A NASA working group finds no basis for changing astronaut selection over patent foramen ovale and calls for tighter EVA medication guidance as Artemis missions move toward lunar operations
A NASA working group has rejected a blanket approach to patent foramen ovale in astronaut selection while leaving a more practical warning at the center of its recommendations: medication protocols before and after spacewalks need clearer clinical rules. The panel reviewed decompression sickness, venous thromboembolism and PFO risks in the setting of NASA's Artemis missions. Its work is a policy and evidence review, not a prospective clinical trial, so it does not establish a new incidence estimate, treatment effect or statistical threshold for individual astronauts.
The review is part of a live Artemis medical-planning process that brings together a September 2024 PFO assessment, an April 2026 venous-thromboembolism working-group report and the updated NASA-STD-3001 Volume 2, Revision F standard for human factors, habitability and environmental health. The scope includes EVA prebreathe testing, documented DCS events, in-flight VTE occurrences and PFO implications for lunar and beyond-lunar missions.
PFO Is Not a VTE Filter
The group found that a PFO should not be treated as a major risk factor for venous thromboembolism formation. It also found no basis for changing astronaut selection criteria on PFO grounds when considering complications from an embolism traveling from an initial formation site in the left internal jugular or cerebrum under normoxic or proposed hypoxic space-habitat conditions.
That conclusion separates two risks that can easily be merged in public discussion. The panel examined whether a PFO could allow a venous gas embolism to reach the arterial side and produce a serious mission health event, but it did not conclude that every PFO meaningfully increases the likelihood of a venous clot forming in the first place. In physiological terms, the opening is being considered primarily as a possible route for venous material to bypass the lungs, not as a demonstrated trigger for thrombosis.
The review did not endorse universal screening or exclusion for astronauts with large PFOs. For crews who are assessed, however, it recommended that members be told their status and offered closure when a large PFO is identified. The recommendation is therefore conditional rather than a new selection rule. It also leaves room for individualized decisions based on anatomy, mission demands, procedural risks and the uncertainty surrounding the protective value of closure.
Bubble Scores Have Limits
The panel also found no definitive link between bubble grades and altitude decompression sickness risk for prebreathe protocols involving partial gravity and ambulation. Minimizing bubbles remains desirable, but bubble scores do not yet provide a reliable standalone forecast of DCS risk, particularly during lunar surface operations.
The distinction matters because EVA prebreathe testing is used to reduce the physiological consequences of pressure changes before an astronaut leaves a spacecraft or habitat. Prebreathe procedures alter the body's inert-gas burden, while ambulation and partial gravity change how circulation and tissue loading behave compared with a stationary laboratory subject. The working group's assessment does not discard that prevention strategy. It limits what can be inferred from one measured feature: a lower bubble score cannot by itself be treated as proof that DCS risk has been eliminated.
In its ground-based research recommendation, the group said subjects should no longer be removed solely because they have LVGE. Studies should balance participant safety with population representativeness, and participants must be informed of their LVGE status and associated risks. That approach preserves a wider research population without removing the requirement for informed participation. It also acknowledges that a test population narrowed by one physiological finding may not represent the crews or operational conditions that future lunar missions are intended to study.
The Evidence Behind Guidance
The recommendations integrate three NASA documents: the September 2024 assessment of PFO as related to DCS in the spaceflight environment and during ground testing, NASA/SP-20240010473; the April 2026 NASA Risk of Venous Thromboembolism in Spaceflight Working Group report, NASA/SP-20260005258/REV1; and the updated DCS prevention standard reviewed by the DCS panel in NASA-STD-3001 Volume 2 Human Factors, Habitability, and Environmental Health, NASA-STD-3001 Vol 2 Rev F.
