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Impacts of systemic hemodynamic factors on cerebral and peripheral perfusion
Changes in physiological factors involving modulation of hemodynamics such as baroreflex sensitivity (BRS), heart rate, and/or blood pressure influence blood flow to downstream tissues leading to changes in response and/or function of tissues. For example, a sufficient increase in heart rate elicits greater accumulative shear stimuli on a per minute basis leading to a greater vasodilatory response of endothelial cells and providing greater perfusion to skeletal muscle. The high-flow and low-impedance nature of the cerebral circulation leads to increased susceptibility to damage from considerable blood pressure fluctuations. For this reason, the cerebrovasculature holds a very narrow range of operation of cerebral autoregulation in response to changes in perfusion pressure. In a nondemented elderly population and patients with Alzheimer’s disease, impaired BRS has been linked with poor cognitive function and a link between high pulsatile components of blood pressure (i.e., pulse pressure) and impaired cognitive function has also been reported. Three research investigations were included in this dissertation study. The first study was to determine the association between heart rate at rest and endothelium-dependent vasodilation as assessed by flow-mediated dilation (FMD) of the brachial artery. The primary findings from study 1 revealed an indirect association between heart rate and FMD through shear stimuli. The studies 2 and 3 sought to determine the association of regional cerebral perfusion with cardiovagal BRS and blood pressure components. A link between cardiovagal BRS and regional cerebral perfusion of the hippocampus was demonstrated in the study 2. This finding may add mechanistic insight to the relationship between impaired BRS and cognitive dysfunction. The primary finding from the study 3 revealed a significant relationship between peripheral pulsatile blood pressure components and regional cerebral perfusion of the hippocampus as well as anterior white matter. This finding highlights the importance of pulsatile pressure on cerebral vascular beds. Taken together, the overall findings from this dissertation study indicate the potential impacts of systemic hemodynamic factors on cerebral and peripheral perfusion. Future longitudinal studies in nondemented elderly and individuals with Alzheimer’s disease are warranted to reveal the causality of these associations.Kinesiology and Health Educatio
Associations of resting heart rate with endothelium-dependent vasodilation and shear rate
Heart rate is an independent risk factor for cardiovascular disease and a hemodynamic factor that can modulate blood flow as it affects the frequency of shear stimuli acting on the arterial wall. However, the association between heart rate and endothelium-dependent vasodilation remains highly controversial. We determined the association between heart rate at rest and endothelium-dependent vasodilation in 98 apparently healthy adults (18–63 years). The mild and positive association between heart rate and flow-mediated dilation (FMD) was no longer significant when age and sex or baseline diameter were controlled for. The path analyses revealed that heart rate was not directly related to FMD but the association was indirectly mediated by shear rate, which was confirmed by a bias-corrected bootstrap 95% CIs (0.0157–0.1056). We concluded that even though heart rate and endothelium-dependent vasodilation were associated with shear rate, there was no independent relation between heart rate and FMD