prebreathe

transitive verb

pre·​breathe (ˈ)prē-ˈbrēt͟h 

prebreathed; prebreathing

: to breathe (pure oxygen) to purge the blood of nitrogen content in preparation for an activity (as a space walk) that involves exposure to a significant change in pressure which might otherwise cause decompression sickness

prebreathing

-iŋ 

noun