Effects of Age, Fitness, and Exercise on the Control of Cerebral Blood Flow

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 (