Table 1.

Comparison of short-term inflight cardiovascular risks and long-term post-career risks: short-term inflight and long-term post-career cardiovascular risks in astronauts

Category	Timing	Typical changes or events	Evidence base	Operational/clinical implications
Short-term inflight risks	During spaceflight	Altered cardiac output and stroke volume; vascular tone changes/systemic vasodilation; fluid shifts toward the head with reduced plasma volume; autonomic/baroreflex changes; early electrophysiologic changes such as ectopy/repolarization changes.	Derived mainly from inflight monitoring, ISS and Shuttle mission studies, and short-duration analog experiments	May impair mission performance, crew safety, and exercise tolerance; require in-mission countermeasures such as exercise regimen, fluid loading, and compression garments.
Long-term in-flight risks	During spaceflight	Cumulative cardiac unloading/remodeling or atrophy (countermeasure-dependent); persistent autonomic/vascular deconditioning; venous stasis or retrograde jugular flow with rare thrombosis; oxidative/inflammatory stress; possible arterial stiffness/endothelial injury; arrhythmia and QT-repolarization concerns; radiation-related vascular injury.	Derived from long-duration ISS cohorts, cardiac MRI/vascular ultrasound studies, NASA risk reports, and limited astronaut epidemiology, terrestrial and animal data.	Important for prolonged-mission monitoring; supports continued exercise countermeasures, venous thrombosis surveillance, rhythm monitoring, and radiation-risk mitigation/shielding for exploration-class missions.
“Acute” post-return risks (landing to several months after return)	Immediately upon return to several months	Orthostatic intolerance/presyncope or collapse; reduced upright arterial pressure and stroke volume; hypovolemia; altered sympathetic/baroreflex responses; functional deconditioning. Most changes improve over days to weeks, often approaching baseline by about 1 month, though some recovery can extend longer.	Based on landing-day and early postflight stand/tilt testing, hemodynamic studies, and short follow-up studies after Shuttle and ISS missions.	Highest risk window for re-entry, landing, and immediate operational tasks; requires supervised rehabilitation, fluid/salt replacement, mobility support, and compression countermeasures.
Long-term post-career risks	Months to decades after career ends	Possible accelerated vascular stiffening, myocardial remodeling/fibrosis, atherosclerotic progression, CAC increase, and late radiation-associated cardiovascular disease; however, persistent LEO-related vascular impairment has not been clearly demonstrated, and the epidemiology is mixed.	Based on longitudinal follow-up studies, event-rate studies, CAC surveillance, and radiation-risk modeling, the strongest uncertainty is for deep-space exposure.	Important for post-mission healthcare, surveillance programs, and guiding occupational health policy

This table summarizes cardiovascular alterations associated with human spaceflight, highlighting factors that were likely unknown at the time of astronaut selection. Short-term in-flight risks include hemodynamic, vascular, autonomic, and electrophysiological changes documented during spaceflight. Long-term post-career risks may involve vascular stiffening, myocardial remodeling, fibrosis, and variable reports of increased cardiovascular events.