Alternations of static cerebral and systemic circulation in normal humans during 14-day head-down bed rest

Background: There have so far been few reports on the static regulations of cerebral and systemic circulation during prolonged head-down bed rest (HDBR). Our aim was to investigate the time course changes in static cerebral and systemic circulation during 14 days of 6° HDBR. Material/Methods: Sixteen subjects participated in the HDBR study. The systolic, mean, and diastolic cerebral blood flow velocities (CBFVs) of the middle cerebral artery were measured using a transcranial Doppler technique. Cerebrovascular bed resistance indices, i.e., resistance index (RI), pulsatility index (PI), and estimated regional cerebrovascular resistance (CVR_est) were calculated. The systemic cardiovascular functions, 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 ^nd day to the last day of the HDBR. The RI and PI showed a rising tendency throughout the HDBR. The CVR_est showed significantly higher levels in the later half of the HDBR. The HR and MAP did not change during the HDBR. Conclusions: The adaptive process of cerebral circulation triggered by HDBR begins very early and leads to a new equilibrium within few days after the onset of HDBR. The alteration of static cerebral circulation with prolonged HDBR, i.e., lowered CBFVs and somewhat higher cerebrovascular bed resistance implies a reduction in the cerebral circulation, but it does not necessarily imply the impaired regulation of cerebral circulation.