Author(s): SAKURAI, A., MURATA, H., TAKANO, D., YAMADA, Y., HORINO, M., FUKUIE, M., SUZUKI, R., MATSUSHIMA, S., HIRASAWA, A., MATSUDA, T., SHIBATA, S., Institution: GRADUATE SCHOOL OF HEALTH SCIENCES, KYORIN UNIVERSITY, Country: JAPAN, Abstract-ID: 1185

INTRODUCTION:
In microgravity environments, body fluids shift toward the head and may cause adverse effects such as optic disc edema and venous thrombosis. A -6° head-down tilt (HDT) is widely used as a ground-based model of microgravity. Previous studies suggest central circulatory responses during prolonged HDT exhibit rapid changes in the early phase, followed by re-equilibration. However, temporal changes in cerebral and cervical circulation during this period remain unclear. This study aimed to investigate temporal changes in cerebral and cervical vascular hemodynamics during a 2-hour -6° HDT exposure.
METHODS:
Thirteen healthy adults (3 females, 27 ± 4 years) participated. Measurements were performed at upright, supine, and at 5, 30, 60, 90, and 120 minutes after -6° HDT onset. Mean middle cerebral artery velocity (MCAV mean) was measured using transcranial Doppler ultrasonography. Cross-sectional area, mean blood flow velocity, and blood flow volume of the internal carotid artery (ICA), external carotid artery (ECA), and bilateral internal jugular veins (IJV) were assessed using ultrasound. Blood pressure, heart rate, end-tidal carbon dioxide pressure, and autonomic nervous system indices were also recorded.
RESULTS:
Postural changes are described using representative values at upright and HDT 5 min, and temporal changes during HDT are described using representative values at HDT 5 min and HDT 120 min.
Postural change from upright to supine and HDT 5 minutes decreased mean arterial pressure (MAP, 90 ± 4 to 84 ± 8 mmHg, p < 0.05) and heart rate (71 ± 6 to 62 ± 9 bpm, p < 0.05), while MCAV mean increased (63.7 ± 11.6 to 67.5 ± 13.7 cm/s, p < 0.05). ICA blood flow volume did not change significantly (260.05 ± 73.26 to 258.27 ± 64.54 mL/min, p = 0.78), whereas ECA blood flow volume increased (153.17 ± 47.75 to 173.00 ± 75.26 mL/min, p < 0.05). In the IJV, cross-sectional area markedly increased (7.73 ± 6.35 to 105.11 ± 56.18 mm2, p < 0.05), while mean blood flow velocity decreased (31.48 ± 12.68 to 10.28 ± 2.52 cm/s, p < 0.05), and blood flow volume increased (161.52 ± 161.75 to 599.98 ± 256.79 mL/min, p = 0.05).
During HDT, MAP increased (84 ± 8 to 90 ± 7 mmHg, p < 0.05), and MCAV mean also increased (67.5 ± 13.7 to 70.5 ± 14.2 cm/s, p < 0.05). In contrast, the ICA, ECA, and IJV showed no major temporal changes, and the dilation and low-flow state of the IJV persisted throughout HDT.
CONCLUSION:
The 2-hour -6° HDT exposure increased cerebral blood flow velocity and induced marked dilation of the internal jugular vein, while arterial inflow through the internal carotid artery remained stable, suggesting that cerebral autoregulation may have been maintained. Although mean arterial pressure and MCAV mean increased during HDT, cervical venous dilation and low-flow state persisted. These findings indicate sustained venous congestion during early simulated microgravity, and the relationship between this venous stasis and a potential increase in thrombotic risk warrants further investigation.