{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,11]],"date-time":"2026-07-11T05:23:28Z","timestamp":1783747408700,"version":"3.55.0"},"reference-count":54,"publisher":"Wiley","issue":"7","license":[{"start":{"date-parts":[[2015,6,30]],"date-time":"2015-06-30T00:00:00Z","timestamp":1435622400000},"content-version":"vor","delay-in-days":0,"URL":"http:\/\/onlinelibrary.wiley.com\/termsAndConditions#vor"}],"funder":[{"name":"Government of Alberta Summer Temporary Employment Program (STEP) grant"},{"name":"NSERC USRA"},{"name":"Petro-Canada Young Innovator Award"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Experimental Physiology"],"published-print":{"date-parts":[[2015,7]]},"abstract":"<jats:sec><jats:title>New Findings<\/jats:title><jats:p><jats:list list-type=\"bullet\">\n<jats:list-item>\n<jats:p><jats:bold>What is the central question of this study?<\/jats:bold><\/jats:p>\n<jats:p>We investigated the effects of superimposed tilt and hypercapnia\u2010induced cerebral arteriolar dilatation on anterior and posterior cerebrovascular CO<jats:sub>2<\/jats:sub> reactivity using hyperoxic rebreathing in human participants.<\/jats:p>\n<\/jats:list-item>\n<jats:list-item>\n<jats:p><jats:bold>What is the main finding and its importance?<\/jats:bold><\/jats:p>\n<jats:p>The main findings are threefold: (i) cerebrovascular CO<jats:sub>2<\/jats:sub> reactivity in the anterior and posterior cerebrovasculature is unchanged with tilt; (ii) cerebral autoregulation is unlikely responsible due to unchanging cerebrovascular resistance reactivity between positions; and (iii) cerebral blood flow is not pressure passive during tilt as it is with pharmacological or lower body negative pressure\u2010induced changes in mean arterial pressure, suggesting that sympathetic activation or balanced transmural pressures during head\u2010down tilt regulate cerebral blood flow.<\/jats:p>\n<\/jats:list-item>\n<\/jats:list><\/jats:p><\/jats:sec><jats:sec><jats:label\/><jats:p>Cerebral autoregulation is a protective feature of the cerebrovasculature that maintains relatively constant cerebral perfusion in the face of static and dynamic fluctuations in mean arterial pressure (MAP). However, the extent that the cerebrovasculature can autoregulate in the face of superimposed steady\u2010state orthostasis\u2010induced changes in MAP (e.g. head\u2010up and head\u2010down tilt; HUT and HDT) and CO<jats:sub>2<\/jats:sub>\u2010mediated arteriolar dilatation is unknown. We tested the effects of steady\u2010state tilt on cerebrovascular CO<jats:sub>2<\/jats:sub> reactivity in the middle and and posterior cerebral artery in the following five body positions: 90 deg HUT, 45 deg HUT, supine, 45 deg HDT and 90 deg HDT on a tilt table during a modified hyperoxic rebreathing test. Absolute and relative cerebrovascular CO<jats:sub>2<\/jats:sub> reactivity [cerebral blood velocity (CBV)\/CO<jats:sub>2<\/jats:sub>], cerebrovascular resistance (CVR) reactivity (CVR\/CO<jats:sub>2<\/jats:sub>) and MAP reactivity (MAP\/CO<jats:sub>2<\/jats:sub>) were quantified using linear regression. Mean arterial pressure was significantly elevated in 90 deg HDT compared with other positions during baseline steady\u2010state tilt (<jats:italic>P<\/jats:italic> &lt; 0.01). Absolute CBV\/CO<jats:sub>2<\/jats:sub> and CVR\/CO<jats:sub>2<\/jats:sub> were greater in the middle cerebral artery than the posterior cerebral artery (<jats:italic>P<\/jats:italic> &lt; 0.01) in all body positions, but relative measures were not different (<jats:italic>P<\/jats:italic> = 0.143 and <jats:italic>P<\/jats:italic> = 0.360, respectively), nor was there any interaction with tilt position. In addition, there was no difference in absolute (<jats:italic>P<\/jats:italic> = 0.556) and relative MAP\/CO<jats:sub>2<\/jats:sub> (<jats:italic>P<\/jats:italic> = 0.308) between positions. Our data demonstrate that cerebral blood flow remains well regulated during superimposed steady\u2010state orthostatic stress and dynamic changes in the partial pressure of end\u2010tidal CO<jats:sub>2<\/jats:sub> during rebreathing. Cerebral autoregulation is likely not the mechanism responsible, but rather sympathetic nervous system activation or a balanced cerebrovascular transmural pressure with HDT maintains resting cerebral blood flow and cerebrovascular CO<jats:sub>2<\/jats:sub> reactivity during rebreathing.<\/jats:p><\/jats:sec>","DOI":"10.1113\/ep085084","type":"journal-article","created":{"date-parts":[[2015,5,13]],"date-time":"2015-05-13T10:17:18Z","timestamp":1431512238000},"page":"839-851","source":"Crossref","is-referenced-by-count":26,"title":["Steady\u2010state tilt has no effect on cerebrovascular CO<sub>2<\/sub> reactivity in anterior and posterior cerebral circulations"],"prefix":"10.1113","volume":"100","author":[{"given":"Michael M.","family":"Tymko","sequence":"first","affiliation":[{"name":"Centre for Heart, Lung, and Vascular Health, School of Health and Exercise Science University of British Columbia  Kelowna British Columbia Canada"},{"name":"Department of Biology, Faculty of Science and Technology Mount Royal University  Calgary Alberta Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Rachel J.","family":"Skow","sequence":"additional","affiliation":[{"name":"Department of Biology, Faculty of Science and Technology Mount Royal University  Calgary Alberta Canada"},{"name":"Faculty of Physical Education and Recreation University of Alberta  Edmonton Alberta Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Christina M.","family":"MacKay","sequence":"additional","affiliation":[{"name":"Department of Biology, Faculty of Science and Technology Mount Royal University  Calgary Alberta Canada"},{"name":"Faculty of Physical Education and Recreation University of Alberta  Edmonton Alberta Canada"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Trevor A.","family":"Day","sequence":"additional","affiliation":[{"name":"Department of Biology, Faculty of Science and Technology Mount Royal University  Calgary Alberta 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