Gap ExMC 1.03: Inadequate information on the individual susceptibility to decompression sickness (DCS) in hypobaric environments relevant to space flight (CLOSED)
Last Published:  07/30/21 01:05:31 PM (Central)
Responsible Element: Exploration Medical Capability (ExMC)
Status: Closed
Closure Rationale

Gap Closure Synopsis
This gap existed to evaluate the pertinent risk factors for decompression sickness (DCS) in altitude exposures, in particular, those related to extravehicular activities (EVAs) that will be conducted during exploration class space missions. Evidence from published reports and training has been documented in the white paper “Individual Susceptibility to Hypobaric Environments: An Update” (Law et al.). This publication provides sufficient closure to this gap by 1) identifying the multitude of risk factors applicable to causing DCS in altitude exposures and 2) indicating the continuation of the current DCS-protective methods used in space flight as further research is conducted to elucidate potential risk factors for DCS in such environments.


There are a litany of conditions, parameters, and factors that investigators have demonstrated to having causality on DCS. An anatomical condition, patent foramen ovale (PFO), is a common remnant of fetal circulation when the atrial septum primum and septum secundum fail to fuse during the first months of life, enabling abnormal blood flow between the left and right atria. This anatomic anomaly is the primary means for arterial gas embolism through the heart, thus suggesting an increase in an individual’s susceptibility to DCS. The susceptibility led NASA’s Medical Operations EVA Integrated Product Team (Med Ops EVA IPT) to review PFO data in 1999 to determine the merit of PFO screening in astronauts and test subjects. These data demonstrated that the relationship between PFO and Type II DCS in altitude exposures is unclear. Also, investigators showed that the identification of PFOs is very difficult regardless of the method, inconsistent and subjective. Taken together, these data led the Med Ops EVA IPT to state that PFO screening would not eliminate the risk of Type II DCS on orbit. Mitigating the risk of Type DCS would be better served by preventing bubble formation by oxygen pre-breathe procedures.

 

Since 1999, data from diving studies has supported a relationship between PFO and Type II DCS. Investigations have also shown that divers have an increase in DCS susceptibility via PFO with age. Contrastingly, a similar relationship has not been observed in studies using altitude exposure which is more applicable to space flight. Once again, the benefit of screening astronauts for PFO remains unclear.

 

Aside from PFO, a variety of individual parameters and environmental factors have been studied in an effort to identify risk factors for autochthonous bubble formation that lead to susceptibility to DCS. Authors have demonstrated that anthropometric and physiologic variables (e.g. age, height, weight, body mass index, percent body fat, aerobic capacity and gender) are not good predictors of DCS susceptibility. In addition, the number of dives and the depth of those dives did not have a significant effect on DCS incidence. Likewise, biochemical studies looking at levels of an individual’s total cholesterol, high-density lipoprotein cholesterol, potassium, phosphate, calcium, magnesium showed that none of them had an impact on the formation of venous gas emboli.

 

On the other hand, exercise has been shown to be a preventative measure for DCS. Investigators have demonstrated in both diving studies and altitude exposure studies the benefits of exercise to decrease venous gas emboli formation. In addition, increased hydration had a preventative effect on DCS susceptibility while temperature’s impact appears to be dependent on an individual’s hydration level.


Rationale for Closure

The ability to predict an individual’s susceptibility to DCS in hypobaric environments would be advantageous for crew selection, assignment, and mission planning. Unfortunately, no definitive risk factors have been identified to help with this process. Diving studies have discerned interesting results regarding these factors but more altitude exposure studies need to be conducted in order to discern any true risk factors for DCS as applicable to exploration space flight. ExMC will continue to monitor the progress of this research, in particular, via Gap 1.01 regarding preflight medical screening. Similarly, ExMC Gap 3.03 will monitor how we will expand DCS treatment in-flight, thus further minimizing the risk to this condition. In the meantime, it will be sensible to continue the current protective measures such as exercise-enhanced pre-breathe periods to minimize the risk of DCS in hypobaric environments.


Task and Supporting Reports Completed

Law, J., and Watkins, S., Individual Susceptibility to Hypobaric Environments: An Update, 5/31/2010, NASA Johnson Space Center.


Closure Documentation:
Description
1)  Initial State
Literature search and data mining report was compiled and published as a NASA technical manuscript.

 

Target for Closure
No Target for Closure available.
Mappings
Risk Risk of Adverse Health Outcomes and Decrements in Performance Due to Medical Conditions that occur in Mission, as well as Long Term Health Outcomes Due to Mission Exposures
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Completed

Multi-Disciplinary Research Plans

Documentation:
No Documentation Available