname: Willie
authors.forename: Nia C
authors.initials: NC
authors.lastname: Lewis
authors.forename: Gordon
authors.initials: G
authors.lastname: Binsted
authors.forename: Damien M
authors.initials: DM
authors.lastname: Bailey
authors.forename: Akke
authors.initials: A
authors.lastname: Bakker
authors.forename: Trevor A
authors.initials: TA
authors.lastname: Day
authors.forename: Philip N
authors.initials: PN
authors.lastname: Ainslie
mesh_terms: D000328:Adult; D001921:Brain / Q000098:blood supply*; D002339:Carotid Arteries / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006706:Homeostasis / Q000502:physiology; D006801:Humans; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D011187:Posture / Q000502:physiology*; D012039:Regional Blood Flow; D055815:Young Adult