Radiographic Diagnosis of Pleural Effusion and Pulmonary Edema in Dogs and Cats

Pleural effusion and pulmonary edema both cause increased soft tissue opacity of the thoracic cavity; however, the disease processes are within different compartments or spaces. An interpretation paradigm that can be used to review the thoracic structures includes extrathoracic structures, pleural space, pulmonary parenchyma, and the mediastinum (including the cardiac silhouette). Abnormally increased soft tissue opacity can border efface with the normal soft tissue opaque structures of the thorax from any of these compartments; however, the focus of this article is recognizing pleural disease (effusion specifically) and pulmonary edema. Some disease processes will cause an increased soft tissue opacity within both the pleural space and the pulmonary parenchyma (e.g., dilated cardiomyopathy, left- and right-sided heart failure). Depending on the assessment and the differential diagnoses (prioritized according to signalment, presenting signs, and clinical physical findings), the specific differentials for pleural effusion and pulmonary parenchymal changes could be the same or totally different. The radiographic evaluation will lead to appropriate next steps for further evaluation, which could include sampling of fluid or pulmonary parenchyma or further evaluation of the thorax by using additional imaging modalities (e.g., ultrasonography, computed tomography).

Normal Anatomy

In the healthy dog, a scant volume of pleural fluid separates the visceral and parietal pleural margins, but this volume is so small that there is no separation between these margins that can be visualized on thoracic radiographs. The interlobar fissures are found in expected anatomic locations that can be associated with the intercostal spaces as found on lateral and ventrodorsal/dorsoventral (VD/DV) radiographs (FIGURE 1).

Figure 1. Ventrodorsal radiograph of a normal dog; white lines indicate areas where a pleural fissure line would occur when an effusion is present.

The pleural space exists between each lung lobe at the interlobar fissure as well as around the lung lobes themselves. The pleural space reflects back on itself at the mediastinum.^1,2 These fissures are not seen on normal thoracic radiographs unless directly tangential to the x-ray beam. The most common pleural fissure noted is the one between the right middle and right caudal lung lobes on a left lateral radiograph (FIGURE 2).

Figure 2. Left lateral radiograph with a pleural fissure line indicated (white arrow). This fissure line is a common finding on left lateral radiographs in dogs. Note that it is a relatively consistent thickness throughout and the fissure does not widen toward the periphery. The cardiac silhouette and diaphragm can be easily visualized, indicating that a pleural effusion is not present.

Occasionally, fat deposits within a fissure will separate the 2 lung lobes, most commonly between the right cranial and right middle lung lobes, as seen on a VD/DV radiograph (FIGURE 3). The pleural fissures are in classic anatomic positions, and one should be familiar with these positions as well as the basic anatomic shape of the lung lobes themselves.

Figure 3. Fat is noted on this ventrodorsal radiograph of a small-breed dog between the right cranial and right middle lung lobes (white arrows). Note that the fat within the fissure is thickest centrally and thinnest peripherally. This appearance is the opposite of that of pleural effusion, in which the widest portion of the pleural fluid is peripheral and thins as it extends medially in the pleural fissure line.

Each lung lobe has a distinct anatomic location. On each lateral radiograph, a dorsal fissure is located around T6 between the cranial and caudal lung lobes. Attempt to identify each lobar bronchus on each radiographic view. On lateral views, the ventrally oriented lobe bronchi may be difficult to visualize (especially the right middle bronchus on a right lateral radiograph). The right cranial lung lobe bronchus exits the carina dorsal to the heart base and turns 90° in a ventral and cranial direction (FIGURE 4). The left cranial lung lobe bronchus exits the ventral trachea at the carina (FIGURE 4). Within approximately a centimeter from its origin, the left cranial lobar bronchus divides into the cranial and caudal segmental bronchi. Just caudal and ventral to the carina, the right middle lung lobar bronchus originates from the ventral and lateral aspect of the right caudal lobe bronchus. The accessory lung lobar bronchus originates from the right caudal lobe bronchus, 2 to 3 cm caudal to the carina in a ventromedial position.

Figure 4. Right lateral radiograph of a normal dog in which the origin of the right cranial lobe bronchus is noted as a radiolucent circle in the cranial aspect of the carina (white arrow). The opening of the left cranial lobe bronchus points straight down (asterisk) from the floor of the carina and then bifurcates into the cranial and caudal subsegments.

Technical Radiographic Considerations

In veterinary medicine, the standard of care has become a minimum of 3 radiographic views of the thorax. There is a “blind spot” ventrally and centrally along the cardiac silhouette on the VD/DV images where pulmonary lesions may not be seen. Recumbent lesions (lesions in the down lung when the patient is in lateral recumbency) will border efface with the surrounding pulmonary parenchyma from atelectasis. These lesions can be up to 5 cm and still not be seen on the image. For emergency cases, a dorsoventral thoracic radiograph can be performed initially but should be followed up with a complete set of thoracic radiographs (right lateral, left lateral, and VD/DV) after the patient has been stabilized.

