title: The effects of superimposed tilt and lower body negative pressure on anterior and posterior cerebral circulations.
abstract: Steady-state tilt has no effect on cerebrovascular reactivity to increases in the partial pressure of end-tidal carbon dioxide (PETCO2). However, the anterior and posterior cerebral circulations may respond differently to a variety of stimuli that alter central blood volume, including lower body negative pressure (LBNP). Little is known about the superimposed effects of head-up tilt (HUT; decreased central blood volume and intracranial pressure) and head-down tilt (HDT; increased central blood volume and intracranial pressure), and LBNP on cerebral blood flow (CBF) responses. We hypothesized that (a) cerebral blood velocity (CBV; an index of CBF) responses during LBNP would not change with HUT and HDT, and (b) CBV in the anterior cerebral circulation would decrease to a greater extent compared to posterior CBV during LBNP when controlling PETCO2 In 13 male participants, we measured CBV in the anterior (middle cerebral artery, MCAv) and posterior (posterior cerebral artery, PCAv) cerebral circulations using transcranial Doppler ultrasound during LBNP stress (-50 mmHg) in three body positions (45°HUT, supine, 45°HDT). PETCO2 was measured continuously and maintained at constant levels during LBNP through coached breathing. Our main findings were that (a) steady-state tilt had no effect on CBV responses during LBNP in both the MCA (P = 0.077) and PCA (P = 0.583), and (b) despite controlling for PETCO2, both the MCAv and PCAv decreased by the same magnitude during LBNP in HUT (P = 0.348), supine (P = 0.694), and HDT (P = 0.407). Here, we demonstrate that there are no differences in anterior and posterior circulations in response to LBNP in different body positions.
authors.affiliation: Centre for Heart, Lung, and Vascular Health, School of Health and Exercise Science University of British Columbia, Kelowna, Canada Department of Biology, Faculty of Science and Technology Mount Royal University, Calgary, Alberta, Canada.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Tymko
authors.affiliation: Institute for Cardiovascular & Metabolic Diseases, University of North Texas Health Science Centre, Fort Worth, Texas.
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Rickards
authors.affiliation: Department of Biology, Faculty of Science and Technology Mount Royal University, Calgary, Alberta, Canada Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Alberta, Canada.
authors.forename: Rachel J
authors.initials: RJ
authors.lastname: Skow
authors.affiliation: Department of Biology, Faculty of Science and Technology Mount Royal University, Calgary, Alberta, Canada.
authors.forename: Nathan C
authors.initials: NC
authors.lastname: Ingram-Cotton
authors.affiliation: Department of Biology, Faculty of Science and Technology Mount Royal University, Calgary, Alberta, Canada.
authors.forename: Michael K
authors.initials: MK
authors.lastname: Howatt
authors.affiliation: Department of Biology, Faculty of Science and Technology Mount Royal University, Calgary, Alberta, Canada tday@mtroyal.ca.
authors.forename: Trevor A
authors.identifier: 0000-0001-7102-4235
authors.initials: TA
authors.lastname: Day
mesh_terms: D000293:Adolescent; D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D001921:Brain / Q000098:blood supply* / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism; D002560:Cerebrovascular Circulation / Q000502:physiology*; D018475:Head-Down Tilt / Q000009:adverse effects*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008150:Lower Body Negative Pressure / Q000009:adverse effects*; D008297:Male; D020768:Middle Cerebral Artery / Q000502:physiology; D011187:Posture / Q000502:physiology; D018667:Tilt-Table Test / Q000379:methods; D017585:Ultrasonography, Doppler, Transcranial / Q000379:methods; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide
keywords: Central hypovolemia; cerebral blood velocity; head‐down tilt; head‐up tilt; lower body negative pressure