Clifford R. Berry1 and Elodie E. Huguet2 1 Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA 2 Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, USA The pleural space is a potential space in the right and left hemithorax created by the serosal mesothelial lining of the thoracic cavity. The pleural space is located on both the right and left sides of the thorax and is bounded by the parietal pleura, along the inside of the thoracic body wall, diaphragm and mediastinum and the visceral pleura covering the surface of the lungs [1–7]. The right and left visceral pleurae reflect at the pulmonary hilum and join with the parietal pleurae, which forms the mediastinum (the space between the right and left pleural cavities) [1–7]. This chapter describes the normal pleural space and abnormalities of the pleural space. The normal pleural cavities represent a potential space, and in the healthy animal, a small amount of pleural fluid is spread over the pleural surface to facilitate the sliding motion between the visceral and parietal pleura as the lungs expand and contract during normal respiration. The visceral pleura lines the lung lobes, including the surface of the lung lobes that form the interlobar fissures (the space between different lung lobes on each side of the thorax), so that the visceral pleurae of adjacent lung lobes are in contact with each other. These interlobar fissures have a specific anatomic location for the divisions between the right cranial and right middle, right middle and right caudal, cranial and caudal subsegments of the left cranial and between the left cranial and left caudal lung lobes (Box 15.1; Figure 15.1). In addition, there is a special fold of pleura in the accessory lung lobe called the plica vena cava that incorporates the caudal vena cava from the level of the diaphragm to its insertion in the caudal aspect of the right atrium along the caudodorsal margin of the cardiac silhouette. The normal pleural space is not visualized on thoracic radiographs in the dog and cat. FIGURE 15.1 Right lateral (A), left lateral (B,C), ventrodorsal (D–F) and schematic drawing (G) of the normal location of the pleural fissure lines (white lines on the images) that would be visualized when a pleural effusion or pneumothorax is present in the dog or cat. The pleural fissure lines are in a similar location in the dog and cat (see Box 15.1). In (C) there are two fissure lines present (white arrows). The first fissure line is between the right cranial and right middle lung lobes and is not routinely seen. The caudal fissure line is between the right middle and right caudal lung lobe and represents mild effusion, pleural thickening or the pleural fissure being oriented tangential to the x‐ray beam and thereby creating a soft tissue opacity. In (E) and (F), ventrodorsal radiographs from normal dogs document pleural fat localization with the interlobar fissures between the right cranial and right middle and the interlobar fissure between the cranial subsegment and caudal subsegment of the left cranial lung lobe (white arrows). (G) Schematic drawing of the location of each of the ventral interlobar fissures relative to the different lung lobes in a dog or a cat. Some basic questions that could be asked about the pleural space would include the following. Each of these changes has specific Roentgen signs and will be reviewed. Abnormal fluid accumulation within the pleural space is called a pleural effusion. In order to be detected on thoracic radiographs, a certain threshold of fluid needs to be present. The Roentgen signs of pleural effusion are summarized in Box 15.2. In acute pleural effusions, the most common Roentgen signs will include retraction of the lung lobes away from the thoracic wall, a decrease in the overall volume present within each lung lobe, leaf‐like appearance of the lung lobe, presence of soft tissue between the thoracic wall (parietal pleural surface) and the visceral pleural surface along the lung lobes, widened interlobar fissures, dorsal elevation of the trachea, and border effacement of the cardiac silhouette and the cranial surface of the diaphragm, particularly ventrally (Figures 15.2–15.4). On a ventrodorsal image, one is more likely to see the interlobar fissures; however, the cardiac silhouette is not border effaced due to the dorsal recumbent positioning and the fluid will collect in the dorsal aspect of the pleural space. However, on a dorsoventral radiograph where the animal is in sternal recumbency, the fluid will collect in the ventral pleural space and thereby result in border effacement of the cardiac silhouette and cranial diaphragmatic cupula (Figure 15.5). FIGURE 15.2 Mild pleural effusion: (A) right lateral and (B) ventrodorsal images from a dog with hypoproteinemia. The effusion results in border effacement of the