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 CO 2 rebreathing in 10 healthy young subjects (age 24Ϯ1 years), 10 healthy elderly subjects (age 72Ϯ3 years), and 10 previously treated hypertensive elderly (age 72Ϯ2 years) subjects. CO 2 reactivity was computed as the slope of cerebrovascular conductance (CVCϭBFV/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. 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 (Ϫ4.7Ϯ0.7 cm/s in hypertensives and Ϫ5.3Ϯ1.2 cm/s in normotensives, PϭNS) compared with younger subjects (Ϫ10.1Ϯ1.1 cm/s, PϽ0.05). CO 2 reactivity was greater in the young subjects (0.19Ϯ0.01) compared with normotensive (0.14Ϯ0.01, PϽ0.05) and hypertensive elderly subjects (0.11Ϯ0.02, PϽ0.05) (PϭNS 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 CO 2 reactivity, elderly normotensive and previously treated hypertensive subjects retain cerebral autoregulatory capacity in response to acute orthostatic hypotension.

Abstract-Transcranial Doppler ultrasound is capable of detecting microembolic material, both gaseous and solid, within the intracranial cerebral arteries. To avoid discrediting this promising and exciting new technique, experts in this field met in January 1997 in Frankfurt, Germany, to discuss the limitations and problems of embolus detection and to determine guidelines for its proper use in clinical practice, as well as in scientific investigations. In particular, the authors suggest that studies report the following parameters: (1) ultrasound device, (2) transducer type and size, (3) insonated artery, (4) insonation depth, (5) algorithms for signal intensity measurement, (6) scale settings, (7) detection threshold, (8) axial extension of sample volume, (9) fast Fourier transform (FFT) size (number of points used), (10) FFT length (time), (11) FFT overlap, (12) transmitted ultrasound frequency, (13) high-pass filter settings, and (14) recording time. There was agreement that no current system of automatic embolus detection has the required sensitivity and specificity for clinical use.(Stroke. 1998;29:725-729.)