Recognizing Atelectasis

Obstructive Atelectasis

• Obstructive atelectasis is associated with the resorption of air from the alveoli through the pulmonary capillary bed distal to an obstructing lesion of the bronchial tree. The most common causes are shown in Table 6.2 .

  • The affected lobe or lung collapses and becomes more opaque because it contains no air. The collapse leads to volume loss in the affected lobe or lung. Because the visceral and parietal pleurae invariably remain in contact with each other as the lung loses volume, this volume loss creates a shift of the slightly mobile structures of the thorax toward the area of atelectasis.

  • The rate at which air is absorbed and the lung collapses depends on its gas content when the bronchus is occluded and how rapidly the occlusion occurs. It takes about 18 to 24 hours for an entire lung to collapse with the patient breathing room air but less than an hour with the patient breathing near 100% oxygen.

  TABLE 6.2

  Most Common Causes of Obstructive Atelectasis

  Cause	Remarks
  Tumors	Includes bronchogenic carcinoma (especially squamous cell), endobronchial metastases, carcinoid tumors
  Mucous plug	Especially in bedridden individuals; postoperative patients; those with asthma, cystic fibrosis
  Foreign body aspiration	Especially peanuts, small toys; following a traumatic intubation

• Most of the signs of obstructive atelectasis are listed in Box 6.1 .

  BOX 6.1

  Signs of Obstructive Atelectasis

  • Displacement of the major or minor fissure ∗

  • Increased density of the atelectatic portion of lung

  • Shift of the mobile structures in the thorax (i.e., the heart, trachea, and/or hemidiaphragms)

  • ∗

    ∗ Toward the atelectasis

    Compensatory overinflation of the unaffected segments, lobes, or lung

• Many of the signs of obstructive atelectasis are shown in a photographically manipulated composite image ( Fig. 6.1 ). They include:

  • Displacement (shift) of the interlobar fissures (major and minor) toward the area of atelectasis.

  • Increase in the density of the affected lung or lobe.

  • Displacement (shift) of the mobile structures of the thorax. The mobile structures are those capable of slight movement due to changes in lung volume and include:

    • The trachea , which is normally midline in location, may shift toward the side of volume loss. There is normally a slight rightward deviation of a portion of the trachea at the site of the left-sided aortic knob .

    • The right heart border normally projects at least 1 cm to the right of the spine on a nonrotated, frontal radiograph but with atelectasis, especially of either of the lower lobes, the heart may shift toward the side of the atelectasis. When the heart shifts toward the right, the left heart border may approach the spine. When the heart shifts toward the left , the right heart border may overlie the spine.

    • The hemidiaphragms may move in response to volume loss. The hemidiaphragm will be silhouetted by atelectasis of either lower lobe, but the position of the left hemidiaphragm may be inferred by noting elevation of the stomach bubble with an atelectatic left lung.

  

Important Points

• Normally, the right hemidiaphragm is almost always higher than the left by about half the distance of the interspace between two adjacent ribs. In about 10% of normal people, the left hemidiaphragm is higher than the right.

• • Overinflation of the unaffected ipsilateral lobes or the contralateral lung. The greater the volume loss and the longer its presence, the more likely the lung on the side opposite the atelectasis or the unaffected lobe(s) in the ipsilateral lung will overinflate to compensate for the volume loss. This may be especially noticeable on the lateral projection by an increase in the size of the retrosternal clear space and on the frontal projection by extension of the overinflated contralateral lung across the midline (see Fig. 6.12B ).