Critical Care & Emergency Medicine -  Respiratory Diseases

Pleural effusion is an abnormal accumulation of fluid in the pleural space. Normally, a small amount of fluid is present, which serves to lubricate the surfaces and prevent friction when the lung expands and contracts. If there is a disturbance in the production or drainage of this fluid, pleural effusion develops. Dogs compensate for small amounts of excess fluid by reducing their activity and increasing their respiratory rate. As the fluid continues to accumulate, lung expansion is restricted, atelectasis can occur, and the dog becomes dyspneic. Eventually, a life-threatening crisis occurs.



  • Causes - A variety of diseases may lead to the development of pleural effusion. Analysis of the fluid and other diagnostic tests help determine the underlying cause of the effusion.
    • Transudative fluid may accumulate in the pleural space in association with decreased oncotic pressure or increased hydrostatic pressure within the blood vascular system.

      Animals with marked hypoalbuminemia (< 1.5 g/dl) have low oncotic pressures that may result in the leakage of fluid into the pleural space. Causes of hypoalbuminemia include protein-losing nephropathy, protein-losing enteropathy, liver disease, portal caval shunts, burns, chronic hemorrhaging, malabsorption and malnutrition.

      Increased hydrostatic pressure is most often associated with right-sided heart failure, cardiac tamponade, and bilateral congestive heart failure in the dog. Overzealous fluid therapy theoretically increases hydrostatic pressure, but pulmonary edema is a more common complication of overhydration, unless right-sided heart disease is present.

      With chronicity, transudates may become modified transudates because of secondary inflammation associated with the fluid accumulation.
    • Inflammation or infection of the pleura and adjacent tissues may result in the accumulation of exudative pleural fluid. These fluids may be septic, such as those associated with penetrating wounds and pyothorax, or nonseptic such as those associated with immune-mediated diseases (e.g. systemic lupus erythematosus, rheumatoid arthritis), lung lobe torsion, and heartworm disease. Other potential causes include pneumonia, esophageal diseases, pulmonary thromboembolism, diaphragmatic hernias, uremia, pancreatitis, lymphoid granulomatosis, and fungal and parasitic infections.
    • Neoplastic effusions are often exudative or hemorrhagic, and may develop with mediastinal lymphoma and thymoma, with pulmonary and chest wall tumors, and with mesothelioma.
    • Obstruction or rupture of the lymphatic thoracic duct and thoracic lymphangiectasia lead to the accumulation of chyle in the pleural space (chylothorax). Chylous effusions may also occur secondary to heart disease, pericardial effusion, certain infections, thromboembolic disease, and neoplasia.
    • Hemorrhagic effusions may occur following trauma and surgery, or in association with clotting abnormalities and pericardial effusion.
    • Eosinophilic effusions have occasionally been recognized in association with pulmonary parasites and neoplasia.
    • Combined pleural and abdominal effusions also develop in the dog, and are most commonly associated with heart, liver and kidney disease; rupture of the biliary tract; and neoplasia.
  • Risk factors
    • Age - No known risk
    • Breed/genetics - Deep-chested dogs with a thin conformation are prone to lung lobe torsions. Afghan hounds and Shiba inus are predisposed to chylothorax. Large breed dogs are more likely to develop mediastinal thymoma and lymphosarcoma than are small breed dogs.
    • Sex - No known risk
    • Geographic/environmental - Outdoor, free-roaming dogs tend to be more prone to traumatic causes of pleural effusion and pyothorax.
    • Other medical disorders - Because pleural effusion is associated with an underlying condition, there are a variety of illnesses that predispose dogs to pleural effusion. These include pancreatitis, neoplasia, heart failure, heartworm disease, liver disease, diaphragmatic hernia, pneumonia, other pulmonary infections, and hypoalbuminemia.
  • Prevention - In general, pleural effusion cannot be prevented. Prompt diagnosis and treatment of the underlying conditions can help reduce the risk of developing pleural effusion. Not allowing dogs to roam can help reduce the risk of traumatic and infectious causes.


  • Species affected - Dogs and cats (See also Pleural Effusion in Cats)
  • Presenting signs and historical problems - Signs associated with pleural effusion may be subtle and vague, particularly early in the disease process or when only small amounts of fluid have accumulated. Animals tend to be lethargic and intolerant of exercise. Eventually, dyspnea, tachypnea and respiratory distress may be evident. Affected animals may sit or crouch in a sternal position with the head and neck extended. The elbows are often abducted from the chest in an attempt to help breathing. Some animals may exhibit open-mouth breathing and have a forceful abdominal component to each inspiration. Other signs include weakness, anorexia, weight loss, pallor, cyanosis, hypothermia, fever or cough.


