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Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas 75231; and The University of Texas Southwestern Medical Center at Dallas, Texas 75235
To determine whether dynamic
cerebral autoregulation is impaired during orthostatic stress, cerebral
blood flow (CBF) velocity in the middle cerebral artery (transcranial
Doppler) and mean arterial pressure (MAP; Finapres) were measured
continuously in 12 healthy subjects during ramped maximal lower body
negative pressure (LBNP) to presyncope. Velocity and
pressure were averaged over 6-min periods of stable data at rest and
during LBNP to examine steady-state cerebral hemodynamics. Beat-to-beat
variability of velocity and pressure were quantified by a "variation
index" (oscillatory amplitude/steady-state mean value) and by power
spectral analysis. The dynamic relationship between changes in pressure
and velocity was evaluated by the estimates of transfer and coherence
function. The results of the study were as follows.
Steady-state MAP remained relatively constant during LBNP, whereas CBF
velocity decreased progressively by 6, 15, and 21% at 
30,
40, and 50 mmHg LBNP, respectively
(P < 0.05 compared with
baseline). At the maximal level of LBNP (30 s before
presyncope) MAP decreased by 9.4% in association with a prominent
reduction in velocity by 24% (P < 0.05 compared with baseline). The variation index of pressure increased
significantly from 3.8 ± 0.3% at baseline to 4.5 ± 0.6% at
50 mmHg LBNP in association with an increase in the variation index of velocity from 6.0 ± 0.6 to 8.4 ± 0.7%
(P < 0.05). Consistently, the low-
(0.07-0.20 Hz) and high-frequency (0.20-0.30 Hz) power of
variations in pressure and velocity increased significantly at high
levels of LBNP (P < 0.05) in
association with an increase in transfer function gain (24% at
50 mmHg, P < 0.05). We conclude that the damping effects of
autoregulation on variations in CBF velocity are diminished
during orthostatic stress in association with substantial falls in
steady-state CBF velocity. We suggest that these changes may contribute
in part to the development of presyncope.
cerebral blood flow; blood pressure; orthostasis; Fourier analysis
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