<unk>  (or gas<unk>  is an abnormal retention of air in the lungs after expiration. It is observed in obstructive lung diseases such as asthma, and chronic obstructive pulmonary disease. The cause is obstruction such that the patient is unable to expel air completely.<unk>  is a cause of obstructive pattern spirometry results, leading to an elevated residual volume.<unk>  is often incidentally diagnosed on computed tomography (CT) scanning. The only way to absolutely differentiate<unk>  from emphysema is by taking expiratory images. On expiratory films, retained hyperlucent gas will be visualised in cases of<unk> .<unk>  represents poorly aerated lung, but on its own is clinically benign. It is is a common problem for smokers who dive. On diving the lung volume collapses and pushes air into the poorly aerated regions. On arising from a deep depth, these air-trapped areas of lung expand. This places great pressure on the lung tissue which can rupture.
Air trapping
Trapped lung
Airway obstruction
Alveolar capillary dysplasia
Emphysema
Post pneumonectomy syndrome
Pulmonary interstitial emphysema
Alveolar lung disease