Selective elevation in external carotid artery flow during acute gravitational transition to microgravity during parabolic flight.

This study sought to determine to what extent acute exposure to microgravity (0 G) and related increases in central blood volume (CBV) during parabolic flight influence the regional redistribution of intra and extra cranial cerebral blood flow (CBF). Eleven healthy participants performed during two parabolic flights campaigns aboard the Airbus A310-ZERO G aircraft. The response of select variables for each of the 15 parabolas involving exposure to both 0 G and hypergravity (1.8 G) were assessed in the seated position. Mean arterial blood pressure (MAP) and heart rate (HR) were continuously monitored and used to calculate stroke volume (SV), cardiac output ([Formula: see text]), and systemic vascular resistance (SVR). Changes in CBV were measured using an impedance monitor. Extracranial flow through the internal carotid, external carotid, and vertebral artery ([Formula: see text], [Formula: see text], and [Formula: see text]), and intracranial blood velocity was measured by duplex ultrasound. When compared with 1-G baseline condition, 0 G increased CBV (+375 ± 98 mL, = 0.004) and [Formula: see text] (+16 ± 14%, = 0.024) and decreased SVR (-7.3 ± 5 mmHg·min·L, = 0.002) and MAP (-13 ± 4 mmHg, = 0.001). [Formula: see text] increased by 43 ± 46% in 0 G ( = 0.030), whereas no change was observed for CBF, [Formula: see text], or [Formula: see text] ( = 0.102, = 0.637, and = 0.095, respectively). Our findings demonstrate that in microgravity there is a selective increase in external carotid artery blood flow whereas global and regional cerebral blood flow remained preserved. To what extent this reflects an adaptive, neuroprotective response to counter overperfusion remains to be established.