Absence of gravity-dependent modulation of straight sinus flow velocity in healthy humans.

The influence of whole-body positions on the cerebral blood flow in normal subjects is unclear. Blood flow in cerebral veins and sinuses is continuous, pulsatile and proportional to cerebral blood flow. We examined young healthy volunteers to evaluate peak mean flow velocity (vm) in the straight sinus (SS) assessed by transcranial Doppler sonography in predefined variations of the whole-body pitch position relative to gravity in the presence of a normal (normocarbia) and an impaired (hypercarbia) cerebral autoregulation. A 2 MHz ultrasound probe was fixed with a headband nearby the protuberantia occipitalis externa. Fifteen subjects were seated in a motorized three-dimensional turntable. Vm-SS, blood pressure and heart rate were monitored in five whole-body pitch positions from upright (0 degrees ) to "20 degrees head-hanging" (110 degrees ): 0, 30, 60, 90 and 110 degrees . The experiment was repeated during the inspiration of 5% CO2. Of 15 subjects, 14 showed reliable ultrasound data; the results of one subject with movement artifacts were excluded. Vm-SS values under normocarbia (hypercarbia) were 23.9 +/- 4.2 cm/s (40.9 +/- 6.7 cm/s) at 0 degrees , 23.1 +/- 5.0 cm/s (38.0 +/- 5.0 cm/s) at 30 degrees , 24.9 +/- 5.1 cm/s (39.9 +/- 3.3 cm/s) at 60 degrees , 29.2 +/- 8.5 cm/s (41.0 +/- 4.7 cm/s) at 90 degrees and 27.0 +/- 11.6 cm/s (43.6 +/- 12.1 cm/s) at 110 degrees . Vm-SS measured under normocarbia (p = 0.09) and hypercarbia (p = 0.25) were not affected while subjects were positioned from upright toward "20 degrees head-hanging", whereas blood pressure and heart rate decreased (p < 0.01). Our results suggest that changes of whole-body position from upright to "20 degrees head-hanging" do not alter cerebral blood flow in healthy subjects.