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

Background and Purpose-Although aging and hypertension may predispose hyper tensive elderly subjects to cerebral hypoperfusion during orthostatic stres s, their effects on the acute cerebral autoregulatory response to hypotensi on are not known. Methods-Continuous middle cerebral artery blood flow velocity (BFV) (transc ranial Doppler ultrasound) and mean arterial pressure (MAP, Finapres) were measured in response to (1) acute hypotension during standing, (2) steady-s tate sitting and standing, and (3) hypercarbia during CO2 rebreathing in 10 healthy young subjects (age 24+/-1 years), 10 healthy elderly subjects (ag e 72+/-3 years), and 10 previously treated hypertensive elderly (age 72+/-2 years) subjects. CO2 reactivity was computed as the slope of cerebrovascul ar conductance (CVC=BFV/MAP) versus end-expiratory CO2. Coherence, transfer magnitudes, and phases between low-frequency MAP and BFV signals were comp uted from their autospectra during 5 minutes of sitting and standing. Results-MAP fell to a similar extent in all groups by an average of 21 to 2 6 mm Hg (22% to 26%) within 30 seconds of standing. Mean BFV also fell in a ll subjects but significantly less in the older subjects (-4.7+/-0.7 cm/s i n hypertensives and -5.3+/-1.2 cm/s in normotensives, P=NS) compared with y ounger subjects (-10.1+/-1.1 cm/s, P<0.05), CO2 reactivity was greater in t he young subjects (0.19+/-0.01) compared with normotensive (0.14+/-0.01, P< 0.05) and hypertensive elderly subjects (0.11+/-0.02, P<0.05) (P=NS between elderly groups). Fewer hypertensive subjects had coherence between MAP and BFV signals; for subjects with coherence, there were no significant group differences in phase or transfer magnitudes in either sitting or standing p ositions. Conclusions-Despite reduced CO2 reactivity, elderly normotensive and previo usly treated hypertensive subjects retain cerebral autoregulatory capacity in response to acute orthostatic hypotension.