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(Stroke. 2001;32:2403.)
© 2001 American Heart Association, Inc.

Original Contributions

Critical Analysis of Cerebrovascular Autoregulation During Repeated Head-Up Tilt

Richard L. Hughson, PhD; Michael R. Edwards, MSc; Deborah D. O’Leary, MSc J. Kevin Shoemaker, PhD

From the Cardiorespiratory and Vascular Dynamics Laboratory, Faculty of Applied Health Sciences, University of Waterloo, Waterloo, Ontario, Canada.

Correspondence to Prof Richard L. Hughson, Cardiorespiratory and Vascular Dynamics Laboratory, Faculty of Applied Health Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada. E-mail hughson{at}uwaterloo.ca

Background and Purpose—— Cerebrovascular autoregulation has been described with a phase lead of cerebral blood flow preceding changes in cerebral perfusion pressure (CPP), but there has been less focus on the effect of CPP on cerebral vascular resistance. We investigated these relations during spontaneous fluctuations (control) and repeated head-up tilt.

Methods—— Eight healthy adults were studied in supine rest and repeated tilt with 10-second supine, 10 seconds at 45° head-up tilt for a total of 12 cycles. Cerebral blood flow was estimated from mean flow velocity (MFV) by transcranial Doppler ultrasound, CPP was estimated from corrected finger pressure (CPP_F), and cerebrovascular resistance index (CVRi) was calculated in the supine position from CPP_F/MFV. Gain and phase relations were assessed by cross-spectral analysis.

Results—— In the supine position, MFV preceded CPP_F, but changes in CVRi followed CPP_F. Gain and phase relations for CPP_F as input and MFV as output were similar in supine and repeated tilt experiments. Thus, changes in cerebrovascular resistance must have had a similar pattern in the supine and tilt experiments.

Conclusions—— Cerebrovascular autoregulation is achieved by changes in resistance in response to modulations in perfusion pressure whether spontaneous or induced by repeated tilt. The phase lead of MFV before CPP_F is a mathematical and physiological consequence of the relation the input variable (CPP_F) and the manipulated variable (cerebrovascular resistance) that should not be taken as an indication of independent control of cerebral blood flow.

Key Words: autoregulation • cerebral blood flow • ultrasonography, Doppler transcranial

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