Abstract des Autors

Ageing and age-related cerebrovascular and neurodegenerative diseases, such as stroke and dementia, are associated with a reduction in cerebral blood flow and cerebrovascular function. Exercise is protective against these negative effects of ageing, where an increase in shear stress on the arterial wall, through increased blood flow, is the likely stimulus for enhanced vascular function. Blood pressure plays an important role in contributing to increases in cerebral blood flow. Older adults show a lower increase in cerebral blood flow during exercise, despite a higher increase in blood pressure, compared to younger adults. This suggests a lack of cerebral blood flow responsiveness with changes in arterial blood pressure in older adults. To understand and maximise the cerebral blood flow response to exercise in older adults, the initial aim of this thesis was to investigate the effect of age on cerebrovascular responses during interval and continuous exercise. Furthermore, this thesis aimed to better understand how the cerebral blood flow – blood pressure relationship is affected by age in response to dynamic stimuli. Three experimental studies were conducted to address these objectives.
Study 1 compared the effects of intensity- and work-matched interval and continuous exercise on the cerebral blood flow response in older and younger men, measured as middle cerebral artery flow velocity (MCAv). The total volume of the response over the duration of exercise and recovery showed a greater overall change in MCAv during interval compared with continuous exercise in both young and older men. This suggests that interval exercise may be an effective alternative for promoting acute increases in MCAv, without excessive rises in blood pressure, particularly in those older adults who may not be able to sustain continuous exercise.
Study 2 was designed to determine the role of mean arterial blood pressure (MAP) on cerebral blood flow responses. The pressure-flow response, assessed as the change in MCAv for a given change in mean arterial blood pressure (%∆MCAv/%∆MAP), and its association with cerebrovascular reactivity to carbon dioxide (CO2), a surrogate measure of cerebral endothelial function, was compared between young and older adults. The rises in blood pressure during rapid, intermittent stand-to-sit manoeuvres resulted in a higher %∆MCAv/%∆MAP response in older adults compared to young. This response was inversely correlated with cerebrovascular CO2 reactivity, suggesting that impaired cerebrovascular function might be associated with a greater cerebral blood flow responsiveness to dynamic changes in systemic blood pressure.
While systemic (e.g. heart) blood pressure responses are typically elevated in older adults, Study 3 compared the changes in MCAv relative to MAP and estimated blood pressure at the brain (MAPbrain) in response to head-up to head-down tilt transitions between younger and older men. The cerebral pressure-flow response, assessed as the change in MCAv for a given change in MAPbrain (%∆MCAv/%∆MAPbrain) and its association with endothelial function, assessed as flow mediated dilation (FMD), was compared between young and older adults. Intermittent head up-down tilt resulted in greater changes in MAPbrain than MAP. Although changes in MAPbrain were similar between age groups, older adults showed a lower MCAv response to head up-down tilt transitions, which resulted in a lower %∆MCAv/%∆MAPbrain than in the young. This lower %∆MCAv/%∆MAPbrain was strongly correlated with FMD in the older group only. This extends the findings of Study 2 and indicates that impaired endothelial function might also be associated with the lower cerebral blood flow responsiveness to changes in cerebral blood pressure in older adults.
In conclusion, interval exercise was more effective than continuous exercise in promoting greater cerebrovascular responses in younger and older adults. Altered systemic and cerebral pressure-flow responses with age during dynamic stimuli were associated with impaired cerebrovascular and endothelial function.