title: Morning reduction of cerebral vasomotor reactivity.
abstract: We measured cerebral vasomotor reactivity during normoventilation, hyperventilation (hypocapnia), and breathing of 6% CO2 (hypercapnia) in 20 normal subjects during the hours of 6 to 8 AM, 1 to 3 PM, and 7 to 9 PM. Cerebral vasomotor reactivity was calculated, using transcranial Doppler, as percent change in the mean blood flow velocity of the middle cerebral artery per mm Hg change in end-tidal CO2 during hypocapnia and hypercapnia. Vasomotor reactivity during hypercapnia was lower in the morning (1.72 +/- 0.66 %/mm Hg) than in the afternoon (2.34 +/- 0.74 %/mm Hg, p < 0.01) and evening (2.31 +/- 0.56 %/mm Hg, p < 0.001). Vasomotor reactivity during hypocapnia did not vary significantly during the three periods (2.34 +/- 0.59 %/mm Hg in the morning, 2.43 +/- 0.51 %/mm Hg in the afternoon, and 2.26 +/- 0.52 %/mm Hg in the evening). This reduced morning response to hypercapnia suggests diminished vasodilator reserve during this period, and may be related to the increased stroke risk during the morning hours.
authors.affiliation: Department of Neurology, University of Southern California School of Medicine, Los Angeles 90033.
authors.forename: S F
authors.initials: SF
authors.lastname: Ameriso
authors.forename: J G
authors.initials: JG
authors.lastname: Mohler
authors.forename: M
authors.initials: M
authors.lastname: Suarez
authors.forename: M
authors.initials: M
authors.lastname: Fisher
mesh_terms: D000328:Adult; D001783:Blood Flow Velocity / Q000502:physiology; D002245:Carbon Dioxide / Q000097:blood*; D002536:Cerebral Arteries / Q000000981:diagnostic imaging / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D006801:Humans; D006985:Hyperventilation / Q000503:physiopathology; D008297:Male; D017585:Ultrasonography, Doppler, Transcranial; D014664:Vasodilation / Q000502:physiology*; D014666:Vasomotor System / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide