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(Stroke. 2000;31:1897.)
© 2000 American Heart Association, Inc.

Original Contributions

Dynamic Regulation of Middle Cerebral Artery Blood Flow Velocity in Aging and Hypertension

Lewis A. Lipsitz, MD; Seiji Mukai, MD; Jason Hamner, BS; Margaret Gagnon, RN Viken Babikian, MD

From the Hebrew Rehabilitation Center for Aged Research and Training Institute (L.A.L., S.M., J.H., M.G.), Beth Israel Deaconess Medical Center Department of Medicine (L.A.L., S.M.), Harvard Medical School Division on Aging (L.A.L., S.M.), Boston University Medical Center Department of Neurology (V.B.), and Boston VA Medical Center (V.B.), Boston, Mass.

Correspondence to Lewis A. Lipsitz, MD, Hebrew Rehabilitation Center for Aged, 1200 Centre St, Boston, MA 02131. E-mail Lipsitz{at}mail.hrca.harvard.edu

Background and Purpose—-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.

Methods—-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 CO₂ 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. CO₂ reactivity was computed as the slope of cerebrovascular conductance (CVC=BFV/MAP) versus end-expiratory CO₂. Coherence, transfer magnitudes, and phases between low-frequency MAP and BFV signals were computed 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 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<0.05). CO₂ reactivity was greater in the 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 positions.

Conclusions—-Despite reduced CO₂ reactivity, elderly normotensive and previously treated hypertensive subjects retain cerebral autoregulatory capacity in response to acute orthostatic hypotension.

Key Words: autoregulation • cerebral blood flow • hypotension, orthostatic • spectrum analysis

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