The four most common processes causing acute ALD are listed in Table 4-1. Pulmonary edema is the most common cause of ALD on chest radiographs. As mentioned in the previous chapter, on ILD, edema can be (a) hydrostatic (from cardiac failure, renal failure, or overhydration); (b) nonhydrostatic, owing to increased capillary permeability (in acute respiratory distress syndrome [ARDS] and fat embolization syndrome); or (c) inflammatory in etiology (as from chemical pneumonitis or eosinophilic pneumonitis). Fat embolization syndrome occurs most commonly after traumatic fracture of long bones, which results in liberated marrow fat entering the pulmonary arterial circulation. Hydrolysis of fat, forming free fatty acids, leads to endothelial damage and increased capillary permeability 12 to 48 hours after trauma. This entity is further discussed in Chapter 8.

The radiographic distinction of pulmonary edema as cardiogenic or noncardiogenic in etiology is not always clear cut (1). Radiographic signs of cardiogenic pulmonary edema include enlargement of the cardiac silhouette (which may be assumed as secondary to cardiomegaly in many cases but is not always distinguishable from pericardial effusion), pleural effusions, pulmonary vascular congestion and redistribution, and interstitial and alveolar opacities. Edema fluid spills into the interstitial spaces and progresses to filling of the airspaces. Often, the chest radiograph shows evidence of interstitial and airspace filling, although occasionally a predominantly interstitial pattern may be seen. Interstitial edema can result in blurring of the margins of blood vessels and hazy thickening of bronchial walls (peribronchial cuffing), thickening of fissures (subpleural edema), and edematous thickening of the interlobular septa (Kerley A and B lines). Subpleural pulmonary edema refers to fluid that accumulates in the loose connective tissue beneath the visceral pleura and is seen radiographically as a thickened fissure; this is sometimes difficult to distinguish from pleural effusion. Chest radiographs are highly sensitive for the diagnosis of pulmonary edema and can show edema in patients who have not yet developed symptoms; conversely, pulmonary edema may be visible radiographically for hours or even days after the hemodynamic factors have returned to normal (2).

The distribution of airspace opacities in alveolar edema is usually patchy, bilateral, and widespread, and the opacities tend to coalesce. Air bronchograms may be evident, particularly when the edema is confluent. Often, alveolar accumulation of fluid in pulmonary edema is most pronounced centrally near the hila, resulting in a "bat's wing" or "butterfly" configuration. A clue to the diagnosis of pulmonary edema, instead
of pneumonia, for example, is rapid change on radiographs taken over short time intervals (several hours); rapid clearing is particularly suggestive of the diagnosis. Edema fluid can also change distribution or shift from one lung to the other as a result of the effect of gravity, as when a patient has been lying on one side.

ARDS is the result of increased pulmonary vascular permeability and develops in response to lung injury. The more common of the many lung insults leading to ARDS include sepsis, pneumonia, aspiration of gastric contents, circulatory shock, trauma, burns, and drug overdose. Often there are multiple overlapping inciting events. The clinical syndrome of ARDS is characterized by acute, severe, progressive respiratory distress, usually requiring mechanical ventilation; widespread pulmonary opacity on chest radiographs; hypoxia despite high inspired oxygen concentration; and decreased compliance of the lungs ("stiff lungs"). Damage to the alveolar capillary membrane leads to increased capillary permeability and leakage of proteinaceous fluid into the alveoli. Eventually, alveolar disruption and hemorrhage occur, surfactant is reduced, and the alveoli tend to collapse. The stages of ARDS are outlined in Table 4-2. The radiographic features may be delayed by up to 12 hours or more following the onset of clinical symptoms - an important difference from cardiogenic pulmonary edema, in which the chest radiograph is frequently abnormal before or coincident with the onset of symptoms. Findings on chest radiography include bilateral, widespread, patchy, ill-defined opacities resembling cardiogenic pulmonary edema, but without cardiomegaly, vascular redistribution, or pleural effusion (Fig. 4-1). Although the lungs appear diffusely involved on chest radiographs, computed tomographic (CT) scanning often shows a more patchy distribution with preservation of normal lung regions (3). If an endotracheal tube is not present on the chest radiograph, the diagnosis of ARDS is unlikely, except in the later stages of healing.

