Carbon monoxide diffusion capacity is decreased in all, except:
Flows remain normal, as does the diffusion capacity of the lung for carbon monoxide (DlCO), unless obstructive sleep apnea (which often accompanies obesity) and associated chronic intermittent hypoxemia have induced pulmonary arterial hypertension,in which case DLCO may be low.
Emphysema
Primary pulmonary hypertension
Alveolar haemorrhage
Infiltrative lung disease