Internal carotid, external carotid and vertebral artery blood flow responses to 3 days of head-out dry immersion.

What is the central question of this study? The extent to which weightlessness associated with a fluid shift from the peripheral to the central circulation influences the blood flow in each cerebral artery remains unknown. The present study was designed to explore the effect of short-term weightlessness conditions on both anterior and posterior cerebral blood flow. What is the main finding and its importance? Short-term weightlessness affects both anterior and posterior cerebral vasculature. However, a heterogeneous cerebral blood flow response in each cerebral artery did not occur during 3 days of dry immersion. We have recently demonstrated that a heterogeneous cerebral blood flow (CBF) response in each cerebral artery might contribute to the maintenance of circulatory homeostasis in the brain. However, the extent to which weightlessness associated with a fluid shift from the peripheral to the central circulation influences the distribution of CBF in each cerebral artery remains unknown. We hypothesized that a dry immersion-induced fluid shift (weightlessness conditions) would cause a heterogeneous CBF response in each cerebral artery. During and after 3 days of dry immersion, the blood flows in the internal carotid (ICA), external carotid (ECA) and vertebral arteries (VA) were measured by Doppler ultrasonography using an 8 MHz linear transducer. Although the 3 days of dry immersion and the 2 days recovery period did not change the blood flow in each cerebral artery, the conductance in both ICA and VA decreased during dry immersion on days 2 and 3 (ICA, 2.95 and 3.23 ml min  mmHg ; VA, 1.10 and 1.05 ml min  mmHg , respectively) from the baseline (ICA, 3.47 ml min  mmHg , P = 0.027; VA, 1.23 ml min  mmHg , P = 0.004). In addition, Pearson correlation analysis demonstrated that the 3 days of dry immersion induced a decrease in cardiac output (P = 0.004) that was associated with changes in ICA (P = 0.046) and VA blood flow (P = 0.021), but not ECA blood flow (P = 0.466). These findings suggest that short exposures to weightlessness, acting via a cephalad redistribution of fluid volume and blood flow in the human body, influenced the cerebral vasculature in each cerebral artery but did not cause a heterogeneous CBF response in each cerebral artery.