Patients with ARDS typically require mechanical ventilation, sometimes with high positive end expiratory pressure because of stiff, noncompliant lungs. This predisposes to barotrauma, with rupture of alveolar walls and subsequent dissection of air into the perivascular bundle sheaths and interlobular septa, resulting in pulmonary interstitial emphysema. Discrete air-filled cysts, or "pneumatoceles," may form in both central and subpleural locations (Fig. 4-2). These air collections can dissect into the mediastinum, causing pneumomediastinum, and can rupture into the pleural space, causing pneumothorax. The lung may be so stiff that it does not collapse easily, even when a pneumothorax is present. Air may dissect from the mediastinum into the neck and chest wall, retroperitoneum, or
peritoneal cavity. The long-term outlook for survivors of ARDS is poorly documented. Mortality is related mainly to multiple organ failure rather than pulmonary dysfunction (4). One study of 109 survivors of ARDS showed that survivors had persistent functional disability 1 year after discharge from intensive care. Most had extrapulmonary conditions, with muscle wasting and weakness being the most prominent (5). Chest radiographs may return to normal or show varying degrees of interstitial lung disease, including pulmonary fibrosis.

Bleeding into the lung parenchyma occurs as the result of a variety of disorders (Table 4-3). A triad of features suggesting pulmonary hemorrhage is hemoptysis, anemia, and airspace opacities on chest radiography. Bleeding into the lung, however, does not always lead to hemoptysis (6). When bleeding into the lung is widespread, the pattern is referred to as diffuse pulmonary hemorrhage (DPH). The pulmonary features of all DPH syndromes are the same, and chest radiographs are generally not helpful in distinguishing among them. Lung opacities range from patchy airspace opacities to widespread confluent opacities with air bronchograms. The lung opacities show a perihilar or middle to lower lung predominance, and they tend to be more pronounced centrally, with sparing of the costophrenic angles and apices. In general, in cases of acute pulmonary hemorrhage (if there are no complicating factors), rapid clearing in 2 to 3 days can be expected. This can aid in narrowing the differential diagnosis when chest radiography shows diffuse ALD (7). When the airspace disease clears, interstitial opacities are often seen on chest radiography, as the result of by-products of blood breakdown being taken up by the septal lymphatics.

Goodpasture syndrome, one of the pulmonary–renal syndromes and the most common cause of DPH, is an anti–basement membrane antibody disease manifesting as DPH and glomerulonephritis. It is a disease of young white men and is only occasionally reported in children (8). The presence of antiglomerular basement membrane antibodies in the serum is a sensitive and specific indicator of the disease. Renal biopsy shows evidence of subacute proliferative glomerulonephritis with linear IgG deposition in the glomeruli. The chest radiograph usually shows bilateral, relatively central, and symmetric ALD, but this is a nonspecific pattern (Fig. 4-3).

Many collagen vascular disorders and systemic vasculitides are associated with DPH, with or without renal disease. The association is most commonly seen with systemic lupus erythematosus (Fig. 4-4) and systemic necrotizing vasculitides of the polyarteritis nodosa type (9).

Wegener granulomatosis (WG) is characterized pathologically by necrotizing granulomatous vasculitis of the upper and lower respiratory tracts, a disseminated small-vessel vasculitis involving both arteries and veins, and a focal, necrotizing glomerulonephritis (10). Mean age at presentation is 50, and there is a slight male predominance. Upper airway involvement with sinusitis, rhinitis, and otitis is the most common clinical presentation. More than 90% of patients with active multiorgan WG have a positive test for cytoplasmic antineutrophil cytoplasmic antibodies (11). There are two characteristic pulmonary radiologic findings: (i) nodules, multiple
or single, ranging from 3 mm to 10 cm in diameter, which may cavitate; and (ii) diffuse areas of lung opacity, representing pulmonary hemorrhage (Fig. 4-5). Occasionally, ill-defined nodular opacities may be present, sometimes appearing as areas of pleural-based, wedge-shaped consolidation, resembling pulmonary infarcts.