Those references give the review a defined technical scope rather than the status of a new flight experiment. The working group assembled experts to examine recent EVA prebreathe testing and DCS events, in-flight VTE occurrences and the implications of PFO for lunar and beyond-lunar missions. Its central operational question was how to reduce risk without treating uncertain indicators as decisive medical filters. This layered approach is consistent with the way NASA and ESA typically manage human-spaceflight hazards: through engineering controls, operational procedures, medical surveillance and mission-specific contingency planning rather than reliance on one screening measurement.
The numerical framework is narrow but important: a small PFO was classified as Grade 1 or 2, while a large PFO was Grade 3 or above. The panel concluded that Grades 1 and 2 do not pose a significant risk, but members held mixed views on whether closing or excluding crew members with Grade 3 or higher would significantly reduce the chance of a VGE reaching the arterial side. The disagreement is consequential because a preventive intervention must be judged against both the hazard it may reduce and the risks introduced by the intervention itself.
Because the package synthesizes technical assessments and standards rather than reporting a single controlled human study, the available context does not support adding a new p-value, confidence interval or population-level effect estimate. That limitation should not be mistaken for a lack of rigor: it identifies the decision as one of evidence integration and operational risk management, not as a claim that PFO closure has been proven beneficial for every astronaut.
Medication Rules Matter
The clearest unresolved operational issue concerns medicines around EVAs. The panel called for clinical guidance covering aspirin use for DCS prevention and pain relief, as well as pre-EVA and post-EVA use of acetaminophen, ibuprofen, naproxen and celecoxib. These analgesics may mask DCS symptoms, making the timing and purpose of their use a direct safety concern rather than a minor medical detail. The concern is not that these medicines automatically cause DCS, but that symptom suppression could complicate recognition, triage and escalation when communications and evacuation options are limited.
That emphasis places protocol design ahead of a simple anatomical screening decision. A crew member's PFO status may be relevant to individualized risk mitigation, but the group did not support universal exclusion. By contrast, medication effects can interfere with recognizing symptoms during a mission, so the rules governing them must be explicit for all crewmembers. A practical protocol would need to specify indication, dose timing, symptom-reporting expectations and the circumstances in which medication should be withheld or reviewed by flight medical personnel.
NASA's broader human-spaceflight planning already depends on layered controls, from prebreathe procedures to medical monitoring and mission-specific operational rules. This review strengthens that logic: where the evidence does not establish a decisive predictive test, Artemis health protocols should combine targeted mitigation with continued data collection rather than impose a universal ban. The framework is closer to adaptive safety management than to a permanent anatomical exclusion policy, and it leaves future decisions open to better ground-test and flight data.
The same safety context applies to NASA's continuing crewed-spaceflight planning, including earlier crew coverage that described transportation and lifeboat services through 2030. Longer-term access to orbit makes consistent medical rules more consequential, but the working group's recommendations remain focused on the specific risks examined here. Reuters has also reported that NASA's discussions with Boeing over possible Starliner missions are continuing, illustrating how vehicle planning and medical policy are evolving in parallel; that broader planning does not change the evidence summarized by the PFO/VTE review.
Regulatory development beyond crew selection presents a related challenge. As commercial space activity expands, including proposals for medicines manufactured in orbit, the absence of a dedicated framework guaranteeing Earth-equivalent pharmaceutical standards shows why spaceflight health policy cannot be separated from quality control, clinical oversight and mission governance. The point is contextual rather than a finding of the PFO review: different parts of the space-medicine ecosystem are still building the rules needed for reliable care beyond Earth.
PFO is best understood in this review as a possible route by which venous gas could pass toward the arterial circulation, not as a demonstrated cause of every clot or DCS event. The evidence supports small-PFO reassurance and rejects routine exclusion, while large-PFO management remains individualized because the benefit of closure is unsettled. NASA's strongest immediate safeguard is therefore not a sweeping selection change but precise EVA medication guidance, disciplined protocol design and continued collection of flight and ground-test data. Future work published in journals such as Nature or PNAS could help refine the biological and operational uncertainties, but those uncertainties should not be filled with assumptions that the current review does not establish.
