Dynamic regulation of middle cerebral artery blood flow velocity in aging and hypertension

Although aging and hypertension may predispose hypertensive elderly subjects to cerebral hypoperfusion during orthostatic stress, their effects on the acute cerebral autoregulatory response to hypotension are not known. Continuous middle cerebral artery blood flow velocity (BFV) (transcranial Doppler ultrasound) and mean arterial pressure (MAP, Finapres) were measured in response to (1) acute hypotension during standing, (2) steady-state sitting and standing, and (3) hypercarbia during CO(2) rebreathing in 10 healthy young subjects (age 24+/-1 years), 10 healthy elderly subjects (age 72+/-3 years), and 10 previously treated hypertensive elderly (age 72+/-2 years) subjects. CO(2) reactivity was computed as the slope of cerebrovascular conductance (CVC=BFV/MAP) versus end-expiratory CO(2). Coherence, transfer magnitudes, and phases between low-frequency MAP and BFV signals were computed from their autospectra during 5 minutes of sitting and standing. MAP fell to a similar extent in all groups by an average of 21 to 26 mm Hg (22% to 26%) within 30 seconds of standing. Mean BFV also fell in all subjects but significantly less in the older subjects (-4.7+/-0. 7 cm/s in hypertensives and -5.3+/-1.2 cm/s in normotensives, P=NS) compared with younger subjects (-10.1+/-1.1 cm/s, P