title: Shear-mediated dilation of the internal carotid artery occurs independent of hypercapnia.
abstract: Evidence for shear stress as a regulator of carotid artery dilation in response to increased arterial CO2was recently demonstrated in humans during sustained elevations in CO2(hypercapnia); however, the relative contributions of CO2and shear stress to this response remains unclear. We examined the hypothesis that, after a 30-s transient increase in arterial CO2tension and consequent increase in internal carotid artery shear stress, internal carotid artery diameter would increase, indicating shear-mediated dilation, in the absence of concurrent hypercapnia. In 27 healthy participants, partial pressures of end-tidal O2and CO2, ventilation (pneumotachography), blood pressure (finger photoplethysmography), heart rate (electrocardiogram), internal carotid artery flow, diameter, and shear stress (high-resolution duplex ultrasound), and middle cerebral artery blood velocity (transcranial Doppler) were measured during 4-min steady-state and transient 30-s hypercapnic tests (both +9 mmHg CO2). Internal carotid artery dilation was lower in the transient compared with steady-state hypercapnia (3.3 ± 1.9 vs. 5.3 ± 2.9%, respectively,P< 0.03). Increases in internal carotid artery shear stress preceded increases in diameter in both transient (time: 16.8 ± 13.2 vs. 59.4 ± 60.3 s,P< 0.01) and steady-state (time: 18.2 ± 14.2 vs. 110.3 ± 79.6 s,P< 0.01) tests. Internal carotid artery dilation was positively correlated with shear rate area under the curve in the transient (r2= 0.44,P< 0.01) but not steady-state (r2= 0.02,P= 0.53) trial. Collectively, these results suggest that hypercapnia induces shear-mediated dilation of the internal carotid artery in humans. This study further promotes the application and development of hypercapnia as a clinical strategy for the assessment of cerebrovascular vasodilatory function and health in humans.NEW & NOTEWORTHYShear stress dilates the internal carotid artery in humans. This vasodilatory response occurs independent of other physiological factors, as demonstrated by our transient CO2test, and is strongly correlated to shear area under the curve. Assessing carotid shear-mediated dilation may provide a future avenue for assessing cerebrovascular health and the risk of cerebrovascular events.
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada; ryanleohoiland@gmail.com.
authors.forename: Ryan L
authors.identifier: http://orcid.org/0000-0002-5657-0059
authors.initials: RL
authors.lastname: Hoiland
authors.affiliation: School of Sport Science, Exercise, and Health, The University of Western Australia, Crawley, Western Australia, Australia.
authors.forename: Kurt J
authors.initials: KJ
authors.lastname: Smith
authors.affiliation: Department of Nutrition, Exercise, and Sports, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Howard H
authors.initials: HH
authors.lastname: Carter
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada.
authors.forename: Nia C S
authors.initials: NCS
authors.lastname: Lewis
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Tymko
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada.
authors.forename: Kevin W
authors.initials: KW
authors.lastname: Wildfong
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada.|Integrative Vascular Biology Laboratory, Department of Integrative Physiology, University of Colorado Boulder, Boulder, Colorado; and.
authors.forename: Anthony R
authors.initials: AR
authors.lastname: Bain
authors.affiliation: School of Sport Science, Exercise, and Health, The University of Western Australia, Crawley, Western Australia, Australia.|Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool, United Kingdom.
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Green
authors.affiliation: Centre for Heart, Lung, and Vascular Health, University of British Columbia Okanagan Campus, School of Health and Exercise Sciences, Kelowna, British Columbia, Canada.
authors.forename: Philip N
authors.initials: PN
authors.lastname: Ainslie
mesh_terms: D001783:Blood Flow Velocity; D002245:Carbon Dioxide / Q000097:blood*; D002343:Carotid Artery, Internal / Q000503:physiopathology*; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology*; D008297:Male; D040542:Mechanotransduction, Cellular; D033081:Shear Strength; D013314:Stress, Mechanical; D014664:Vasodilation; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide
keywords: carbon dioxide; cerebral blood flow; flow-mediated dilation; shear stress; transcranial Doppler; ultrasound