  • General
    • Attitude - Most affected animals are lethargic. Some may present moribund or comatose.
    • Body condition - Some animals are in poor body condition.
    • Vital signs - Depending on the underlying cause of the effusion, some animals may be febrile and some may be hypothermic. Tachycardia may be present. Dyspnea, tachypnea and open-mouth breathing are common.
    • Mucous membranes - Some affected animals may have pale mucous membranes from anemia. Cyanosis may be noted. Petechiae and ecchymoses may occur with clotting abnormalities.
    • Hydration status - Many animals are dehydrated.
  • Head and neck - If the cause of the pleural effusion is due to cardiac disease, jugular venous distention and/or jugular pulses may be present.
  • Eyes - Often unremarkable unless the underlying disease also produces chorioretinitis.
  • Oral cavity - Often unremarkable
  • Thorax (cardiopulmonary) - Heart and breath sounds are usually muffled or dull ventrally. Breath sounds dorsally are usually normal or only slightly muffled. Chest percussion often reveals dullness in the area of fluid accumulation. Auscultation of the heart may reveal a murmur or arrhythmia, especially if the pleural effusion is cardiogenic. Gallop rhythms are uncommonly heard. External evidence of trauma may be detected.
  • Abdomen (gastrointestinal/urinary) - Abdominal palpation may reveal pain if pancreatitis is present. Ascites may also be evident.
  • Reproductive system - Often unremarkable
  • Lymph notes - Peripheral lymph nodes may be enlarged, particularly if lymphosarcoma or systemic infections are the underlying causes.
  • Integumentary system - Ecchymoses and abrasions or lacerations may be present if pleural effusion is due to trauma.
  • Neurologic examination - Mental status varies from dullness and depression to coma in severe cases.
  • Musculoskeletal examination - Muscle weakness is common. Traumatic injuries may also be noted.


  • Special examination techniques - Analysis of the fluid is crucial in determining the underlying cause of the pleural effusion. The fluid is obtained by thoracocentesis. Cytology is performed on both a direct smear and a centrifuged sample.

    Fluid analysis involves evaluation of the color, turbidity, odor, specific gravity, pH, and total protein content of the fluid. If chylothorax is suspected, triglyceride and cholesterol levels are determined on both the pleural fluid and serum. Other chemical analyses can also be performed. A portion of the fluid is submitted for aerobic and anaerobic culture and sensitivity testing.
  • Clinical laboratory tests
    • CBC - A neutrophilia with a left shift may be present if pyothorax, lung lobe torsion, pulmonary infections, or neoplasia is the cause of the effusion. Lymphopenia is often associated with chylothorax. Anemia may be associated with lymphoma.
    • Serum biochemical tests - Biochemistry tests may reveal a variety of abnormalities, depending on the underlying cause. A low albumin is present in animals with hypoalbuminemia. Animals with pancreatitis may have elevated amylase and lipase. Alterations in liver enzyme activities, BUN and creatinine may also be noted.
    • Urinalysis - Animals with protein-losing nephropathy often have severe proteinuria.
    • Coagulation profile - PT, PTT and ACT are often prolonged in animals with pleural effusion due to a coagulopathy.
  • Pleural Fluid Analysis - Fluid analysis helps classify the fluid as a transudate, modified transudate, septic exudate, nonseptic exudate, chylous effusion, hemorrhagic effusion, and sometimes a neoplastic effusion. These categories are based upon the gross appearance of the fluid, on the protein content and viscosity of the fluid, on the white blood cell count and the cytologic features of the fluid. Although categorizing the fluid does not always lead to a specific diagnosis, it does allow the differential diagnoses to be narrowed.

    • A transudate is defined as a fluid with a protein level < 1.5 g/dl and a white blood cell count < 1,000/ul. Viscosity of the fluid is low and specific gravity is < 1.018. The fluid is usually clear and colorless and has no odor. A small number of lymphocytes, mesothelial cells, neutrophils and macrophages may be seen on cytology. Transudates arise most commonly with hypoalbuminemia and heart disease.
    • A modified transudate is defined as a fluid with a protein level between 2.5 to 4 g/dl and a white blood cell count of 1,000 to 5,000/ul. Specific gravity often ranges from 1.018 to 1.030. The fluid may be clear to moderately cloudy, with a serous to serosanguineous color. Variable numbers of red blood cells, neutrophils, lymphocytes, macrophages and mesothelial cells are seen. Modified transudates can be difficult to distinguish from nonseptic exudates. Diseases associated with modified transudates in the dog include congestive heart failure, chronic hypoalbuminemia, lung lobe torsion, pericardial effusion, heartworm disease, pulmonary thromboembolism, neoplasia and diaphragmatic hernia.
    • Septic exudate is defined as a fluid with a protein level of 3 to 7 g/dl and a white blood cell count of 5,000 to 300,000/ul. The fluid is typically turbid and opaque and may have a foul odor. The color is variable: from white to yellow to red. Numerous degenerative neutrophils are found and may contain bacteria. Free bacteria may also be noted. Diseases associated with septic exudates include pyothorax (e.g. Actinomyces and Nocardia), bacterial and fungal infections of the lungs, mediastinum and esophagus, migrating foreign bodies, and bacteremia. Effusions associated with Actinomyces and Nocardia may contain flocculent material and sulfur-like granules.
    • Nonseptic exudate is defined as a fluid with a protein level of 3 to 6 g/dl and a white blood cell count of 5,000 to 20,000/ul. The color ranges from serous to serosanguineous to yellow. The fluid may be hazy or turbid. It is thick and viscous with FIP. With FIP, protein levels may be > 6.0 g/dl. No bacteria are seen on cytology. The distribution of cells is variable, and samples may contain neutrophils, plasma cells, lymphocytes, macrophages and red blood cells. Nonseptic exudates may arise from lymphatic or venous obstruction, or aseptic inflammation. They can be difficult to distinguish from modified transudates. Diseases associated with nonseptic exudates include certain immune-mediated diseases, uremia, pancreatitis, inflammation in adjacent pulmonary or mediastinal organs, lymphoid pulmonary granulomatosis, pulmonary thromboembolism, lung lobe torsion, neoplasia, and diaphragmatic hernia.
    • Chylous effusion is defined as a fluid containing chylomicrons and with a triglyceride concentration greater than the serum triglyceride concentration. In true chylous effusions, the pleural fluid cholesterol:triglyceride ratio is < 0.15. Chylous effusions typically have a protein level of 2 to 6.5 g/dl and a variable white blood cell count (often 1,000-20,000/ul). The fluid is opaque and either white or pink in color. Lymphocytes predominate, and some neutrophils may also be seen on cytology. True chylothorax is extravasation or leakage of intestinal lymph from an obstructed or ruptured thoracic duct. It may also arise with thoracic lymphangiectasia, or be idiopathic in nature.

