Effects of Artificial Gravity on the Vascular System during 60 days of Strict Head-Down Tilt Bedrest

During spaceflight, the lack of gravity-induced hydrostatic pressure gradients in the body results in a headward fluid shift hypothesized to play a role in pathophysiological changes in the cardiovascular, ocular, and central nervous systems. Here, 24 subjects (n=8 control) underwent 60 days of strict 6° head-down tilt (HDT) bedrest, an analog of spaceflight where we tested the ability of daily 30 min artificial gravity (AG) exposure via short-arm human centrifugation (intermittent AG, n=8, and continuous AG, n=8) to reverse the HDT-induced headward fluid shift. Right internal jugular vein (IJV) cross-sectional area (2D ultrasound), and near-infrared spectroscopy measures of total hemoglobin concentration ([HbT]) in the prefrontal cortex and gastrocnemius muscle were measured before, during, and after centrifugation. IJV area decreased from 0.88 cm2 (95% CI: 0.62 - 1.13) pre-AG to 0.33 cm2 (0.24 - 0.42, P<0.001) in the first 2 min of continuous AG and remained partially collapsed while exposed to AG, with similar results seen during application of intermittent AG; [HbT] in the gastrocnemius muscle increased by 78.7µM (13 - 177 µM, p<0.0001) during AG, with smaller decreases of [HbT] in the head (p<0.001). Overall, AG resulted in a rapid shift of blood volume from the upper to the lower body; however, beneficial fluid shifting towards the lower limbs during AG appear to only be present during active countermeasure application, regardless of intermittent or continuous AG application. Thus, AG may be an effective countermeasure to physiological changes resulting from the spaceflight-induced headward fluid shift, dependent on duration and dose of AG.