|dc.description.abstract||The maintenance of adequate cerebral blood flow (CBF) is important both acutely and chronically, is highly dependent on mean arterial pressure (MAP) and the partial pressure of arterial carbon dioxide (PaCO2), and can be affected by age, fitness, and exercise. The primary aim of this thesis was to assess the effects of age, fitness and exercise on the control of CBF.
Resting CBF undergoes a gradual decline with increasing age. Although there is a higher risk of syncope with older age, the effect of fitness on orthostatic tolerance remains controversial. Furthermore, syncope has been widely reported following prolonged exercise (>4 h), but not following exercise of shorter durations. The purpose of the first study was to assess the cardiorespiratory and cerebrovascular responses to orthostatic stress [60° head-up tilt (HUT); i.e., assessment of the control of CBF during perturbations in MAP] in young and older trained and untrained individuals at rest and following moderate (30 min), and prolonged (4 h; trained groups only) exercise. Across the study, neither age nor fitness had a differential effect on orthostatic tolerance at rest or following exercise. Although tolerance was uncompromised following 30 min of exercise, all groups exhibited greater postural reductions in MAP and middle cerebral artery blood flow velocity (MCAv; an index of CBF), potentially indicating the early stages of reduced post-exercise cardiovascular and cerebrovascular control. Syncope developed in ~80% of participants following prolonged exercise. Although the progression of syncope was not affected by age, the cardiovascular mechanisms responsible for syncope differed between the young and older athletes. The young athletes exhibited greater reductions in stroke volume (SV) alongside a maintained total peripheral resistance (TPR), whereas the older displayed lesser reductions in SV combined with a reduced TPR. Despite a 32% higher resting MCAv in the young, MCAv at the point of syncope was the same in both young and older adults. Therefore, as determined by perturbations in MAP (by HUT), neither age, nor fitness differentially affect the maintenance of CBF at rest or following exercise.
Study One also provided the opportunity to examine the previously suggested – but untested – notion that the magnitudes of both post-exercise hypotension (PEH) and initial orthostatic hypotension (initial (<30 s) hypotensive response to HUT; IOH) are related to syncope following prolonged exercise. Neither PEH nor IOH were related to time to syncope following prolonged exercise, indicating that although a reduction in MAP (and consequently CBF) are ultimately responsible for syncope, the magnitudes of PEH and IOH do not predict the extent of orthostatic intolerance.
The purpose of the second study was to assess the effects of age and fitness on the control of CBF (during perturbations in PaCO2; termed cerebrovascular reactivity) at rest and during exercise. Based on cross-sectional data, irrespective of age, CBF is chronically elevated with fitness. The effect of age on cerebrovascular reactivity is controversial, and the effect of fitness on reactivity is also unknown. In addition, the effect of age on the control of CBF during exercise has not been reported. Therefore, the aim of this study was to assess the effect of a 12-week aerobic-based exercise intervention on CBF and CO2 reactivity at rest and during sub-maximal exercise in young and older individuals. Findings showed that, irrespective of age, both resting MCAv (~+7%) and cerebrovascular reactivity (~+10-15%) were elevated following the training. The increase in MCAv in response to sub-maximal exercise was attenuated in the older individuals. Furthermore, cerebrovascular reactivity was elevated during sub-maximal exercise, although not differentially with age. Collectively, these findings support the use of chronic exercise as an effective tool in the prevention of cerebrovascular disease, and may provide a physiological link in the positive relationship between exercise training and stroke.
In conclusion, the control of CBF is well maintained in healthy older age. Assumption of the upright posture following an acute bout of exercise can compromise the ability to maintain adequate CBF, leading to syncope; however, conversely, the control of CBF seems to be improved during exercise and following chronic exercise training.||en_NZ