The thoracic radiographs should be taken during peak inspiration and should be centered over the cardiac silhouette so that the thoracic inlet and the caudodorsal portion of the caudal lung lobes appear on the same radiograph. Doing so may not be possible with a 14” × 17” or a 17” × 17” DR/CR (digital radiography/computed radiography) plate on a large breed dog (e.g., Great Dane), for which 2 images would be required for each view (cranial thorax and caudal thorax), ensuring that there is overlap in the middle. The technique typically involves high kVP and low mA; the highest mA and fastest time setting(s) used to obtain mAs are in the 1 to 5 range. For a small dog or cat, the kVP may still be in the 60 to 70 range.

Straight positioning is essential for accurate interpretation. The VD/DV images should be straight. On lateral views, the sternum should align with the thoracic vertebrae, and each left and right rib head should be superimposed over the other. Sometimes for lateral radiographic views, a triangular sponge positioning device is required to lift the sternum away from the table to get the sternum and spine at a similar height before taking the radiograph. Peak inspiration means that the cupola of the diaphragm is drawn caudal to the cardiac silhouette and the caudodorsal lung margins reach to the level of T12 to T13 in dogs and T13 to L1 in cats. An expiratory radiograph will negatively affect interpretation in 2 primary ways. First, the cardiac silhouette will always look big relative to the overall thoracic volume; and second, the lungs will look whiter (more opaque) than is typical because the decreased pulmonary volume results in less gas between the pulmonary vascular structures. For a small dog being evaluated for a cough, if only a right lateral radiograph is taken on expiration, it is likely that the cardiac silhouette will appear enlarged and opacity of the pulmonary parenchyma increased, which could be mistaken for pulmonary edema. If you were to repeat the radiograph on inspiration and take the dorsoventral or ventrodorsal views, you may realize that the thorax could be normal.

Pleural Effusion

If you believe that the pleural space is abnormal, ask the following questions: Is there pleural fluid or pleural air? Are there any extrapleural signs or masses? Are the ribs and osseous structures of the thorax normal? Are there any diaphragmatic abnormalities such that a diaphragmatic rupture should be considered? If the answer to any of these questions is yes, then details regarding the abnormality should be described. See BOX 1 for radiographic interpretations of pulmonary lesions. For example, a large, expansile mass associated with the right 7th rib causes focal medial retraction of the pleural space and lung margins with a moderate volume of focal pleural effusion. An extrapleural sign is an area of increased soft tissue opacity that appears to be within the lung field and is being caused by indentation from an extrapleural structure. The structure could be normal (e.g., costochondral junction of a chondrodystrophic dog, such as a basset hound) or a primary abnormality (e.g., rib fracture, rib tumor).

Common causes of pleural effusion (transudate, modified transudate, or exudate) are listed in BOX 2.^2,3

The radiographic diagnosis of pleural effusion (FIGURES 5–7) is based on the radiographic abnormalities listed in BOX 3.

Figure 5. A right lateral radiograph of a dog with a mild pleural effusion that is collecting ventrally. There is mild dorsal elevation of the pulmonary lobes ventrally with widening of the ventral pleural fissures in the caudoventral thorax over the region of the apex of the cardiac silhouette.

Figure 6. A right lateral radiograph of a dog with a chylothorax creating a moderate pleural effusion. There are similar features as in FIGURE 5 as well as border effacement of the cardiac silhouette and rounding of the caudodorsal lung lobes. The metallic staples are from a procedure for occlusion of the thoracic ducts.

Figure 7. Right lateral radiograph of a dog with a severe pleural effusion secondary to hypoproteinemia. There is border effacement of the cardiac silhouette and diaphragm along with the other features of pleural effusion described in the article text and shown on FIGURES 5 and 6. Three central catheters are noted: central venous line (bottom left), nasogastric feeding tube (middle), and pleural chest tube (top right).

In patients with effusion, soft tissue opacity or fluid accumulation will be present within the interlobar fissures, which will be widest peripherally and thinner centrally. The costophrenic and lumbar diaphragmatic angles will be rounded or blunted as a result of fluid accumulation in these regions. There will be partial or complete border effacement of the cardiac silhouette and diaphragm (FIGURE 6). In patients with severe pleural effusion, the trachea may be dorsally elevated (FIGURE 7).

Unilateral pleural effusions are usually secondary to exudates because the mediastinum in dogs and cats is considered fenestrated. A transudate or a modified transudate should pass freely between the right and left pleural space through the mediastinal fenestrations. With an exudative effusion, these fenestrations will become plugged, resulting in a unilateral pleural effusion (e.g., chylothorax, pyothorax, hemothorax, neoplastic effusion). In a reactive exudative effusion, such as one secondary to chylothorax, the visceral pleural surface will be thickened and the lung lobe margins rounded. This restrictive pleuritis results in the inability of the lung lobes to reinflate to their normal shape or volume (FIGURE 8).

Figure 8A. Right lateral (A) and ventrodorsal (B) radiographs of a cat with severe chronic chylothorax and pneumothorax

Figure 8B. Right lateral (A) and ventrodorsal (B) radiographs of a cat with severe chronic chylothorax and pneumothorax. There is marked thickening of the visceral pleura and rounding of the lung lobe borders. There are multiple chronic rib fractures with callus formation consistent with a “thoracic bellows effect” from the chronic chylothorax. Subcutaneous emphysema is noted on the left side of the thorax consistent with recent thoracentesis. There is also dorsal bowing of the caudal sternum (pectus excavatum).

