Decompression Sickness and Spacewalk (EVA) Medicine
Also: spacewalk decompression sickness · prebreathe · EVA injuries · altitude DCS
Hostile/Closed EnvironmentNeurological & CognitiveMusculoskeletalPulmonaryCardiovascularEVA & Surface OpsIn-flightLEO long-durationLunar surfaceMars transit & surfaceClinicianOperator / SFP
- Going from cabin pressure (historically 14.7 psia) to a low-pressure suit (historically 4.3 psia) can release nitrogen bubbles — the same DCS seen in divers and high-altitude aviators. ◆◆◆◆ High · E
- Prevention is by oxygen prebreathe (with exercise or a lowered-pressure 'campout'); NASA requires protocols to show less than 4% DCS probability with 95% confidence in ground chamber testing. ◆◆◆◆ High · E A
- NASA reports it has never had a Type II (serious, neurological/cardiopulmonary) DCS event in spaceflight. ◆◆◆◇ Moderate · F E
- Diagnosis is clinical; in-flight treatment is repressurization (suit plus cabin pressure, analogous to Navy treatment tables) with 100% oxygen and fluids — fluids are avoided in pulmonary DCS. ◆◆◆◇ Moderate · E
- Every EVA needs medical clearance; other EVA hazards include CO2 buildup, water intrusion in the helmet, thermal stress and hand/shoulder injuries. ◆◆◆◆ High · E F
Proposed certainty (◆ High → ◇ Very low) and evidence-type tags: F flight · A analog · M model/animal · C computational · E expert/operational/standards. Grades are AI-proposed and pending medical reviewer confirmation. How we grade.
What to do
Recognize DCS (after EVA, altitude chamber or suit pressure test)
| NASA category | Features |
|---|---|
| Type I | Itching, musculoskeletal (joint) pain, mild rash |
| Type II | Neurological symptoms (numbness, weakness, cognitive issues), inner ear problems, cardiopulmonary symptoms, cutis marmorata (mottled skin) |
Per OCHMO-TB-037 [1]. Most cases present 60–240 minutes after decompression [1].
- Diagnosis is clinical — bubbles on Doppler/ultrasound (venous gas emboli, VGE) are common without symptoms, and symptoms can occur without detectable bubbles [1].
- Treat early: 100% oxygen, recompression (on Earth: hyperbaric chamber per U.S. Navy treatment tables, e.g., TT6), and oral or IV fluids — except in pulmonary DCS [1].
- Consult a hyperbaric/dive medicine service (e.g., via a diving emergency hotline) for any suspected DCS on Earth.
Red flags (Type II / arterial gas embolism)
- Stroke-like symptoms, confusion, loss of consciousness, chest pain, dyspnea, cough (“chokes”).
- Any neurological symptom after a decompression exposure = emergency recompression.
- A patent foramen ovale (present in about 25% of people) can let venous bubbles reach the arterial side; NASA does not currently screen out people with a PFO [1].
Commercial and clinical counseling
- Patients who scuba dive before an altitude or space exposure: apply standard “no flying after diving” intervals; ask the operator about their decompression plan.
- Hydration and avoiding vigorous or resistive exercise right after decompression reduce risk [1].
Overview: why EVAs cause DCS
At cabin pressure, nitrogen is dissolved in tissues. When the suit pressure is much lower (historically 4.3 psia for the U.S. EMU), tissue nitrogen comes out of solution as bubbles [1]. A low-pressure suit is used because higher suit pressure stiffens the suit and limits dexterity [1].
Prebreathe washes nitrogen out by breathing 100% oxygen before decompression. NASA protocols have included [1]:
| Protocol | Key features |
|---|---|
| In-suit 4-hour prebreathe | 100% O2 in the suit at 14.7 psia for 4 h |
| Campout | Overnight in the airlock at 10.2 psia, 26.5% O2 (limited to 8 h 40 min), shortening in-suit prebreathe |
| Exercise prebreathe | Short intense exercise on 100% O2, plus in-suit prebreathe at 10.2 psia |
| In-suit light exercise (ISLE) | 40 min 100% O2 by mask, 20-min depressurization to 10.2 psia, then mild in-suit exercise |
Each protocol must be validated on the ground: less than 4% DCS probability with 95% confidence, no serious DCS, and VGE assessment — for example, 74 exposures without DCS under frequentist statistics, or 47 using Bayesian methods that incorporate historical data [1][4].
Risk factors: dehydration, higher body fat (nitrogen is more soluble in fat), exercise timing, PFO [1].
Treatment in space
There is no hyperbaric chamber in orbit. NASA’s approach combines increased suit pressure and cabin pressure — a minimum of about 16.4 psia may alleviate symptoms — with 100% oxygen, in principle like the Navy tables; the protocol is scenario-dependent [1]. Vehicles must provide this treatment capability [V2 6009].
Other EVA hazards
- CO2 buildup when suit scrubbers fail, and helmet fogging [2].
- Water intrusion: on ISS-36 (2013), a large amount of water collected in an astronaut’s helmet about an hour into an EVA; he was assisted back to the airlock [2].
