In scuba divers the presence of gas emboli (bubbles) in the bloodstream as a result of decompression is well-known and can be common after normal dives with no subjective signs of decompression sickness. The occurrence of decompression sickness in astronauts following decompression in connection with extravehicular activity (EVA) is not known. It has though been demonstrated that using the corresponding decompression techniques on the ground gives rise to symptoms of decompression sickness in approximately 6% of the cases. This suggests a much higher frequency of gas emboli without clear symptoms of decompression sickness. A non-invasive and simple technique for assessing current decompression techniques before and after EVA would be beneficial.
In this experiment astronauts will perform a simple inhalation-exhalation procedure (as in the NOA-1 protocol) as late as possible before starting standard EVA preparations and again as soon as possible after completing the EVA. An increased level of expired Nitric Oxide compared to pre-procedure levels will indicate the presence of gas emboli, suggesting a need for adaptation of existing EVA procedures. (Description provided by ESA: Astrolab Mission)