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Articles in PresS, published online ahead of print April 4, 2002
Am J Physiol Regu Physiol, 10.1152/ajpregu.00452.2001
Submitted on July 29, 2001
Accepted on April 2, 2002
1 Cardiorespiratory and Vascular Dynamics Laboratory, University of Waterloo, Waterloo, Canada
2 School of Kinesiology, University of Western Ontario, Waterloo, Canada
* To whom correspondence should be addressed. E-mail: hughson{at}uwaterloo.ca.
Transfer function analysis of the arterial blood pressure (BP) and mean flow velocity (MFV) relationship describes an aspect of cerebrovascular autoregulation. We hypothesized the transfer function relating BP to cerebrovascular resistance (CVRi) would be sensitive to low frequency changes in autoregulation induced by head up tilt (HUT) and altered arterial PCO2. Nine subjects were studied supine and HUT with end-tidal PCO2 kept constant at normal levels, +5 mmHg and -5 mmHg. The BP
MFV relationship had low coherence at low frequencies and there were significant effects of HUT on gain only at high frequencies and of PCO2 on phase only at low frequencies. BPCVRi had coherence above 0.5 from very low to low frequencies. There was a significant reduction of gain with increased PCO2 in the very low and low frequencies and with HUT in the low frequency region. Phase was affected by PCO2 in the very low frequencies. Transfer function analysis of BPCVRi provides direct evidence of altered cerebrovascular autoregulation under HUT and higher levels of PCO2.
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