Different responses of cerebral vessels to-30 degrees head-down tilt in humans

This study explored changes of the cerebral circulation and evaluated the r esponses to weightlessness in 12 volunteers (18-22 yr of age). The velociti es, diameters and blood flow volume of the common carotid artery (CCA), int ernal carotid artery (ICA), vertebral artery (VA) and internal jugular vein (IJV) were measured with color Doppler echogram before and during simulate d weightlessness. -30 degrees head-down tilt (HDT) for 45 min was used as a weightlessness simulation model. When the subjects' positions were changed from the supine to -30 degrees HDT, blood flow velocities along the CCA, I CA and IJV decreased significantly (p < 0.05), and their diameters were inc reased. The subjects were divided into two groups according to measured cha nges in flow volume of the ICA and IJV: group I with a net in-flow of cereb ral blood flow (CBF) and group II with a net out-flow of CBF during HDT. Sy mptoms were recorded in the two groups during HDT (nasal congestion, sensat ion of head fullness, headache, and others) and graded on a four-point scal e, from absent to serious. Results showed that group I had a higher symptom s score while group II had a lower symptoms score. Although this difference did not reach statistical significance, it suggests that cerebral blood fl ow changes may be partly responsible for the symptoms observed in subjects during HDT.