DPH can occur as a result of various coagulopathies, including thrombocytopenia (such as in leukemia or after bone marrow transplantation), anticoagulation, coronary thrombolysis, and diffuse intravascular coagulation. Infectious hemorrhagic necrotizing pneumonias or hemorrhagic neoplasms can result in diffuse, focal, or multifocal patchy areas of pulmonary hemorrhage. Pulmonary hemorrhage related to chest trauma is discussed in Chapter 8.

Alveolar proteinosis typically presents in a patient who feels relatively well, in striking contrast to the markedly abnormal radiograph, which shows bilateral diffuse or multifocal patchy opacities. The opacities represent a phospholipoproteinaceous material that fills the alveolar spaces and clears after bronchioalveolar lavage (12). Recurrence of disease can result in a chronic pattern of ALD, with serial chest radiographs showing varying patterns of recurrent ALD with interval clearing (Fig. 4-6). Alveolar proteinosis is associated with an increased incidence of lymphoma and infection with Nocardia (13,14).

Infectious pneumonia is the most common cause of focal ALD, and bacteria are the most common inciting agents. Fungal, mycobacterial, parasitic, and even viral pneumonias can all produce focal or diffuse airspace opacities on chest radiography (Figs. 4-7, 4-8, 4-9). Opacity of more than half a lobe with no loss of volume is virtually diagnostic of pneumonia, and common causes are Streptococcus pneumoniae or Mycoplasma pneumoniae (Figs. 4-10 and 4-11). Lobar consolidation with expansion of the lobe, although uncommon, strongly suggests bacterial pneumonia (particularly S. pneumoniae, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus pneumonias). A round consolidative process is likely to be caused by pneumonia (Fig. 4-12). Organisms most likely to cause round pneumonia are S. pneumoniae, S. aureus, K. pneumoniae, P. aeruginosa, Legionella pneumophila or L. micdadei, Mycobacterium tuberculosis, and several fungi. The development of air–fluid levels within an area of consolidation that is known or presumed to be pneumonia strongly suggests necrotizing pneumonia with abscess formation, and likely pathogens include S. aureus, Klebsiella sp, Proteus sp, and Pseudomonas

sp, as well as mixed infections (Fig. 4-13). Multifocal pneumonia can be caused by numerous organisms, but the "bat's wing pattern" in the immunocompetent patient should suggest aspiration pneumonia, Gram-negative bacterial pneumonia (Fig. 4-14), and nonbacterial pneumonias such as mycoplasma, viral, and rickettsial pneumonia. Pneumonia in the immunocompromised host often results in the bat's wing pattern from opportunistic organisms such as Pneumocystis jiroveci and various fungi.

The radiologic manifestation of aspirated material into the lungs is dependent on the type and volume of material aspirated, the immune status of the patient, and the presence or absence of pre-existing lung disease. Aspiration of bland substances such as blood or neutralized gastric contents does not incite an inflammatory process, and associated lung opacities clear rapidly with ventilation therapy or coughing. Aspiration of acidic gastric contents and other irritating substances causes inflammation of the lung. Within several hours of aspirating such substances, chest radiographs usually show progressive airspace opacity in the gravitationally dependent regions of the lungs (Fig. 4-15). Radiologic improvement is generally seen within a few days unless the patient develops superimposed infection or ARDS.

Nasogastric or endotracheal intubation, diminished levels of consciousness, and supine positioning predispose patients to aspirate. Acute aspiration may be accompanied by fever, shortness of breath, and hypoxemia, which can make aspiration difficult to distinguish from bacterial pneumonia.