The head-down tilt(HDT) position infuses changes in cerebral blood flow, intracranial pressure, hemodynamic and respiratory system. This study was performed to evaluate the changes in cerebral blood flow and the onset of autoregulation according to the different degree of HDT.

The subjects were 12 healthy adult female volunteers. They were divided two groups : 10° HDT(group 1) and 15° HDT(group 2). The systolic, diastolic and mean blood pressure, heart rate, end-tidal CO₂ concentration and cerebral blood flow velocity on middle cerebral artery by transcranial Doppler were measured before positioning and 1,2,3,5,7,9,11 minute after positioning.

There was no significant changes in cerebral blood flow velocities statistically according to the HDT under 15 degrees. In group 1, vean arterial blood pressure were increased at 5 minutes and returned to control value at 7 minutes after HDT with statistical significances. Diastolic blood pressure in group 1 were increased at 1 and 2 minutes after HDT with statistical significances. In froup 2, systolic blood pressure were increased at 5,7,9,11 minutes after HDT statistically significantly.

There were no significant changes of cerebral blood flow under less than 15° HDT. But systolic blood pressure were increased with 15° HDT in the healthy adults statistically significantly(p<0.05) not but clinically. So, we suggested that if HDT is required, we should take care of the partients more than 10 minutes after HDT.