<unk> , also called gas<unk> , is an abnormal retention of air in the lungs where it is difficult to exhale completely. It is observed in obstructive lung diseases such as asthma, bronchiolitis obliterans syndrome and chronic obstructive pulmonary diseases such as emphysema and chronic bronchitis.<unk>  is not a diagnosis but is a presentation of an illness, and can be a guide to the appropriate differential. __TOC__
Air trapping
Autoinflation
Mediastinal fibrosis
Airway obstruction
Reactive airway disease
Pulmonary hyperplasia
Extrapulmonary restriction
Diffuse alveolar damage