title: Factors affecting the determination of cerebrovascular reactivity.
abstract: OBJECTIVE
  Cerebrovascular reactivity (CVR), measures the ability of the cerebrovasculature to respond to vasoactive stimuli such as CO2. CVR is often expressed as the ratio of cerebral blood flow change to CO2 change. We examine several factors affecting this measurement: blood pressure, stimulus pattern, response analysis and subject position.
  
  
  METHODS
  Step and ramp increases in CO2 were implemented in nine subjects, seated and supine. Middle cerebral artery blood flow velocity (MCAv), and mean arterial pressure (MAP) were determined breath-by-breath. Cerebrovascular conductance (MCAc) was estimated as MCAv/MAP. CVR was calculated from both the relative and absolute measures of MCAc and MCAv responses.
  
  
  RESULTS
  MAP increased with CO2 in some subjects so that relative CVR calculated from conductance responses were less than those calculated from CVR calculated from velocity responses. CVR measured from step responses were affected by the response dynamics, and were less than those calculated from CVR measured from ramp responses. Subject position did not affect CVR.
  
  
  CONCLUSIONS
  (1) MAP increases with CO2 and acts as a confounding factor for CVR measurement; (2) CVR depends on the stimulus pattern used; (3) CVR did not differ from the sitting versus supine in these experiments; (4) CVR calculated from absolute changes of MCAv was less than that calculated from relative changes.
authors.affiliation: Department of Physiology, University of Toronto Toronto, ON, M5S 1A8, Canada.
authors.forename: Rosemary E
authors.initials: RE
authors.lastname: Regan
authors.affiliation: Department of Physiology, University of Toronto Toronto, ON, M5S 1A8, Canada ; Department of Anaesthesiology, University of Toronto Toronto, ON, Canada ; University Health Network Toronto, ON, Canada.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Fisher
authors.affiliation: Department of Physiology, University of Toronto Toronto, ON, M5S 1A8, Canada ; Department of Anaesthesiology, University of Toronto Toronto, ON, Canada ; University Health Network Toronto, ON, Canada.
authors.forename: James
authors.initials: J
authors.lastname: Duffin
mesh_terms: D000328:Adult; D062186:Arterial Pressure / Q000502:physiology*; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology; D002245:Carbon Dioxide / Q000097:blood; D002560:Cerebrovascular Circulation / Q000502:physiology*; D006801:Humans; D020768:Middle Cerebral Artery / Q000502:physiology*; D011187:Posture; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide
keywords: Blood pressure; CO2; cerebral blood flow; humans; hypoxia