Gravitational effects on carotid and jugular characteristics in graded head-up and head-down tilt

Altered-gravity affects hemodynamics and blood flow in the neck. At least one incidence of jugular venous thrombosis has been reported in an astronaut on the International Space Station. This investigation explores the impact of changes in the direction of the gravitational vector on the characteristics of the neck arteries and veins. Twelve subjects underwent graded tilt from 45° head-up to 45° head-down in 15° increments in both supine and prone positions. At each angle, the cross-sectional area of the left and right common carotid arteries, ACCA, and internal jugular veins, AIJV, were measured using ultrasound. Internal jugular venous pressure, IJVP, was also measured using compression sonography. Gravitational dose-response curves were generated from experimental data. ACCA did not show any gravitational dependence. Conversely, both AIJV and IJVP increased in a non-linear fashion with head-down tilt. AIJV was significantly larger on the right side than the left side at all tilt angles. In addition, IJVP was significantly elevated in the prone position compared to the supine position, most likely due to raised intrathoracic pressure whilst prone. Dose-response curves were compared with existing experimental data from parabolic flight and spaceflight studies, showing good agreement on an acute timescale. The quantification of jugular hemodynamics as a function of changes in the gravitational vector presented herein provides a terrestrial model to reference spaceflight-induced changes, contributes to the assessment of the pathogenesis of spaceflight venous thromboembolism events, and informs the development of countermeasures.