Recognizing the Causes of an Opacified Hemithorax

Atelectasis of the Entire Lung

• Atelectasis of an entire lung usually results from complete obstruction of the right or left main bronchus. With bronchial obstruction, no air can enter the lung. The remaining air in the lung is absorbed into the bloodstream through the pulmonary capillary system, leading to loss of volume of the affected lung.

• In adults, such atelectasis might be caused by an obstructing neoplasm , like a bronchogenic carcinoma. In younger individuals, asthma may produce mucous plugs that obstruct the bronchi. A foreign body may be aspirated (in children, peanuts are a frequent culprit) that also leads to bronchial obstruction. Critically ill patients also develop atelectasis from mucous plugs.

• In obstructive atelectasis (also called resorptive atelectasis ), even though there is volume loss within the affected lung, the visceral and parietal pleura almost never separate from each other. That is an important fact about atelectasis and is sometimes confusing to beginners who try to picture atelectasis and a pneumothorax as both producing collapse of a lung without understanding why they look completely different radiographically ( Fig. 5.1 , Table 5.1 ).

  • Because the visceral and parietal pleura do not separate from each other in atelectasis, mobile structures in the thorax are “pulled toward” the side of the atelectasis producing a shift (movement) of these mobile thoracic structures toward the side of opacification.

  • The most visible mobile structures in the thorax are the heart , the trachea , and the hemidiaphragms. In obstructive atelectasis , one or all of these structures will shift toward the side of opacification (toward side of volume loss) ( Fig. 5.2 ).

    

  • Table 5.2 summarizes the movement of the mobile structures in the thorax in patients with obstructive atelectasis.

    TABLE 5.2

    Recognizing a “Shift” in Atelectasis/Pneumonectomy

    Structure	Normal Position	Right-Sided Atelectasis or Pneumonectomy	Left-Sided Atelectasis or Pneumonectomy
    Heart	Midline	Heart moves rightward; left heart border may come to lie near left side of spine	Heart moves leftward; right heart border overlaps the spine
    Trachea	Midline	Shifts toward right	Shifts toward left
    Hemidiaphragm	Right slightly higher than left	Right hemidiaphragm moves upward and may disappear (silhouette sign)	Left hemidiaphragm moves upward and may disappear (silhouette sign); shadow of stomach bubble may be elevated

  
  TABLE 5.1

  Pneumothorax Versus Obstructive Atelectasis

  Feature	Pneumothorax	Obstructive Atelectasis
  Pleural space	Air in the pleural space separates the visceral from the parietal pleura.	The visceral and parietal pleura do not separate from each other.
  Density	The pneumothorax itself will appear “black” (air density). The hemithorax may appear more lucent than normal.	Atelectasis is the absence of air in the lung. The hemithorax will appear more opaque (“whiter”) than normal.
  Shift	There is never a shift of the heart or trachea toward the side of a pneumothorax.	There is almost always a shift of the heart and trachea toward the side of the atelectasis.