The Vascular Frontier: Exploring the diagnosis and management of vascular conditions in spaceflight

Commentary on: Pavela J, Sargsyan S, Bedi D, et al. Surveillance for jugular venous thrombosis in astronauts. Vasc Med 2022; 27: 365-372.

Humankind is at the cusp of major changes in human spaceflight. The National Aeronautics and Space Administration (NASA) Space Shuttle Program retired in July 2011. In parallel, the private spaceflight sector has expanded exponentially to tailor to evolving needs from supporting programs of national space agencies to developing commercial space tourism. The final frontier of space, which used to be accessible to only a select few tied to space agencies, will soon be accessible to anyone with the financial means to purchase their golden ticket. ¹

Since the dawn of the space race, the impact of the spaceflight environment on human physiology has been investigated and explored. Human physiology has evolved and adapted to Earth’s gravity; however, it is completely altered in the space environment. ² Space is a remote and hostile environment where different gravitational pulls (microgravity, lunar gravity, Martian gravity, etc.) result in pathophysiological alterations to human physiology adapted to terrestrial gravity. ³ There is a possibility of adverse medical events requiring medical intervention in space, and these adverse events may increase as space becomes more accessible to the public. ⁴

There is a myriad of changes that happen to human physiology in spaceflight, with the most notable being cephalad fluid shifts with fluid redistribution occurring because of a lack of the terrestrial gravitational pull. After more than 50 years of spaceflight, the first documented venous thromboembolism (VTE), an internal jugular deep venous thrombosis (DVT), occurred on the International Space Station (ISS) in 2019. ¹ An unidentified astronaut (no demographics were available for privacy reasons) was identified with an obstructive left internal jugular vein thrombosis 2 months into a mission aboard the ISS. ⁵ Following multispecialty consultation, treatment was started with the only anticoagulant aboard the ISS – enoxaparin (1.5 mg/kg once daily for 33 days, followed my 1 mg/kg once daily to extend therapy until apixaban could be transported to the ISS). ⁵ Apixaban, protamine and prothrombin complex were supplied to the spacecraft at the earliest opportunity. Apixaban 5 mg twice daily was started 42 days after the diagnosis of the VTE, and the dose was reduced to 2.5 mg twice daily 3 months after diagnosis. ⁵ Subsequent sonographic surveillance demonstrated progressive reduction of the thrombus in the left internal jugular vein. Venous thrombosis was previously an unrecognized risk of spaceflight, and Dr James Pavela and colleagues ⁶ are to be commended on further developing an occupational surveillance program to screen for VTE in spaceflight crew members.

In this issue of Vascular Medicine, Pavela and colleagues report on duplex ultrasound findings in a small cohort of 11 astronauts. ⁶ They describe abnormal flow characteristics in the internal jugular veins (most prominent on the left) with mild-to-moderate echogenicity of intravenous spontaneous echo contrast, which may be prognostic for thrombus formation and subsequent development of DVT. ⁶ Such point-of-care ultrasound performed by spaceflight crew members is a valuable tool in the spaceflight medical armamentarium to evaluate and diagnose DVT. Studies have previously demonstrated that ultrasound evaluation of several conditions can be performed successfully in microgravity, including pneumothorax ⁷ and sinus fluid levels. ⁸ Thus, the work of Pavela and colleagues further supports the evidence that diagnostic ultrasound evaluation could be performed in microgravity by nonexperts.

Ultrasound screening for the development of VTE in astronauts highlights unique complexities of space medicine, such as the need for evidence-based clinical decision making, the need for crew member-performed ultrasonography, and the need for adequate medical treatment with the limited pharmacy aboard ISS. ⁵ Active astronaut screening for VTE, as proposed by Pavela et al., is an important consideration for the evaluation, diagnosis, and management strategy for VTE during spaceflight. Other cardiovascular conditions that may be life-threatening in space include cardiac arrythmias, myocardial infarctions, accelerated atherosclerosis, and orthostatic intolerance to name a few. ⁹ This will become even more critical as the next stages of human space travel concretize to other planetary bodies, such as the moon and Mars.