ventral cardiac silhouette and cranioventral margin of the diaphragm and separation of the interlobar fissures with soft tissue opacity (red and white arrows). (C) Right lateral radiograph from a cat with a mild to moderate pleural effusion. Similar features are noted as described for the dog. In this cat, the effusion was determined to be a modified transudate. FIGURE 15.3 Moderate pleural effusion: right lateral (A) and ventrodorsal images (B) with multiple pleural fissure lines and retraction of the lung lobes away from the thoracic wall. The cardiac silhouette can be visualized on the VD image and is not enlarged. There is a pneumoperitoneum present consistent with prior surgical intervention. Multiple catheters are in place. FIGURE 15.4 Right lateral (A), left lateral (B), and ventrodorsal radiographs (C) from a cat with a severe pleural effusion secondary to a chylothorax. There is retraction of all lung lobes away from the thoracic walls with multiple pleural fissures lines filled with soft tissue opacity. On the ventrodorsal image, there is collapse of the right cranial and left cranial segments of the left cranial lung lobes with air bronchograms noted centrally. The cardiac silhouette and diaphragm are border effaced by the pleural fluid. On the ventrodorsal image, there is enlargement of the caudal lobar pulmonary vessels due to preferential shunting of blood to the lung lobes that are aerated for oxygenation of the blood. Right lateral (D), left lateral (E), and dorsoventral (F) views from a dog with a cranial mediastinal mass and severe pleural effusion. There are similar changes noted as for the cat related to the pleural space. An unstructured interstitial pulmonary pattern is present throughout the lung lobes consistent with partial atelectasis from the pleural effusion. Additionally, there is an air bronchogram within the right cranial lung lobe on the left lateral projection consistent with complete collapse of the right cranial lobe. On the ventrodorsal image, there is caudal and medial displacement of the left cranial lung lobe. The cranial mediastinal mass was a thymoma with a left‐sided distribution. FIGURE 15.5 Ventrodorsal (left) and dorsoventral (right) from a dog with a mild to moderate pleural effusion. The effusion was secondary to a cranial mediastinal mass within the left cranioventral mediastinal reflection. The fine needle aspirate was consistent with a thymoma. On the VD image, the pleural fluid is in the dorsal thorax (dorsal recumbency) and the cardiac silhouette and cranial mediastinal mass can be visualized. On the DV image, the pleural fluid is in the ventral thorax and border effaces the cardiac silhouette and the cranial mediastinal mass such that they are not visualized. Radiographically, one cannot differentiate between the different causes of pleural effusion (transudate, modified transudate, and exudate). If the effusion is unilateral, then the type of effusion is most likely an exudate, resulting in blockage of normal mediastinal fenestrations between the right and left pleural spaces as noted normally in dogs and cats (Figure 15.6). The most common exudates would include pus, chyle, hemorrhage or a neoplastic effusion. In chronic exudative effusions, particularly pyothorax and chylothorax, the visceral pleural surface can become thickened, resulting in a rounded lung lobe (called a restrictive pleuritis; Box 15.3, Figures 15.7 and 15.8). This thickening of the visceral pleura can also block the normal mediastinal fenestrations and restrict the lung lobe from reexpanding when the effusion is removed, also known as a pleural peel sign. Additionally, there can be chronic rib fractures associated with a chylothorax that is secondary to a thoracic bellows effect (Figure 15.9). FIGURE 15.6 Moderate to severe right‐sided pleural effusion and a mild left‐sided pleural effusion with rounding of the caudal lung lobes consistent with a predominantly unilateral effusion. This cat (right lateral, left lateral, and ventrodorsal images (A–C)) had an exudate consistent with a pyothorax on cytology of the pleural fluid. The right cranial and right middle lung lobes are collapsed as noted on the left lateral and ventrodorsal images. FIGURE 15.7 A cat that has a history of a chronic chylothorax. In this cat (right lateral, left lateral, and ventrodorsal images (A–C)), there is a bilateral severe pneumothorax with rounding of the visceral pleural margins of the lung lobes consistent with restrictive pleuritis and pleural peel. All lung lobe margins are rounded and thickened with dorsal elevation of the cardiac silhouette away from the sternum. There is enlargement of the sternal lymph node. Dorsal elevation of the trachea is also noted. The pneumothorax is presumed to be iatrogenic after the pleural effusion was removed secondary to leakage