- Hand injuries from glove pressure and repetitive tasks; eye irritation; thermal stress [2].
- Fitness: spacewalkers must meet minimum aerobic capacity [V1 4001]–[V1 4002].
Mission-type differences
| Mission | Notes |
|---|---|
| LEO (ISS) | Mature prebreathe protocols; return to airlock in minutes |
| Lunar surface | Frequent EVAs; NASA has evaluated an “exploration atmosphere” (8.2 psia, 34% O2) to cut prebreathe time; dust and terrain injuries [3] |
| Mars | Same plus long transit deconditioning and no evacuation |
| Commercial | The same physiology and requirements apply to any commercial spacewalk |
Terrestrial parallels
- Diving DCS and arterial gas embolism — identical pathophysiology and recompression principles [5].
- Altitude DCS in high-altitude aviators and hypobaric chamber workers — the closest analog (decompression from sea level rather than from depth) [3][5].
- Fitness-to-dive assessments — PFO, lung disease and fitness discussions transfer directly.
Gaps
- Optimal protocols for frequent lunar-surface EVAs and exploration atmospheres [3].
- Role of PFO screening for EVA crew [1].
- In-flight treatment effectiveness has not been tested in real cases.
Deeper dive: the tissue ratio (R-value)
Decompression stress is often expressed as R = tissue nitrogen pressure ÷ suit pressure; historically the 4.3 psia EMU used R ≤ 1.65. Nitrogen washout during prebreathe is modeled as exponential with a 360-minute tissue half-time chosen because it best correlated R with DCS incidence in NASA and U.S. Air Force chamber tests. NASA also uses a bubble-growth model (tissue bubble dynamics) that outputs a bubble growth index predictive of DCS and VGE [1].
Commercial spaceflight notes
For any commercial EVA, the same principles apply: validated prebreathe, medical clearance before each EVA, and a treatment plan. Clinicians seeing a participant after a private spacewalk should ask about joint pain, numbness or skin changes in the hours afterwards.
Clinical pearls
- Any neurological symptom after a decompression exposure is DCS until proven otherwise — oxygen and recompression first.
- Fluids help, except in pulmonary DCS [1].
Evidence profile
- Overall certainty
- ◆◆◆◇ Moderate (proposed)
- Evidence types
- E · Expert / operational / standardsA · Terrestrial analogF · Flight data
- Literature through
- 2026-10-14
NASA-STD-3001 requirements referenced 6
| Requirement | Title | Plain-language meaning |
|---|---|---|
| [V2 6006] | Total Pressure Tolerance Range for Indefinite Crew Exposure | Cabin total pressure kept within a range tolerable indefinitely (5–15 psia). |
| [V2 6007] | Rate of Pressure Change | Limits how fast pressure may change (no more than 13.5 psi/min). |
| [V2 6008] | DCS Risk Identification | Mission-specific mitigation achieving less than 4% DCS probability at 95% confidence in ground tests; no serious DCS; VGE assessed. |
| [V2 6009] | DCS Treatment Capability | The system must provide DCS treatment capability. |
| [V1 6006] | EVA Medical Clearance | Every crewmember medically cleared by ground medical support before each EVA. |
| [V1 4001]–[V1 4002] | EVA Aerobic Capacity | Minimum VO2max for microgravity and surface EVAs. |
IDs as published in NASA-STD-3001 Vol. 1 Rev C (2023-09-15) and NASA-STD-3001 Vol. 2 Rev F (2026-07-14). NASA-STD-3001 binds NASA programs; it is not a regulation for commercial flights. Paraphrased; read the standard for exact wording. Standards quick reference.
References 6
- NASA OCHMO. Decompression Sickness (DCS) Risk Mitigation. Technical Brief OCHMO-TB-037. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-037-decompr… NASA E
- NASA OCHMO. EVA Mishaps. Technical Brief OCHMO-TB-040. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-040-eva-mis… NASA E
- NASA Human Research Program. Evidence Report: Risk of Hypobaric Hypoxia and Decompression Sickness (2024). humanresearchroadmap.nasa.gov/evidence/reports/Evidence%20Re… NASA A
- NASA OCHMO. NASA-STD-3001, Volume 2, Revision F (2026), Section 6.2. standards.nasa.gov/standard/NASA/NASA-STD-3001_VOL_2 Standard E
- Vann RD, Butler FK, Mitchell SJ, Moon RE. Decompression illness. Lancet. 2011;377(9760):153-164. doi:10.1016/S0140-6736(10)61085-9 Peer-reviewed E
- NASA OCHMO. NASA-STD-3001, Volume 1, Revision C (2023). www.nasa.gov/wp-content/uploads/2023/11/nasa-std-3001-vol-1-… Standard E
Links and DOI metadata were machine-checked on 2026-10-14 (URL resolves; DOI title/journal/year matched via Crossref). Human verification of each cited claim against its source is still required before review. NASA documents are U.S. Government works (public domain) and are summarized with attribution; journal articles are summarized, not reproduced.
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