EVA Prebreathe Bubble Growth Modeling
This study addressed an important symptom of decompression sickness (DCS) that can affect astronauts conducting extravehicular activity (EVA) in relatively low-pressure spacesuits. To improve mobility and reduce risk of suit injury during spacewalks, spacesuits are operated at a low-pressure (4.3 psi in the EMU), 100% Oxygen environment. When tissue is exposed to this environment after the 14.7 psi cabin, the Nitrogen gas that is dissolved in the body begins to form bubbles and leave the body, leading to DCS symptoms. Current EVA protocol involves a 100% O2 prebreathe to reduce the N2 in the body prior to the spacewalk, but as exploration-class missions come online and EVA frequency is significantly increased, DCS continues to be a risk to astronauts. Modeling this phenomenon serves as a means for evaluating the risk to EVA astronauts and testing possible treatment protocols.
This was a project completed for ASEN 6519: EVA at CU Boulder in Spring 2021 with Dr. Allison Anderson.
Tissue Bubble Dynamics Model (TBDM)
This study is driven by the empirical TBDM, formulated in Gernhardt’s 1991 doctoral dissertation and implemented in a 2015 paper by Conkin et al. (cited below and accessed here), which serves as a means for simulating tissue bubble growth under DCS conditions. The TBDM is a differential equation that does not have a closed-form solution and must be computed numerically. The model is defined as:
where the constants α = 0.0125 cm3gas/cm3tis is the Ostwald nitrogen solubility (no relation), the diffusion coefficient D = 2.0 × 10−8 cm2/s, bubble film thickness h = 3.0 × 10-4 cm, surface tension γ = 30 dyne/cmtis modulus of elasticity for articular cartilage M = H/cm3tis dyne/cm2, and the sum metabolic gas tension is Pmet = 1.76×105 dyne/cm2. The remaining parameters (the initial ambient pressure, PB,tis, nitrogen tension Pt = Ptis,N2, and ascent rate, v) are determined by the particular conditions of each simulation. The model uses a 360-minute half-life and an initial micronucleation site of r0 = 3μm.
Bubble Growth Index (BGI)
BGI is the ratio of final bubble radius to the initial radius calculated by the TBDM, assumed to be 3μm, and serves as a measure of DCS risk. The Part 1 and Part 2 plots show the difference between an instantaneous transition to the low-pressure environment and one that includes a 120-minute resting prebreathe and 6-minute pressure ascent to 4.3 psi. A BGI of 15.0 corresponds to the onset of joint pain.
Varied Prebreathe Protocols
Exercising during a 100% O2 prebreathe accelerates oxygen absorption and tissue denitrogenation. The Part 4 figures compare different initial condition scenarios. Scenario 4a shows a 30-minute exercise, 30-minute resting prebreathe with results similar to that of a 120-minute resting prebreathe. Scenario 4b shows only a 30-minute exercise or 30-minute resting prebreathe, compared to the 120-minute resting baseline, both of which are worse, as expected. Finally Scenario 4d shows the BGI effect of using a “zero-prebreathe” suit, such as the 8.3 psi NASA Mark III suit, compared with the 4.3 psi EMU, both with a 60-minute resting prebreathe.
Gernhardt ML. Development and evaluation of a decompression stress index based on tissue bubble dynamics [dissertation]. Philadelphia (PA): University of Pennsylvania; 1991.
Conkin J, Abercromby AF, Dervay JP, Feiveson AH, Gernhardt ML, Norcross JR, Ploutz-Snyder R, Wessel JH 3rd. Hypobaric Decompression Sickness Treatment Model. Aerosp Med Hum Perform. 2015 Jun;86(6):508-17. doi: 10.3357/AMHP.4178.2015. PMID: 26099121.