from one of the lung lobes after trauma related to the thoracocentesis. Progression to a tension pneumothorax is possible but did not occur in this cat. FIGURE 15.8 Right lateral (A), left lateral (B), and ventrodorsal (C) images from a 7‐year‐old DSH with a chronic chylothorax and bilateral pleural effusion. There is severe thickening of the visceral pleural margins of the lung lobes particularly noted caudodorsally on the ventrodorsal image (consistent with a pleural peel). There are multiple cranial lobar broncholiths also seen. FIGURE 15.9 Thoracic bellows effect seen in two different cats ((A,B) and (C,D) – right lateral and ventrodorsal images for each). In cat 1, there is a bilateral pleural effusion and multiple healing rib fractures (white arrows). Multiple air bronchograms are noted in the right and left cranial lung lobes on both projections. The pleural effusion was a chylothorax. In cat 2 (C,D), there is a bilateral pneumothorax with severe rounding of the lung lobes and a pleural peel consistent with a restrictive pleuritis. In addition, on peak inspiration there is dynamic narrowing of the thorax in the ventrodorsal view with an apparent dynamic pectus excavatum noted on the lateral view. Left‐sided subcutaneous emphysema is present consistent with prior thoracocentesis. In evaluating the thorax for a pleural effusion, one should look for certain secondary changes in thoracic structures that might explain the effusion. First, be sure to evaluate all of the ribs for any areas of expansile lesions that are aggressive in nature (lots of lysis) as pleural masses can cause a pleural effusion (Figure 15.10). The aggressive lesion could also originate from the thoracic spine or sternum. Evaluate the cardiac silhouette for the presence of a mediastinal shift that might indicate a mass lesion in one side of the hemithorax. Additionally, a diaphragmatic rupture can result in pleural effusions and if significant cranial abdominal organ displacement is present, this can result in a mediastinal (contralateral) shift away from the abdominal viscera. A thoracic ultrasound might be indicated in order to evaluate the pleural space for a mass or abdominal contents. Evaluate the major airways, particularly the right middle, left cranial, and right cranial bronchi. If the airway appears twisted or pinched off at the pulmonary hilum, then a lung lobe torsion should be considered as the cause of the pleural effusion. Cardiomegaly secondary to right heart disease or a pericardial effusion can result in right heart failure and ascites and a pleural effusion in the dog or just a pleural effusion in the cat. Evaluate the position of the carina on the lateral radiographs in a cat. If the carina is located at the seventh intercostal space (abnormal caudal position), then the possibility of a cranial mediastinal mass should be considered (Figure 15.11). FIGURE 15.10 A three‐view thorax (right lateral, left lateral, and dorsoventral views (A–C)) from a 10‐year‐old mixed‐breed dog that presented for respiratory distress. There is an expansile, osteolytic lesion associated with the fourth right rib that results in a mass within the right cranial thorax. A bilateral pleural effusion is present. There are multiple pulmonary nodules noted consistent with pulmonary metastatic disease. The primary diagnosis on histology was osteosarcoma of the rib with pleural and pulmonary metastasis. FIGURE 15.11 Right lateral (A), left lateral (B), and dorsoventral (C) radiographs from a 9‐year‐old DSH that presented for respiratory distress. There is a mild to moderate bilateral pleural effusion. There is a cranial mediastinal mass with dorsal elevation of the trachea, caudal displacement of the carina (to the seventh intercostal space) and the cardiac silhouette. Fine needle aspirates of the cranial mediastinal mass and pleural fluid were consistent with lymphoma. There are many different causes of pleural effusion, some of which include hypoproteinemia, pancreatitis, fluid overload, trauma, right heart failure in a dog, congestive heart failure in a cat, mediastinal or pleural masses, lung lobe torsion, chylothorax, pyothorax (tends to be unilateral and young cats), coagulopathy, vasculitis, diaphragmatic translocation of abdominal effusions, and diaphragmatic ruptures. If a pleural effusion is identified, one should tap and drain the fluid then repeat the thoracic radiographs to determine if there are any Roentgen signs that would support a specific etiology as the cause of the effusion. If chest tubes are placed, follow‐up thoracic radiographs are indicated to document chest tube placement. The Roentgen signs associated with a pneumothorax (gas within the pleural space) are like those described for pleural effusion, except rather than a soft tissue opacity separating the visceral and parietal