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1 Cardiorespiratory and Vascular Dynamics Laboratory, Faculty of Applied Health Sciences, University of Waterloo, Waterloo N2L 3G1; and 2 Neurovascular Research Laboratory, School of Kinesiology, University of Western Ontario, London, Ontario, Canada N6A 3K7
Transfer function analysis of the
arterial blood pressure (BP)-mean flow velocity (MFV) relationship
describes an aspect of cerebrovascular autoregulation. We hypothesized
that 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 in supine and
HUT positions with end-tidal PCO2
(PETCO2) kept constant at normal levels:
+5 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. BP 
CVRi had coherence >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 at the low frequency. Phase was affected by
PCO2 in the very low frequencies. Transfer
function analysis of BP CVRi provides direct evidence of altered
cerebrovascular autoregulation under HUT and higher levels of
PCO2.
transfer function analysis; Doppler ultrasound; brain blood flow; orthostasis
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