Atelectasis
Atelectasis is marked by incomplete expansion of lobules (clusters of alveoli) or lung segments, which may result in partial or complete lung collapse. The collapsed areas are unavailable for gas exchange; unoxygenated blood passes through these areas unchanged, thereby producing hypoxia.
Atelectasis may be chronic or acute and occurs to some degree in many patients undergoing upper abdominal or thoracic surgery. The prognosis depends on prompt removal of any airway obstruction, relief of hypoxia, and reexpansion of the collapsed lung.
Causes
Atelectasis commonly results from bronchial occlusion by mucus plugs and is frequently a problem in patients with chronic obstructive pulmonary disease, bronchiectasis, or cystic fibrosis and in those who smoke heavily. (Smoking increases mucus production and damages cilia.) Atelectasis may also result from occlusion by foreign bodies, bronchogenic carcinoma, and inflammatory lung disease.
Other causes include respiratory distress syndrome of the neonate (hyaline membrane disease), oxygen toxicity, and pulmonary edema, in which alveolar surfactant changes increase surface tension and permit complete alveolar deflation.
External compression, which inhibits full lung expansion, or any condition that makes deep breathing painful may also cause atelectasis. Such compression or pain may result from upper abdominal surgical incisions, rib fractures, pleuritic chest pain, tight
dressings around the chest, or obesity (which elevates the diaphragm and reduces tidal volume).
dressings around the chest, or obesity (which elevates the diaphragm and reduces tidal volume).
Atelectasis may also result from prolonged immobility, which causes preferential ventilation of one area of the lung over another, or mechanical ventilation using constant small tidal volumes without intermittent deep breaths.
Central nervous system depression (as in drug overdose) eliminates periodic sighing and is a predisposing factor of progressive atelectasis.
