In October 2024, NASA’s Office of the Chief Health and Medical Officer (OCHMO) took a significant step towards understanding and addressing a critical health risk associated with space travel: the risk of venous thromboembolism (VTE) during spaceflight. To tackle this issue, they initiated a comprehensive working group, which convened for two days at NASA’s Johnson Space Center. A subsequent meeting took place in December 2024 at the European Space Agency (ESA) facility in Cologne, Germany, further reinforcing the collaborative approach to this complex challenge.
The working group comprised a diverse panel of experts, including internal NASA subject matter experts (SMEs), members of the NASA OCHMO Standards Team, stakeholders from both NASA and ESA, and external specialists. Among the latter were physicians and medical professionals from leading universities and medical centers in the U.S. and Canada, highlighting the interdisciplinary and international effort to address VTE risks.
Spaceflight Venous Thrombosis (SVT) refers to the formation of a thrombus, or blood clot, in the internal jugular vein and related vasculature during spaceflight. These clots can be symptomatic, causing visible swelling in the internal jugular vein, facial edema beyond the usual adaptation to space, eyelid swelling, and headaches, or they can be asymptomatic. Though the incidence of obstructive thrombi has been identified in only a small number of crewmembers, the potential consequences necessitate a thorough investigation and proactive measures.
Importantly, crewmembers who have experienced thrombosis were able to complete their missions successfully. Anticoagulants, medications that help prevent clot formation, were used but discontinued several days before landing to minimize the risk of bleeding if a traumatic injury occurred. In some cases, the thromboses resolved completely after landing, while others required additional treatment.
The pathogenesis of VTE is often explained by Virchow’s triad, which suggests that VTE results from one or more of the following factors:
- Alterations in blood flow, also known as stasis.
- Injury or changes to the vascular endothelium, the thin layer of cells lining the blood vessels.
- Changes in the blood constituents leading to hypercoagulability, meaning an increased tendency for blood clot formation, which can be due to hereditary or acquired conditions.
Blood stasis, or venous stasis, occurs when blood flow in the veins slows down, leading to pooling. This slowing can result from damaged or weak vein valves, immobility, or lack of muscular contractions. Symptoms associated with venous stasis include swelling, skin changes, varicose veins, and slow-healing sores or ulcers. On Earth, venous thrombosis is often linked to factors like aging, obesity, a sedentary lifestyle, pregnancy, use of estrogen, and genetic predispositions, among others.
In the context of spaceflight, there are unique physiological changes that could contribute to VTE development. The microgravity environment of space leads to a redistribution of bodily fluids, causing them to move from the legs toward the head. This headward fluid shift results in facial puffiness, decreased leg volume, increased cardiac stroke volume, and decreased plasma volume, potentially affecting blood flow.
Astronauts have reported altered blood flow during missions, including retrograde venous blood flow (RVBF), where blood flows backward towards the brain, and stasis or a slowdown in blood flow. While the causes of these phenomena are not fully understood, their potential role in thrombus formation necessitates further study.
Another area of concern related to fluid shifts is Spaceflight-Associated Neuro-Ocular Syndrome (SANS), which involves changes in the eyes and brain due to chronic weightlessness. This syndrome can lead to optic nerve swelling, changes in the retina, flattening of the back of the eye, and brain swelling. Some astronauts experience mild changes, while others face significant clinical outcomes. The long-term health implications of SANS are still being explored, but the risk is particularly concerning for long-duration missions, such as a potential mission to Mars.
To address the risk of thrombosis in space, experts have developed an algorithm to guide in-mission assessment and treatment. This protocol involves early in-flight ultrasound testing to evaluate the flow characteristics of the left internal jugular vein and its associated vasculature.
The working group has proposed several areas for further research to enhance the understanding and management of thrombosis in spaceflight:
Improved detection capabilities to identify thrombus formation during missions.
A deeper understanding of the pathophysiology and factors leading to thrombosis in the space environment.
Development of effective countermeasures and treatments to mitigate the risk.NASA’s proactive approach to studying VTE in space not only aims to ensure the safety and health of astronauts but also contributes valuable insights to the broader field of medicine. The unique conditions of space provide a rare opportunity to study human physiology in an environment radically different from Earth, potentially leading to discoveries that can benefit medical science as a whole.
For more information on NASA’s efforts to address venous thromboembolism during spaceflight, you can visit their official website.
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