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Risk of Reduced Crew Health and Performance Due to Hypoxia


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Preparations for Next Moonwalk Simulations Underway (and Underwater)

astronauts go through a "pre-breathing" period before eva
Astronauts Michael R. (Rich) Clifford and Linda M. Godwin, the assigned space-walking mission specialists for STS-76, go through a “pre-breathing” period on the Space Shuttle Atlantis’ middeck. This practice is normal procedure for space-walkers in preparation for their Extravehicular Activity (EVA) and the wearing of their Extravehicular Mobility Units (EMU). The photograph was taken with a 35mm camera by one of the crew members.

Human exploration missions will require robust, flexible Extravehicular Activity (EVA) architecture protocols that include the use of a reduced-pressure cabin atmosphere enabling staged denitrogenation. Use of this atmosphere could result in compromised health and performance to the crewmember due to exposure to mild hypobaric hypoxia; of most concern are the potential effects on the increased intracranial pressure, visual impairment, cognitive performance, sensorimotor dysfunction, oxidative damage, and sleep quality. In addition to hypobaric hypoxia associated with staged denitrogenation, there are additional factors that can result in hypoxic exposure to the crewmember, such as cabin depressurization, Environmental Control, and Life Support System (ECLSS) failure, toxic exposure, or crewmember illness/injury.

astronauts test a Personal Cabin Pressure Altitude Monitor
Jan Zysko (left) and Rich Mizell (right) test a Personal Cabin Pressure Altitude Monitor in an altitude chamber at Tyndall Air Force Base in Florida. Zysko invented the pager-sized monitor that alerts wearers of a potentially dangerous or deteriorating cabin pressure altitude condition, which can lead to life-threatening hypoxia. Zysko is chief of the KSC Spaceport Engineering and Technology directorate’s data and electronic systems branch. Mizell is a Shuttle processing engineer. The monitor, which has drawn the interest of such organizations as the Federal Aviation Administration for use in commercial airliners and private aircraft, was originally designed to offer Space Shuttle and Space Station crew members added independent notification about any depressurization
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+ DAG File Information (HSRB Home Page)

+ Hypoxia Risk DAG and Narrative (PDF)

+ Hypoxia Risk DAG Code (TXT)

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+ Risk of Reduced Crew Health and Performance Due to Hypoxia

+ 2015 November Evidence Report (MSWord)

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Last Updated
Mar 11, 2025
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Robert E. Lewis

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      Background
      Spaceflight Venous Thrombosis (SVT)
      Spaceflight Venous Thrombosis (SVT) refers to a phenomenon experienced during spaceflight in which a thrombus (blood clot) forms in the internal jugular vein (and/or associated vasculature) that may be symptomatic (thrombus accompanied by, but not limited to, visible internal jugular vein swelling, facial edema beyond “nominal” spaceflight adaptation, eyelid edema, and/or headache) or asymptomatic. Obstructive thrombi have been identified in a very small number of crewmembers, as shown in the figure below.

      Note that the figure below is for illustrative purposes only; locations are approximate, and size is not to scale.

      Approximate location of identified thrombi in crewmembers.Source: Modified from Cerebral Sinus Venous Thrombosis – University of Colorado Denver With treatment, crewmembers were able to complete their mission, and anticoagulants were discontinued several days prior to landing to minimize the risk of bleeding in the event of a traumatic injury. Some thromboses completely resolved post landing, and some required additional treatment.
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      Spaceflight Considerations
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      Risks Concept Risk is inherent in human spaceflight. However, specific risks can and should be understood, managed, and mitigated to reduce threats posed to astronauts. Risk management in the context of human spaceflight can be viewed as a trade-based system. The relevant evidence in life sciences, medicine, and engineering is tracked and evaluated to identify ways to minimize overall risk to the astronauts and to ensure mission success. The Human System Risk Board (HSRB) manages the process by which scientific evidence is utilized to establish and reassess the postures of the various risks to the Human System during all of the various types of existing or anticipated crewed missions. The HSRB operates as part of the Health and Medical Technical Authority of the Office of the Chief Health and Medical Officer of NASA via the JSC Chief Medical Officer.
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      Directed Acyclic Graphs 
      Summary
      The HSRB uses Directed Acyclic Graphs (DAG), a type of causal diagramming, as visual tools to create a shared understanding of the risks, improve communication among those stakeholders, and enable the creation of a composite risk network that is vetted by members of the NASA community and configuration managed (Antonsen et al., NASA/TM– 20220006812). The knowledge captured is the Human Health and Performance community’s knowledge about the causal flow of a human system risk, and the relationships that exist between the contributing factors to that risk.
      DAGs are:
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      Details
      At NASA, the Human System Risks have historically been conceptualized as deriving from five Hazards present in the spaceflight environment. These are: altered gravity, isolation and confinement, radiation, a hostile closed environment, and distance from Earth. These Hazards are aspects of the spaceflight environment that are encountered when someone is launched into space and therefore are the starting point for causal diagramming of spaceflight-related risk issues for the HSRB.
      These Hazards are often interpreted in relation to physiologic changes that occur in humans as a result of the exposure; however, interaction between human crew (behavioral health and performance), which may be degraded due to the spaceflight environment – and the vehicle and mission systems that the crew must operate – can also be influenced by these Hazards.
      Each Human System Risk DAG is intended to show the causal flow of risk from Hazards to Mission Level Outcomes. As such, the structure of each DAG starts with at least one Hazard and ends with at least one of the pre-defined Mission Level Outcomes. In between are the nodes and edges of the causal flow diagrams that are relevant to the Risk under consideration. These are called ‘contributing factors’ in the HSRB terminology, and include countermeasures, medical conditions, and other Human System Risks. A graph data structure is composed of a set of vertices (nodes), and a set of edges (links). Each edge represents a relationship between two nodes. There can be two types of relationships between nodes: directed and undirected. For example, if an edge exists between two nodes A and B and the edge is undirected, it is represented as A–B, (no arrow). If the edge were directed, for example from A to B, then this is represented with an arrow (A->B). Each directed arrow connecting one node to another on a DAG indicates a claim of causality. A directed graph can potentially contain a cycle, meaning that, from a specific node, there exists a path that would eventually return to that node. A directed graph that has no cycles is known as acyclic. Thus, a graph with directed links and no cycles is a DAG.  DAGs are a type of network diagram that represent causality in a visual format.
      DAGs are updated with the regular Human System Risk updates generally every 1-2 years. Approved DAGs can be found in the NASA/TP 20220015709 below or broken down under each Human System Risk.
      Documents
      Directed Acyclic Graph Guidance Documentation – NASA/TM 20220006812 Directed Acyclic Graphs: A Tool for Understanding the NASA Human Spaceflight System Risks – NASA/TP 20220015709 Publications
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      Last Updated Mar 11, 2025 EditorRobert E. LewisLocationJohnson Space Center Related Terms
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