The differential diagnoses for chronic bilateral pleural effusion that should be ruled out include chronic diaphragmatic hernia, thoracic malignancy (commonly a rib tumor), and lung lobe torsion (may cause the effusion or be secondary to a chronic effusion).

Pulmonary Edema

Pulmonary edema is the abnormal accumulation of fluid within the interstitial space of the lungs, based on abnormalities of Starling’s forces across the capillary bed and interstitium.⁴ Common causes of pulmonary edema are listed in BOX 4. Net flow of fluid into the pulmonary interstitial space can result from alterations in plasma albumin (plasma oncotic pressure), elevated venous hydrostatic pressure (elevated left atrial pressures), increased capillary permeability (membrane breakdown), and increased osmotic (oncotic) pressures within the interstitial space.

Radiographic signs of pulmonary edema can be found in BOX 5.

In patients with cardiogenic pulmonary edema, elevated venous hydrostatic pressure will lead to retention of interstitial fluid (unstructured interstitial pulmonary pattern). In dogs, cardiogenic pulmonary edema is secondary to left-sided cardiac failure; therefore, radiographs will usually show left-sided cardiomegaly, elevation of the carina and caudal thoracic trachea, and a caudodorsal distribution of the increased pulmonary opacity (most commonly unstructured interstitial) that starts from a central or hilar location and spreads peripherally (FIGURE 9).

Figure 9A. Right lateral (A) and ventrodorsal (B) radiographs of a Doberman pinscher with congestive heart failure secondary to dilated cardiomyopathy.

Figure 9B. Right lateral (A) and ventrodorsal (B) radiographs of a Doberman pinscher with congestive heart failure secondary to dilated cardiomyopathy. There is a pleural effusion (mild in severity) noted secondary to right-sided heart failure as evidenced by multiple pleural fissure lines and mild retraction of the lung lobes away from the thoracic walls (arrows). There is cardiomegaly and pulmonary venous enlargement. There is an unstructured interstitial pulmonary pattern noted, consistent with cardiogenic pulmonary edema.

This caudodorsal distribution of the pulmonary changes is seen in dogs with mitral valve disease/insufficiency but not necessarily in dogs with left-sided cardiac failure secondary to cardiomyopathy, which could be more randomly distributed. In some dogs with mitral valve degenerative disease, the pulmonary edema will be distributed in the right caudal lobe due to the relative relationship between the right caudal lobe pulmonary vein and the direction of the regurgitant jet from the left ventricle into the left atrium (FIGURE 10). An unstructured interstitial pulmonary pattern can progress to an alveolar pulmonary pattern (part of a continuum of the same disease process, exhibiting different patterns depending on the severity of the disease). In dogs with dilated cardiomyopathy, congestive right- and left-sided heart failure can result in pleural effusion and pulmonary edema, respectively.

Figure 10A. (A) Right lateral and (B) ventrodorsal (VD) radiographs of a dog with chronic mitral valve degenerative disease.

Figure 10B. (A) Right lateral and (B) ventrodorsal (VD) radiographs of a dog with chronic mitral valve degenerative disease. There is moderate to severe left-sided cardiomegaly with left atrial enlargement and dorsal elevation of the trachea and carina. There is an increased unstructured interstitial pulmonary pattern within the right caudal lung lobe, best visualized on the VD radiograph, consistent with cardiogenic pulmonary edema (arrow).

Cardiogenic pulmonary edema in cats will be distributed more randomly than just caudodorsally (FIGURE 11); it can be distributed ventrally and multifocally. Cats with primary or secondary myocardial disease can exhibit left-sided heart failure (causing pulmonary edema and pleural effusion), right-sided heart failure (causing pleural and peritoneal effusion), or both.

Figure 11A. Right lateral (A) and ventrodorsal (B) radiographs of a cat with cardiogenic pulmonary edema.

Figure 11B. Right lateral (A) and ventrodorsal (B) radiographs of a cat with cardiogenic pulmonary edema. The cardiac silhouette is severely enlarged and there is a randomly distributed mixed pulmonary pattern consisting of an unstructured interstitial to alveolar pulmonary patterns. The pulmonary vessels are enlarged centrally but not in the peripheral aspect of the lung field.

Summary

Pulmonary edema should not be confused with pleural effusion. Although congestive right- and left-sided heart failure can result in both, usually the disease processes that will cause pulmonary edema versus pleural effusion differ. Recognizing pleural effusion is based on seeing radiographic pleural fissure lines with retraction of the visceral pleural surface away from the parietal surface. Recognizing pulmonary edema as a pulmonary pattern is also critical for identifying parenchymal lesions versus pleural space abnormalities. Radiography is an essential part of classifying thoracic disease processes. Diagnosis can be facilitated by dividing the thorax into the 4 radiographic compartments (extrathoracic structures, pleural space, pulmonary parenchyma, and the mediastinum) and combining those findings with signalment, presenting complaint, and physical examination results.