Evaluation of PFO in Relation to DCS in Spaceflight Settings and Ground Testing

In June 2024, the NASA Office of the Chief Health and Medical Officer (OCHMO) Standards Team convened an independent assessment working group to evaluate ongoing research and clinical efforts aimed at reducing the risk of decompression sickness (DCS) associated with patent foramen ovale (PFO) during space missions and related ground testing involving human subjects.

Decompression sickness occurs when dissolved gases, primarily nitrogen, form bubbles within the bloodstream and tissues due to abrupt decreases in ambient pressure, a risk often encountered in activities like scuba diving or high-altitude flight. The presence of these gas bubbles can disrupt blood flow, lead to inflammation, and harm tissue, ultimately manifesting various symptoms. NASA categorizes DCS into two primary types: Type I, marked by milder symptoms such as joint or muscle pain and skin rashes, and the more severe Type II, which can involve neurological, auditory, and cardiopulmonary issues. In the context of spaceflight, DCS risk is particularly pronounced during extravehicular activities (EVAs) where astronauts operate outside their spacecraft in a lower pressure environment, necessitating specific mitigation protocols that address systemic nitrogen levels.

A patent foramen ovale is a persistent opening between the heart’s right and left atria, remnants of fetal circulation that usually close after birth. Generally, around 75% of individuals will have their PFOs close within the first two years of life. For the remaining 25%, approximately 6% present with larger PFOs, defined as those exceeding 2 mm in diameter. Such openings pose risks, especially during activities that elevate right atrial pressure, potentially allowing venous gas emboli to bypass the lungs and enter the arterial system. This process could lead to serious neurological events like strokes.

The working group’s objectives were clear: to assess the increased risk PFOs may pose during decompression scenarios in both ground and spaceflight settings, to evaluate the merits of PFO screening among astronaut candidates and current crew members, to explore risk reduction strategies for individuals identified as high-risk for DCS due to their PFO status, and to recommend further research or technology development that could mitigate risks associated with PFO-related DCS.

Held over two days at NASA’s Johnson Space Center, the meeting gathered a diverse group of stakeholders, including NASA experts and external specialists from various fields such as cardiology, hypobaric medicine, and military health. Attendees reviewed existing literature regarding the correlation between PFOs and DCS, evaluating NASA’s current practices and decision-making processes. The discussions led to a collaborative exchange of insights, culminating in a comprehensive summary report now accessible on the public NASA OCHMO Standards Team website.

Key findings from the assessment highlighted several critical points:

• In high-risk scenarios, excluding individuals with PFOs or treating existing PFOs may not significantly lower DCS risk, emphasizing that while some incremental differences might exist, risks cannot be entirely eliminated due to various other physiological factors that also contribute to DCS susceptibility.
• Current evidence does not support the necessity of PFO screening for spaceflight or ground testing participants under existing protocols. Optimal DCS risk reduction should focus on creating safe environments through effective prebreathe procedures and ensuring rapid availability of treatment options if DCS symptoms arise.
• No immediate further research on PFOs in relation to DCS and altitude exposure is deemed necessary, given the low incidence risk combined with the implementation of robust safety protocols and treatment readiness on both ground and in space.
• For engineering protocols conducted on the ground, maintaining equivalent treatment capabilities as those in research protocols is essential to address potential DCS cases promptly, significantly contributing to test subject safety.

The detailed summary report encompasses valuable background context, discussion outcomes from the working group, and comprehensive recommendations. The insights gleaned will guide key stakeholders in their decisions regarding future ground testing and spaceflight operations, with a concerted focus on safeguarding crew health and safety—an overarching goal vital for mission success.