pleural surfaces, a gas opacity will be present and contrast with radiopaque lungs (Box 15.4). The most common Roentgen findings of a pneumothorax include retraction of the lung lobes away from the parietal pleural surfaces with visualization of the visceral pleural surface of the lung lobe, separation of the cardiac silhouette from the sternum, and gas will be present in the pleural space, as characterized by the absence of pulmonary vessels and airways normally seen in fully inflated lung lobes contacting the parietal pleural space (Figure 15.12). The visceral pleural surface of the lungs will be seen at the level of the pneumothorax, depending on the degree of pneumothorax that is present. As the pneumothorax progresses, more extensive and increased degrees of atelectasis are seen within the lung lobes. FIGURE 15.12 Five different patients with varying degrees of pneumothorax. All cases were secondary to trauma. In cat 1 (right and left lateral images (A)), there is mild retraction of the caudal lung lobes from the diaphragm (white arrows). Additionally, there is a focal alveolar pulmonary pattern associated with the caudodorsal lung lobe consistent with a pulmonary contusion. The cardiac silhouette is enlarged with atrial and ventricular enlargement and an indentation along the caudal border of the cardiac silhouette. This cat was determined to have hypertrophic cardiomyopathy on echocardiography. In cat 2 (left lateral and ventrodorsal images (B,C)), there is retraction of the lung lobes away from the thoracic wall. There is visualization of the lateral margins of the visceral pleural surface of the lung lobes. This space is filled with air consistent with a pneumothorax. There is mild dorsal elevation of the cardiac silhouette on the left lateral. In dog 1 (left lateral only (D)), there is a mild pneumothorax with dorsal elevation of the cardiac silhouette away from the sternum and pleural air is present within this space consistent with a pneumothorax. In dog 2 (left lateral and ventrodorsal images (E,F)), there is retraction of the lungs away from the thoracic walls and elevation of the cardiac silhouette away from the sternum consistent with a moderate pneumothorax (P). There is an unstructured interstitial pulmonary pattern within the atelectatic lung lobes; however, this change in the peripheral lung lobes is consistent with the degree of the bilateral pneumothorax. In dog 3 (right lateral and dorsoventral images (G,H)), there is a mild to moderate bilateral pneumothorax. However, there is a generalized severe unstructured interstitial to early alveolar pulmonary pattern noted throughout the lung lobes. These changes are consistent with pulmonary contusions in this trauma case and would be considered severe. There is a focal bulla seen in the lateral aspect of the right cranial lung lobe at the fifth right rib on the dorsoventral view. A metal opaque structure (foreign body) is noted in the cranial abdomen. The most common cause of a pneumothorax is trauma. In a tension pneumothorax, the opening into the pleural space acts as a one‐way valve and air continues to accumulate, typically on one side. This results in extreme atelectasis of the affected lung lobes, a contralateral mediastinal shift away from the side of the tension pneumothorax, and flattening of the diaphragm on the side of the tension pneumothorax (Figure 15.13). FIGURE 15.13 Three examples of a tension pneumothorax. In dog 1 (right lateral and dorsoventral images (A,B)), there is a severe bilateral pneumothorax with collapse of the lung lobes (atelectasis) centrally. The cardiac silhouette is elevated dorsally. There are soft tissue curved structures seen along the sternum on the lateral view consistent with muscle attachments to the costal cartilages. On the dorsoventral image, there is “tenting” of the diaphragm with central positioning of the cardiac silhouette (due to the bilateral pneumothorax) consistent with a tension pneumothorax. In cat 1 (dorsoventral and right lateral views (C,D)), there is a unilateral severe pneumothorax with leftward shift of the cardiac silhouette and collapse of the right lung lobes. These changes are consistent with a unilateral tension pneumothorax. In cat 2 (right lateral and ventrodorsal images (E,F)), there is a severe unilateral left‐sided pneumothorax with a rightward contralateral mediastinal shift away from the central mediastinum. In addition, there is a right‐sided pleural effusion with rounding of the right caudal lung lobe. On the ventrodorsal view, there is “tenting” of the left side of the diaphragm. The effusion was determined to be a pyothorax post thoracocentesis.
CHAPTER 15
Pleural Space
Overview
Pleural Anatomy and Physiology
Abnormalities of the Pleural Space
Pleural Effusion
Pneumothorax
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