Pulmonary atelectasis is a reduction in the volume or air content of one or more lung segments or lobes. As a result of intra-alveolar gas absorption, pulmonary atelectasis is usually accompanied by reduced translucency of the affected area, aggregation of adjacent structures (bronchi, pulmonary vessels, interstitial lung) into the atelectasis area, sometimes visible as solid alveolar cavities, and compensatory emphysema of other lung tissue. Pulmonary atelectasis can be classified as either congenital or acquired. Congenital pulmonary atelectasis is a condition in which the infant is born without gas filling the alveoli, with severe clinical dyspnea and cyanosis, and the child mostly dies from severe hypoxia after birth. The main cause of recurrent pulmonary atelectasis is recurrent endobronchial obstruction, commonly due to mucus plugs, tumors, granulomas, or foreign bodies formed by mucousy bronchial secretions. Pulmonary atelectasis can also be caused by bronchial stenosis or distortion or by exogenous compression of bronchi by enlarged lymph nodes, tumors or hemangiomas or exogenous compression of lung tissue by fluids and gases such as pleural effusion and pneumothorax. Small pulmonary atelectasis may be asymptomatic. Pulmonary atelectasis with more than one lobe often has dyspnea, paroxysmal cough, chest pain, cyanosis, tachycardia, and sometimes shock; in slow onset, right chest pain and cough due to traction on the pleura and mediastinum by negative chest pressure; in some cases with infection, fever, purulent sputum, etc. may occur. Massive pulmonary atelectasis when bronchial obstruction, narrowing or depression of the rib space on the affected side, reduced or absent respiratory motion, highly turbid or solid sounds, reduced or absent interstitial and phonatory sounds; high clear sounds on the healthy side; displacement of the heart and mediastinum to the affected side and rise of the diaphragm.