Recurrent pulmonary atelectasis is a decrease in the volume or air content of one or more lung segments or lobes. As a result of gas absorption within the alveoli, pulmonary atelectasis is usually accompanied by decreased translucency of the affected area, aggregation of adjacent structures (bronchi, pulmonary vessels, interstitium) toward the area of atelectasis, sometimes visible solid alveolar cavities, and compensatory emphysema of other lung tissues. Pulmonary atelectasis can be categorized as either congenital or acquired. Congenital atelectasis is a condition in which infants are born without gas filling the alveoli, with clinically severe dyspnea and cyanosis, and most of the children die of severe hypoxia after birth. In adults, the main cause of acute or chronic atelectasis is endobronchial obstruction, which is commonly caused by mucus plugs formed by mucousy bronchial secretions, tumors, granulomas, or foreign bodies. Lung atelectasis can also be caused by narrowing or twisting of the bronchi or by exogenous compression of the bronchi by enlarged lymph nodes, tumors, or hemangiomas, or by exogenous compression of the lung tissues by fluids and gases (e.g., pleural effusion and pneumothorax). The clinical manifestations of pulmonary atelectasis vary mainly depending on the etiology, the degree and extent of atelectasis, the time of occurrence, and the severity of complications. One-sided lobar atelectasis with rapid onset may be characterized by chest tightness, shortness of breath, dyspnea, and dry cough. When combined with infection, it may cause chest pain on the affected side, sudden dyspnea and cyanosis, cough, wheezing, hemoptysis, purulent sputum, chills and fever, tachycardia, elevated temperature, decreased blood pressure, and sometimes shock. Slow-onset pulmonary atelectasis or small areas of atelectasis may be asymptomatic or mildly symptomatic, such as atelectasis of the middle lobe of the right lung. Physical examination of the chest shows decreased or absent thoracic motion at the site of the lesion, shifting of the trachea and heart to the affected side, turbid to solid sounds on percussion, and decreased or absent breath sounds. Diffuse microscopic atelectasis may cause dyspnea, shallow breathing, hypoxemia, and decreased lung compliance, and is often an early manifestation of respiratory distress syndrome in adults and neonates. Chest auscultation may be normal or audible with twisting, dry rales, or rales. When the extent of lung atelectasis is large, there may be cyanosis, turbidity on percussion in the lesion area, and decreased breath sounds. On inspiration, dry or wet rales may be heard.