Case Study: Dose-Dependent Internal Jugular Vein Response to Lower Body Negative Pressure in Microgravity Quantified by the Flow Directionality Index

Microgravity produces a headward fluid shift that alters venous hemodynamics and has been associated with stagnant or retrograde flow in the internal jugular vein (IJV), raising concern for thrombosis risk during spaceflight. Lower body negative pressure (LBNP) has emerged as a promising countermeasure to partially restore hydrostatic gradients, yet its physiological effects in true microgravity remain poorly characterized. In this case study, we applied graded LBNP (0, -20, and -30 mmHg) during parabolic flight in a single female participant and obtained matching supine baseline measurements on the ground. Bilateral IJV cross-sectional area and flow were quantified using ultrasound, and IJV pressure was measured using compression sonography. To enhance characterization of venous flow, we developed the continuous Flow Directionality Index (FDI), integrating time spent in antegrade, retrograde, and stagnant flow. Compared with 1 g baseline, microgravity was associated with larger IJV cross-sectional area and lower IJV pressure. Increasing LBNP reduced both variables across gravity conditions. In microgravity, IJV flow at LBNP of 0 mmHg included periods of stagnation, whereas -20 and -30 mmHg progressively improved venous return, reflected by higher FDI values. FDI provided greater granularity than traditional qualitative grading, revealing differences in flow quality otherwise difficult to detect. These findings demonstrate the feasibility and physiological relevance of graded LBNP during true microgravity to mitigate cephalad fluid shift-related alterations in jugular venous hemodynamics and introduce a sensitive quantitative approach for evaluating venous flow. This work establishes a foundation for future multi-subject studies aimed at optimizing countermeasures for long-duration spaceflight.