26-072 Venous Pressure, Cardiac Preload and Gravity: An Integrated Study - Caen - CNES

Description

Mission

INTRODUCTION

In microgravity, from the standing position, the loss of hydrostatic gradient causes a cephalad fluid shift, leading to jugular vein distension and increase in thoracic blood volume. The increase in internal jugular venous pressure results in transient venous engorgement. Conversely, although mean central venous pressure (CVP) in microgravity is slightly lower than in the supine 1-g condition (Lawley et al., 2017), intrathoracic pressure decreases even more, maintaining or enhancing transmural cardiac filling pressure and thus modifying stroke-volume regulation. Over time, this redistribution may alter cerebral venous drainage, promoting venous remodeling and intracranial fluid accumulation—two hallmark features of Spaceflight-Associated Neuro-Ocular Syndrome (Marshall-Goebel et al., 2019; Kramer et al., 2020).

Direct CVP monitoring could improve understanding of central volume regulation and cardiovascular adaptation, but it is not feasible in space due to technical and ethical constraints: invasive catheterization, the clinical reference, is impractical for operational use. Ultrasound-based surrogates such as inferior vena cava diameter or collapsibility remain qualitative and operator-dependent. Controlled compression sonography, introduced by Thalhammer et al , relies on the principle that the external pressure required to collapse a vein equals its internal pressure. This method has been validated in peripheral veins (Uthoff et al., 2011) and applied to the internal jugular vein during parabolic flight (Martin et al., 2016) and bed rest (Hearon et al., 2023), using a device developed by Compremium (Switzerland). However, CVP estimates from the internal jugular vein can be affected by intrathoracic and perivascular pressures, potentially overestimating central venous pressure. In addition, the Compremium system is proprietary, with no access to acquisition or processing parameters, which limits reproducibility in research.

Building on these findings, this PhD project aims to develop and validate a non-operator-dependent ultrasound compression technique for venous pressure assessment, applicable across gravitational environments. In addition to the internal jugular vein, the external jugular vein will be investigated as a complementary site. Its superficial, extrathoracic location may provide a more accurate estimate of venous load in both physiological and clinical contexts.

DESIGN

Phase 1 – In vitro validation

A jugular venous segment from a large-animal model will be mounted within a controlled perfusion circuit allowing precise manipulation of intraluminal pressure and flow resistance. Simultaneous invasive and compression-derived pressure recordings will validate the physical equivalence between collapse and venous pressure. Ultrasound probes from different manufacturers will be tested to confirm portability and inter-system consistency.

Phase 2 – Human validation

In healthy volunteers, jugular collapse pressure will be measured non-invasively during graded head-down tilt and supine conditions, reproducing microgravity-like cephalad fluid shifts without invasive instrumentation. The relationship between hydrostatic column height, collapse pressure, and echocardiographic indices of right-heart filling will be analyzed. Comparison between internal and external jugular responses will quantify the influence of extravascular tissue pressure on venous compliance.

Phase 3 – Parabolic flight

During parabolic flight campaign, venous collapse pressure will be measured on both internal and external jugular veins while echocardiography monitors right-heart function and filling. This environment enables direct assessment of how rapid gravity transitions modify venous return and preload, offering dynamic validation of the non-invasive method under real microgravity conditions. These data will help establish its robustness and operational relevance for future space missions.

Phase 4 – Clinical and translational validation

In patients scheduled for right-heart catheterization at Caen University Hospital, non-invasive jugular collapse pressure will be recorded simultaneously with invasive right-atrial pressure and echocardiographic indices of cardiac function and filling. This phase will extend validation to clinical populations, quantifying how pathological variations in cardiac function and venous congestion influence the accuracy and interpretation of non-invasive pressure estimates.

Summary

This PhD project aims to understand how gravity affects the interplay between venous return, cardiac filling, and cerebral drainage. Operationally, it will deliver a reproducible and universal protocol for non-invasive venous pressure monitoring, compatible with ultrasound systems used both in spaceflight and clinical practice on Earth. Overall, this project will establish an integrative framework linking venous collapse pressure, cardiac preload, and gravitational state.