Cerebral blood flow responses to severe orthostatic stress in fit and unfit young and older adults

Background: Animal models suggest that aging negatively affects the cerebral blood flow (CBF) response to a hypotensive challenge while research with humans suggests that chronic physical activity may positively affect age-associated alterations in CBF. The extent to which chronic exercise affects the CBF responses to orthostatic stress in older subjects is unknown. Objectives: The purpose of this study was to determine the extent to which differences in age and cardiovascular fitness, reflective of differences in chronic exercise training, affect cerebral blood flow responses to severe orthostatic stress. Methods: Subjects were divided into four groups: old fit (OF; 72 +/- 2 year-olds; 38.7 +/- 1.6 ml . kg(-1) . min(-1)), old unfit (OU; 71 +/- 1 year-olds; 27.7 +/- 2.9 ml . kg(-1) . min(-1)), young fit (YF; 23 +/- 0.5 year-olds; 56.1 +/- 1.8 ml . kg(-1) . min(-1)) and young unfit (YU; 23 +/- 1 year-olds; 41.1 +/- 1.6 ml . kg(-1) . min(-1)). Fitness was quantified with maximal graded exercise testing. Middle cerebral artery blood flow velocity (CBF velocity; transcranial Doppler) responses to graded orthostatic stress (lower body negative pressure) to presyncope were assessed. Results: Orthostatic tolerance did not differ between groups (p = 0.633). The groups differed in fitness (YF > YU = OF > OU; p < 0.001) but not in either resting CBF velocity (p = 0.364) or pulsatility index (Pl; p = 0.251). CBF velocity decreased below rest in the last completed stage of submaximal LBNP (p < 0.001) but did not differ between groups (p = 0.182). PI did not change with submaximal LBNP within groups. At presyncope, mean CBF velocity decreased below that of the last completed stage of LBNP in all groups (p < 0.001) while the PI increased in the young fit group only (p < 0.001). Conclusion: Marked differences in age and fitness did not affect cerebral blood flow responses to severe orthostatic stress. While severe orthostatic stress can compromise CBF velocity in all age groups, older adults appear to autoregulate CBF as well as much younger subjects.