Chapter 11 Intensive Care Unit

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SECTION 1 • Techniques and Methods in Critical Care

diaphragm sign.” An unnaturally sharp heart border is the first indicator of pneumomediastinum, a sign that must be distinguished from the “kinetic halo” seen at the heart or diaphragm border of an edema- tous lung. The mediastinal pleura, outlined by gas on both sides of a thin radiodense line, can often be detected. On a lateral film, pneumomediastinum usually appears as a thin crescent of gas outlining the ascending aorta. Not uncommonly, extrapleural gas extends from the mediastinum, lifting the pari- etal pleura off the diaphragm, or outlining the infe- rior pulmonary ligament. Pneumomediastinum is an important harbinger of pneumothorax, which follows in up to 30% of mechanically ventilated patients. In doubtful cases where progression is feared, defini- tive diagnosis can be established by CT. Subcutaneous Gas In the adult, subcutaneous gas, also known as subcu- taneous emphysema, usually has important diagnos- tic but little physiologic significance. Subcutaneous gas produces lucent streaks or bubbles in the soft tissues that contrast with the normal densities of the chest and neck. However, there is almost no limit to the path the gas may take, as it may track into the retroperitoneum, the peritoneal cavity, and even the scrotum. During mechanical ventilation, bilateral subcutaneous gas usually results from alveolar rup- ture and medial gas dissection, indicating both a via- ble decompression pathway and an increased risk of pneumothorax. Once pneumothorax has occurred, progressive accumulation of gas in the subcutaneous tissue suggests the presence of a bronchopleural fis- tula or a malfunctioning chest tube, especially if the gas is bilateral. Ipsilateral subcutaneous gas detected shortly after chest tube placement generally entered

via the tube track itself. Subcutaneous gas detected immediately after blunt chest trauma should raise the possibility of tracheobronchial or esophageal dis- ruption (see Chapter 36). Pulmonary Edema Without invasivemonitoring, distinguishing between normal permeability (fluid overload and congestive heart failure [CHF]) and high-permeability pulmo- nary edema, ARDS can be difficult. Considerable overlap exists in the radiographic findings of these entities, but certain CXR findings may be helpful in determining the etiology of excess lung water. These forms of edema are often distinguished by three fea- tures: size of the heart and great vessels, distribution of vascular markings, and the pattern of infiltration (Table 11-1). CHF and volume overload are char- acterized by a widened vascular pedicle, an even or inverted pattern of vascular markings, and a ten- dency toward a gravitational distribution of edema (“bat wing” or basilar). Pleural effusions, particularly those of substantial size, are also more common with CHF than ARDS. The vascular pedicle is measured at the point the superior vena cava crosses the right main bronchus to a perpendicular dropped from the point of takeoff of the left subclavian artery from the aorta. Kerley B lines, because of perilymphatic interstitial fluid, are common in established CHF (usually of several days’ to weeks’ duration), whereas crisp air bronchograms are unusual. Conversely, the less mobile infiltrates of ARDS are widely scat- tered, patchy, and often interrupted by distinct air bronchograms. These criteria are better for correctly classifying CHF and volume overload edema and less accurate for identifying ARDS. Although useful

Table 11-1.  Radiographic Features of Pulmonary Edema Characteristics Cardiogenic or Volume Overload Edema

High-Permeability Edema

Heart size

Enlarged

Normal

Vascular pedicle Flow distribution

Normal/enlarged Balanced/cephalad Normal/increased

Normal/small Basal/balanced

Blood volume Septal lines

Normal Absent

Common

Peribronchial cuffing Air bronchograms Edema distribution

Very common Uncommon

Uncommon Very common

Even/central/gravitational Very common/moderate–large

Patchy/peripheral/nongravitational

Pleural effusion

Infrequent/small