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Reductions in cerebral blood flow (CBF) and the CBF response to hypercapnia (cerebrovascular reactivity) are implicated in the progression of Alzheimer's disease and other related dementias. Recent data suggest that 20%–30% of individuals exhibit vertebral artery hypoplasia (VAH), and as a result, have a significant difference in left and right vertebral artery blood flow. Consequently, in those with VAH, the regions of the brain that the posterior circulation supplies may be at risk of hypoperfusion. Furthermore, people with VAH may rely more on the systemic circulation to regulate CBF, rather than on local regulation. The purpose of this study was to determine the effect of VAH on cerebral blood flow regulation in habitually active, sex and age matched adults. We hypothesized that during normoxia, and in response to hypercapnia, adults with VAH (n=10) would have lower global flow and lower global cerebrovascular reactivity, basilar artery (BA) reactivity, and internal carotid artery (ICA) reactivity compared with adults that did not exhibit VAH (Control, n=10). Participants underwent a 4D flow MRI scan in the supine position to determine regional and global blood flow during normoxia and hypercapnia (4% and 6% CO2). 4D flow MRI allows for flow analysis of multiple vessels with full volumetric coverage of any vascular region of interest. Reactivity was calculated as the linear relationship between blood flow and end‐tidal CO2. Global flow was calculated as the sum of the flow in the BA and ICAs. Global flow was significantly lower in VAH participants compared with control participants during normoxia (VAH: 478±26 ml/min; Control: 580±44 ml/min; p<0.05), 4% CO2 (VAH: 493±25 ml/min; Control: 631±47 ml/min; p<0.05), and 6% CO2 (VAH: 533±31 ml/min; Control: 682±48 ml/min; p<0.05). Compared to control participants, VAH participants had significantly lower global cerebrovascular reactivity (VAH: 6.2±2.0 ml/min/mmHg; Control: 11.0±1.6 ml/min/mmHg; p<0.05), BA reactivity (VAH: 1.2±0.5 ml/min/mmHg; Control: 2.6±0.4 ml/min/mmHg; p<0.05), and ipsilateral (same side as hypoplastic vertebral artery) ICA reactivity (VAH: 2.4±0.8 ml/min/mmHg; Control: 4.3±0.7 ml/min/mmHg; p<0.05) to hypercapnia. Additionally, there was a trend for lower contralateral ICA reactivity (VAH: 2.6±0.8 ml/min/mmHg; Control: 4.2±0.6 ml/min/mmHg; p=0.06) in VAH participants compared with control participants. These results suggest that individuals with VAH are not only affected in the immediate posterior cerebral circulation, but CBF is also affected globally, despite the bilateral symmetry of the brain and the collateral flow provided by the Circle of Willis. Future studies should consider the prevalence of hypoplasia in the cerebrovasculature in order to determine its relevance to cerebrovascular pathologies.Support or Funding InformationNIH HL 118154, Wisconsin Alumni Research Foundation, Hilldale Undergraduate/Faculty Research FellowshipThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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