title: Fundamental relationships between blood pressure and cerebral blood flow in humans.
abstract: Cerebral blood flow responses to transient blood pressure challenges are frequently attributed to cerebral autoregulation (CA), yet accumulating evidence indicates vascular properties like compliance are also influential. We hypothesized that middle cerebral blood velocity (MCAv) dynamics during or following a transient blood pressure perturbation can be accurately explained by the windkessel mechanism. Eighteen volunteers underwent blood pressure manipulations, including bilateral thigh-cuff deflation and sit-to-stand maneuvers under normocapnic and hypercapnic (5% CO2) conditions. Pressure-flow recordings were analyzed using a windkessel analysis approach that partitions the frequency-dependent resistance and compliance contributions to MCAv dynamics. The windkessel was typically able to explain more than 50% of the MCAv variance, as indicated by R(2) values for both the flow recovery and postrecovery phase. The most consistent predictors of MCAv dynamics under the control condition were the windkessel capacitive gain and high-frequency resistive gain. However, there were significant interindividual variations in the composition of windkessel predictors. Hypercapnia consistently reduced the capacitive gain and enhanced the low-frequency (0.04-0.20 Hz) resistive gain for both thigh-cuff deflation and sit-to-stand trials. These findings indicate that 1) MCAv dynamics during acute transient hypotension challenges are dominated by cerebrovascular windkessel properties independent of CA; 2) there is significant heterogeneity in windkessel properties between individuals; and 3) hemodynamic effects of hypercapnia during transient blood pressure challenges primarily reflect changes in windkessel properties rather than pure CA impairment.
authors.affiliation: Cardiovascular Systems Laboratory, University of Otago, Wellington South, New Zealand; Centre for Translational Physiology, University of Otago, Wellington South, New Zealand; shieak.tzeng@otago.ac.nz.
authors.forename: Y C
authors.initials: YC
authors.lastname: Tzeng
authors.affiliation: Cardiovascular Systems Laboratory, University of Otago, Wellington South, New Zealand; Centre for Translational Physiology, University of Otago, Wellington South, New Zealand;
authors.forename: B A
authors.initials: BA
authors.lastname: MacRae
authors.affiliation: Centre for Heart, Lung and Vascular Health, School of Health and Exercise Sciences, University of British Columbia, Okanagan, Kelowna, British Columbia, Canada; and.
authors.forename: P N
authors.initials: PN
authors.lastname: Ainslie
authors.affiliation: Graduate School of Biomedical Engineering, University of New South Wales, Sydney, New South Wales, Australia.
authors.forename: G S H
authors.initials: GS
authors.lastname: Chan
mesh_terms: D001783:Blood Flow Velocity / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006439:Hemodynamics / Q000502:physiology*; D006706:Homeostasis / Q000502:physiology; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D008297:Male; D020768:Middle Cerebral Artery / Q000503:physiopathology*; D017585:Ultrasonography, Doppler, Transcranial; D055815:Young Adult
keywords: blood pressure; cerebral blood flow; circulation; compliance