Atelectasis

Atelectasis refers either to the incomplete expansion of the lungs (neonatal atelectasis) or to the collapse of the previously inflated lung, producing areas of relatively airless pulmonary parenchyma. Acquired atelectasis, encountered principally in adults, may be divided into resorption (or obstruction), compression, and contraction atelectasis
Resorption atelectasis is the consequence of a complete obstruction of an airway, which in time leads to resorption of the oxygen trapped in the dependent alveoli, without impairment of blood flow through the affected alveolar walls. Since lung volume is diminished, the mediastinum shifts toward the atelectatic lung. Resorption atelectasis is caused principally by excessive secretions (e.g., mucus plugs) or exudates within smaller bronchi and is, therefore, most often found in bronchial asthma, chronic bronchitis, bronchiectasis, postoperative states, aspiration of foreign bodies, and, rarely, bronchial neoplasms. Compression atelectasis results whenever the pleural cavity is partially or completely filled by fluid exudate, tumor, blood, or air (the last-mentioned constituting pneumothorax) or, with tension pneumothorax, when air pressure impinges on and threatens the function of the lung and mediastinum, especially the major vessels. With compressive atelectasis, the mediastinum shifts away from the affected lung. Contraction atelectasis occurs when local or generalized fibrotic changes in the lung or pleura prevent full expansion.
Significant atelectasis reduces oxygenation and predisposes to infection. Because the collapsed lung parenchyma can be re-expanded, atelectasis is a reversible disorder (except that caused by contraction).

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