title: Transcranial Doppler estimation of cerebral blood flow and cerebrovascular conductance during modified rebreathing.
abstract: Clinical transcranial Doppler assessment of cerebral vasomotor reactivity (CVMR) uses linear regression of cerebral blood flow velocity (CBFV) vs. end-tidal CO(2) (Pet(CO(2))) under steady-state conditions. However, the cerebral blood flow (CBF)-Pet(CO(2)) relationship is nonlinear, even for moderate changes in CO(2). Moreover, CBF is increased by increases in arterial blood pressure (ABP) during hypercapnia. We used a modified rebreathing protocol to estimate CVMR during transient breath-by-breath changes in CBFV and Pet(CO(2)). Ten healthy subjects (6 men) performed 15 s of hyperventilation followed by 5 min of rebreathing, with supplemental O(2) to maintain arterial oxygen saturation constant. To minimize effects of changes in ABP on CVMR estimation, cerebrovascular conductance index (CVCi) was calculated. CBFV-Pet(CO(2)) and CVCi-Pet(CO(2)) relationships were quantified by both linear and nonlinear logistic regression. In three subjects, muscle sympathetic nerve activity was recorded. From hyperventilation to rebreathing, robust changes occurred in Pet(CO(2)) (20-61 Torr), CBFV (-44 to +104% of baseline), CVCi (-39 to +64%), and ABP (-19 to +23%) (all P < 0.01). Muscle sympathetic nerve activity increased by 446% during hypercapnia. The linear regression slope of CVCi vs. Pet(CO(2)) was less steep than that of CBFV (3 vs. 5%/Torr; P = 0.01). Logistic regression of CBF-Pet(CO(2)) (r(2) = 0.97) and CVCi-Pet(CO(2)) (r(2) = 0.93) was superior to linear regression (r(2) = 0.91, r(2) = 0.85; P = 0.01). CVMR was maximal (6-8%/Torr) for Pet(CO(2)) of 40-50 Torr. In conclusion, CBFV and CVCi responses to transient changes in Pet(CO(2)) can be described by a nonlinear logistic function, indicating that CVMR estimation varies within the range from hypocapnia to hypercapnia. Furthermore, quantification of the CVCi-Pet(CO(2)) relationship may minimize the effects of changes in ABP on the estimation of CVMR. The method developed provides insight into CVMR under transient breath-by-breath changes in CO(2).
authors.affiliation: Department of Geriatric Medicine, Radbound University Nijmegen Medical Center, The Netherlands.
authors.forename: Jurgen A H R
authors.initials: JA
authors.lastname: Claassen
authors.forename: Rong
authors.initials: R
authors.lastname: Zhang
authors.forename: Qi
authors.initials: Q
authors.lastname: Fu
authors.forename: Sarah
authors.initials: S
authors.lastname: Witkowski
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Levine
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D001794:Blood Pressure / Q000502:physiology*; D002245:Carbon Dioxide / Q000502:physiology; D002560:Cerebrovascular Circulation / Q000502:physiology*; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006935:Hypercapnia / Q000000981:diagnostic imaging / Q000503:physiopathology*; D006985:Hyperventilation / Q000000981:diagnostic imaging / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D017585:Ultrasonography, Doppler, Transcranial; D014666:Vasomotor System / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide