In the course of lung and respiratory disease, gas exchange is increasingly restricted. This leads to a situation in which not enough oxygen can be taken in [hypoxia]. The retaining of CO2 [hypercapnia] might occur later on. It is important to determine which type occurs at which time and if they only occur during exercise or at rest, as well.

According to this, one undergoes (e.g. on a bicycle ergometer) a BGA at a certain work rate (e.g. 50 watts) and blood is drawn in this situation under stress. Sometimes this exertion is created by walking (e.g. on a treadmill or in a 6-minute walking test. This way, the question can be answered as to whether it is reasonable to take in additional oxygen during exercise or not.