Chest Radiography in the Intensive Care Unit

Radiography

The left lower lobe is the most common site of atelectasis (66%), followed by the right lower lobe (22%) and right upper lobe (11%). When atelectasis is linear or plate-like (i.e., subsegmental atelectasis), it is often located in the gravity-dependent lower lobes ( Figs. 69.1 and 69.2 ). Although atelectasis is most commonly subsegmental in ICU patients, it may involve complete pulmonary segments or lobes ( Fig. 69.3 ). Direct signs of atelectasis, such as displacement of interlobar fissures or crowding of vessels/bronchi may prove useful in determining the location of atelectasis. On occasion, atelectasis can be ill-defined and appear similar to a layering pleural effusion. In recumbent patients radiographic evidence of volume loss may not be as evident.

In lobar atelectasis the presence or absence of air bronchograms is useful to determine the potential effectiveness of airway clearance techniques. If air bronchograms are present, the atelectasis is most likely related to small airways collapse, and an inspiratory flow device, such as an incentive spirometer, may be useful in opening the collapsed alveoli. If air bronchograms are absent in lobar atelectasis, however, mucoid impaction is the most likely etiology, and airway clearance techniques, such as chest physiotherapy or acapella devices, have a high likelihood of providing clinical benefit. Noninvasive airway clearance techniques are just as effective in clearance of airway secretions as fiberoptic bronchoscopy; therefore bronchoscopy is generally reserved only for intractable cases.

Computed Tomography

Computed tomography (CT) findings are similar to those seen on radiographs. Atelectasis is characterized on CT by increased density with volume loss and crowding of bronchi and vasculature, often within a defined anatomic distribution (e.g., subsegmental, segmental, or lobar) (see Fig. 69.2B ). An obstructing mucus plug may be identified proximal to the collapsed lung (see Fig. 69.3C ). Intravenous contrast can aid in distinguishing atelectasis from consolidation: Atelectasis should enhance greater than skeletal muscle, whereas consolidation often enhances either similar to or less than skeletal muscle ( Fig. 69.4 ). It is important to note that vessels enhance in both conditions and should not be confused for parenchymal enhancement.

Radiography

Aspiration may manifest as a focal or multifocal consolidative pattern ( Fig. 69.5 ). Poorly defined nodules in a small airway distribution also are commonly identified ( Fig. 69.6 ), resulting in a tree-in-bud appearance often seen on CT. Airway thickening, bronchiolar impaction, and volume loss also are frequently identified. Although the radiographic appearance of aspirated material varies, the chest radiograph may be normal in 10% of cases.

Aspiration pneumonitis may progress over the first day but usually begins to clear within a few days ( Fig. 69.7 ). Progression of consolidation with persistent/worsening clinical symptoms suggests the development of pneumonia. In a recumbent patient, aspiration is typically located in the posterior segments of the upper lobes, the superior segments of the lower lobes, and the posterior basilar segments of the lower lobes. This distribution results in a central predominance on a supine AP radiograph.

Computed Tomography

CT findings are similar to the radiographic findings of consolidative or tree-in-bud opacities in a dependent distribution (see Figs. 69.5B and 69.6B ). CT is more sensitive, however, in depicting the tree-in-bud pattern, the extent of aspiration, and the exact distribution. In addition, CT can show small airway filling and obstruction (see Fig. 69.5B ). It also better identifies complications such as lung abscess.

Radiography

The chest radiograph typically shows patchy areas of consolidation or poorly defined opacities that are often multifocal ( Fig. 69.8 ). Air bronchograms are helpful for the diagnosis of pneumonia but also occur with atelectasis. In contrast to atelectatic air bronchograms, pneumonia-related air bronchograms are not characterized by volume loss and “crowding” of the bronchi. Cavitation can occur and is a more specific finding of pneumonia when present. Radiographic changes in pneumonia typically develop more slowly than in atelectasis, aspiration, or pulmonary edema. Although pneumonia is commonly present in patients with ARDS, the chest radiograph has been found to be only 57% accurate in differentiating pneumonia without ARDS from non–pneumonic-induced ARDS.

Computed Tomography

Radiographs alone are generally adequate to evaluate pneumonia, but CT can be an asset in evaluating potential complications of pneumonia, such as developing cavitation or in determining the extent and character of parapneumonic effusions. In the setting of new ill-defined opacities on a background of preexisting consolidation, CT better differentiates the potential etiology of multifocal opacities and whether they stem from one or more than one process. Contrast-enhanced studies of pneumonia often show only mild or no enhancement of the involved parenchyma; however, hypoenhancement of parenchyma should raise concern for necrotizing pneumonia (see Fig. 69.4 ).