Background: There have so far been few reports on the static regulationsof cerebral and systemic circulation during prolonged head-down bed rest (HDBR). Our aim was to investigatethe time course changes in static cerebral and systemic circulation during 14 days of 6° degrees HDBR.Material/Methods: Sixteen subjects participated in the HDBR study. The systolic, mean, and diastoliccerebral blood flow velocities (CBFVs) of the middle cerebral artery were measured using a transcranialDoppler technique. Cerebrovascular bed resistance indices, i.e., resistance index (RI), pulsatility index(PI), and estimated regional cerebrovascular resistance (CVR[sub]est[/sub])) were calculated. The systemic cardiovascularfunctions, i.e, heart rate (HR), mean arterial pressure (MAP), left ventricular ejection time (LVET),stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were measured or calculated.Results: All CBFVs consistently showed significant decreases from the 2[sup]nd[/sup] day to the last day of theHDBR. The RI and PI showed a rising tendency throughout the HDBR. The CVR[sub]est[/sub] showed significantly higherlevels in the later half of the HDBR. The HR and MAP did not change during the HDBR. Conclusions: Theadaptive process of cerebral circulation triggered by HDBR begins very early and leads to a new equilibriumwithin few days after the onset of HDBR. The alteration of static cerebral circulation with prolongedHDBR, i.e., lowered CBFVs and somewhat higher cerebrovascular bed resistance implies a reduction in thecerebral circulation, but it does not necessarily imply the impaired regulation of cerebral circulation.

Keywords: Bed Rest, Blood Flow Velocity, Cardiovascular Physiological Phenomena, Cerebrovascular Circulation, Head-Down Tilt - physiology, Hemodynamics