Seventeen people were put into Earth’s orbit in the year 2023, marking a prominent era of human space flight. However, astronauts don’t always remain confined to their space shuttle or station, as they often perform extravehicular activities like space walks or moon walks. While outside in space they are exposed to extreme conditions like cold temperatures and low air pressure, also known as hypobaric decompression.
Astronauts who experience hypobaric decompression can have health issues, such as damage to brain processes that help us experience our external environment. But scientists don’t fully understand the bodily processes that cause these astronauts to get sick.
To figure out what causes hypobaric decompression sickness, investigators from the UK examined biological molecules that humans release in response to severe stress, called biomarkers. They performed experiments on people under simulated conditions of high altitudes on Earth. Humans also experience hypobaric decompression sickness at high altitudes, since air pressure decreases the higher you go in the atmosphere. So researchers can perform experiments on people at high altitudes to study the same biomarkers.
The researchers studied 15 healthy men, aged 20 to 50 years old. First they used 100% oxygen to wash out the nitrogen contained in each participants’ lungs so they wouldn’t actually get sick, a process called denitrogenation. Then they equipped each participant with tanks of 100% oxygen, which they breathed for the entirety of their climb. They asked the men to climb to an equivalent pressure altitude of 25,000 feet (about 8 km) twice, where they remained for 60 and 90 minutes, separated by 1 hour of breathing air normally at ground level.
The investigators collected blood samples from each participant’s veins before they started their ascent, which they referred to as time 0 (T0). They also collected blood samples after the participants’ second ascent (T8), and the next morning (T24).
The researchers analyzed the components of the blood samples, including biomarkers for inflammation and brain injury, using a biochemical method known as an enzyme-linked immunosorbent assay. They coupled this method with a second technique that detects biomarkers by measuring the concentration of colored compounds in solution, and a third technique that sorts different types of blood cells based on their fluorescence.
The researchers found the participants experienced a rapid change in their blood composition, although their blood mostly returned to normal by T24. The participants still had elevated biomarkers even at T24, including protein and peptide biomarkers that signal inflammation and traumatic brain injury, which were 10% to 100% elevated over normal levels. They also found the participants had small gas bubbles called microbubbles forming in their veins.
The researchers concluded hypobaric decompression sickness was mainly caused by inflammation throughout the participants’ bodies. They explained some inflammation persisted after the participants were brought back to normal pressure, suggesting that astronauts could suffer some degree of permanent brain damage even once they are relieved of duty.
The researchers cautioned they were unable to tell how much other high altitude stressors also experienced by astronauts, like microbubbles, could have influenced their results. They suggested future researchers could test their conclusions by conducting experiments with a larger number of participants, examining additional biomarkers, and collecting more frequent biomarker data throughout the experiments.