title: Relationship between middle cerebral artery blood velocity and end-tidal PCO2 in the hypocapnic-hypercapnic range in humans.
abstract: This study examined the relationship between cerebral blood flow (CBF) and end-tidal PCO2 (PETCO2) in humans. We used transcranial Doppler ultrasound to determine middle cerebral artery peak blood velocity responses to 14 levels of PETCO2 in a range of 22 to 50 Torr with a constant end-tidal PO2 (100 Torr) in eight subjects. PETCO2 and end-tidal PO2 were controlled by using the technique of dynamic end-tidal forcing combined with controlled hyperventilation. Two protocols were conducted in which PETCO2 was changed by 2 Torr every 2 min from hypocapnia to hypercapnia (protocol I) and vice-versa (protocol D). Over the range of PETCO2 studied, the sensitivity of peak blood velocity to changes in PETCO2 (CBF-PETCO2 sensitivity) was nonlinear with a greater sensitivity in hypercapnia (4.7 and 4.0%/Torr, protocols I and D, respectively) compared with hypocapnia (2.5 and 2.2%/Torr). Furthermore, there was evidence of hysteresis in the CBF-PETCO2 sensitivity; for a given PETCO2, there was greater sensitivity during protocol I compared with protocol D. In conclusion, CBF-PETCO2 sensitivity varies depending on the level of PETCO2 and the protocol that is used. The mechanisms underlying these responses require further investigation.
authors.affiliation: Departments of Physiology and Biophysics, Faculty of Medicine, Heritage Medical Research Bldg.Rm. 212,University of Calgary, Calgary, Alberta, Canada. poulin@ucalgary.ca
authors.forename: Kojiro
authors.initials: K
authors.lastname: Ide
authors.forename: Michael
authors.initials: M
authors.lastname: Eliasziw
authors.forename: Marc J
authors.initials: MJ
authors.lastname: Poulin
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity; D002245:Carbon Dioxide / Q000097:blood*; D002560:Cerebrovascular Circulation; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000000981:diagnostic imaging / Q000503:physiopathology*; D016857:Hypocapnia / Q000000981:diagnostic imaging / Q000503:physiopathology*; D008297:Male; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000502:physiology*; D012123:Pulmonary Ventilation; D013990:Tidal Volume; D017585:Ultrasonography, Doppler, Transcranial; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide