Dynamic modulation of cerebrovascular resistance as an index of autoregulation under tilt and controlled PET(CO(2)).
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 PCO(2). Nine subjects were studied in supine and HUT positions with end-tidal PCO(2) (PET(CO(2))) 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 PCO(2) 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 PCO(2) in the very low and low frequencies and with HUT at the low frequency. Phase was affected by PCO(2) in the very low frequencies. Transfer function analysis of BP --> CVRi provides direct evidence of altered cerebrovascular autoregulation under HUT and higher levels of PCO(2).
Cardiorespiratory and Vascular Dynamics Laboratory, Faculty of Applied Health Sciences, University of Waterloo, Waterloo N2L 3G1, Ontario, Canada N6A 3K7.
Michael R
MR
Edwards
J Kevin
JK
Shoemaker
Richard L
RL
Hughson
D000328:Adult; D002245:Carbon Dioxide / Q000097:blood*; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology; D016857:Hypocapnia / Q000503:physiopathology; D008297:Male; D016683:Supine Position; D014655:Vascular Resistance / Q000502:physiology*
D002245:Carbon Dioxide