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Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, and University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75231
To determine the dependence of cerebral blood flow (CBF)
on arterial pressure over prolonged time periods, we measured
beat-to-beat changes in mean CBF velocity in the middle cerebral artery
(transcranial Doppler) and mean arterial pressure (Finapres)
continuously for 2 h in six healthy subjects (5 men and 1 woman,
18-40 yr old) during supine rest. Fluctuations in velocity and
pressure were quantified by the range [(peak
trough)/mean] and coefficients of variation (SD/mean) in the time
domain and by spectral analysis in the frequency domain. Mean velocity
and pressure over the 2-h recordings were 60 ± 7 cm/s and 83 ± 8 mmHg, associated with ranges of 77 ± 8 and 89 ± 10% and
coefficients of variation of 9.3 ± 2.2 and 7.9 ± 2.3%,
respectively. Spectral power of the velocity and pressure was
predominantly distributed in the frequency range of 0.00014-0.1 Hz
and increased inversely with frequency, indicating characteristics of
an inverse power law (1/f
). However,
linear regression on a log-log scale revealed that the slope of
spectral power of pressure and velocity was steeper in the
high-frequency (0.02-0.5 Hz) than in the low-frequency range
(0.002-0.02 Hz), suggesting different regulatory mechanisms in
these two frequency ranges. Furthermore, the spectral slope of pressure
was significantly steeper than that of velocity in the low-frequency
range, consistent with the low transfer function gain and low coherence
estimated at these frequencies. We conclude that 1) long-term
fluctuations in CBF velocity are prominent and similar to those
observed in arterial pressure, 2) spectral power of CBF
velocity reveals characteristics of
1/f
, and 3) cerebral
attenuation of oscillations in CBF velocity in response to changes in
pressure may be more effective at low than that at high frequencies,
emphasizing the frequency dependence of cerebral autoregulation.
blood pressure; spectral analysis; Doppler
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