Definition 
Lung parenchyma without air. Airless lung parenchyma may lead to hypoxemia or infection (due to inadequate drainage of atelectatic lung).

Types

• Obstructive: excessive secretions, such as mucous plugs in asthma, and foreign bodies.
• Compressive: pleural effusions (CHF and pleural metastases), blood, air, subdiaphragmatic accumulation of fluid (pus).
• Patchy.