Morning reduction of cerebral vasomotor reactivity.

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.