Atelectasis
Alveolar clusters (lobules) or lung segments that expand incompletely may produce a partial or complete lung collapse. Known as atelectasis, this phenomenon effectively removes certain regions of the lung from gas exchange. This allows unoxygenated blood to pass unchanged through these regions and produces hypoxia.
Atelectasis may be chronic or acute. The prognosis depends on prompt removal of any airway obstruction, relief of hypoxia, and reexpansion of the collapsed lung.
Causes
Atelectasis can result from bronchial occlusion by mucus plugs, bronchiectasis, or cystic fibrosis. Mucus plugs may also affect lung expansion in patients who smoke heavily. It may also result from occlusion caused by foreign bodies, bronchogenic cancer, and inflammatory lung disease.
Other causes include idiopathic respiratory distress syndrome of the newborn, oxygen toxicity, and pulmonary edema.
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, and obesity (which elevates the diaphragm and reduces tidal volume).
Furthermore, lung collapse or reduced expansion may accompany prolonged immobility (which promotes ventilation of one lung area over another) or mechanical ventilation (which supplies constant small tidal volumes without intermittent deep breaths). Central nervous system depression (resulting from drug overdose, for example) eliminates periodic sighing and predisposes the patient to progressive atelectasis.
Complications
Atelectasis may cause hypoxemia and acute respiratory failure. In addition, static secretions from atelectasis may lead to pneumonia.
Assessment
Clinical effects vary with the causes of lung collapse, the degree of hypoxia, and the underlying disease. If atelectasis affects a small lung area, the patient’s symptoms may be minimal and transient. However, with massive collapse, the patient may report severe symptoms—for example, dyspnea and pleuritic chest pain.
Inspection may disclose decreased chest wall movement, cyanosis, diaphoresis, substernal or intercostal retractions, and anxiety.
Palpation may detect decreased fremitus and mediastinal shift to the affected side. Percussion may disclose dullness or flatness over lung fields. Auscultation findings may include crackles during the last part of inspiration and decreased (or absent) breath sounds with major lung involvement. Auscultation may also disclose tachycardia.
