A blood-gas infuser was a medical device used to supply a body with oxygen. It could not, however, replace the function of the lungs on a long-term basis.
In 2371, The Doctor used a blood-gas infuser when Neelix's lungs were removed by a Vidiian. (VOY: "Phage")
Background information Edit
This medical instrument was designed by Rick Sternbach. (The Official Star Trek: Voyager Magazine issue #1, p. 72)