    • Other effusions that may be discovered upon fluid analysis include peritoneal fluid that has crossed the diaphragm, and eosinophilic effusions.
  • Pleural fluid biochemistry analysis - Assays of fluid pH, glucose and other enzymes may be helpful in further characterizing the effusion.
    • A pH < 6.9, a glucose value < 10 mg/dl, and a high neutrophil count are consistent with a pyothorax.
    • A pH > 7.2 and a neutrophil count < 45 percent of the total WBC count are consistent with a malignant effusion.
    • Lactate dehydrogenase concentrations < 200 IU/L are consistent with an exudate.
    • Cardiogenic effusions often have fibronectin concentrations < 31.5 percent of plasma fibronectin values, whereas malignant effusions often have levels > 31.5 percent.
  • Serology/immunologic tests - Heartworm antigen test is performed in endemic areas.
  • Microbiology - Bacteria associated with septic pleural effusions include Actinomyces, Nocardia, Fusobacterium, E.coli, Bacteroides, Pasteurella, Peptostreptococcus, Klebsiella, etc. Blood cultures may also be indicated if bacteremia is suspected
  • Diagnostic imaging
    • Radiographs (thoracic/abdominal) - Thoracic radiographs are indicated to confirm the diagnosis of pleural effusion. Radiography may need to be delayed until after thoracocentesis is performed in animals with severe respiratory distress.

      Evidence of pleural effusion on radiography includes separation of the lung lobes from the parietal pleura and sternum with a fluid density between these structures, scalloping of the edges of the lungs, interlobar fissure lines, rounding of the lung margins at the costophrenic angles on the ventrodorsal view, and inability to identify the cardiac and diaphragmatic silhouettes.

    • The effusion is commonly bilateral because the mediastinum is very fenestrated in the dog. Focal, unilateral effusions are rarely seen with pyothorax, lung lobe torsion, hemothorax, and diaphragmatic hernias.

      The mediastinum may be widened with anterior mediastinal neoplasia. The cranial middle lung lobe is atelectatic with lung lobe torsion. Cardiomegaly may be noted with heart disease.

      Radiographs are repeated after pleural drainage to evaluate the thoracic structures. If a unilateral lesion is suspected, both left and right lateral radiographs are taken, along with ventrodorsal or dorsoventral views. Horizontal beam radiography allows detection of small amounts of fluid. Radiographs of the abdomen may reveal concurrent ascites, particularly if hypoalbuminemia, diaphragmatic hernia, pancreatitis, or disseminated neoplasia is present
    • Ultrasound (thoracic/ECHO) - Thoracic ultrasonography is helpful prior to removal of pleural effusion to visualize structures within the thorax. It may identify pulmonary, mediastinal or pleural masses, and the presence of a diaphragmatic hernia. Ultrasonography can also be useful to guide the positioning of the needle during thoracentesis.

      Echocardiography is performed after thoracentesis and when the dog is stable. It helps to confirm the presence, and to characterize the type of, any existing heart disease.
  • Electrodiagnostics
    • ECG - An electrocardiogram is recommended if cardiac arrhythmias are ausculted.


  • Differential diagnosis - A variety of illnesses can cause dyspnea and lethargy in the dog. Some of these include:
    • Congestive heart failure
    • Pulmonary edema
    • Pulmonary contusions
    • Pulmonary neoplasia
    • Heartworm disease
    • Pulmonary thromboembolism
    • Pulmonary infections with the systemic mycoses
    • Diaphragmatic hernia
    • Pneumonia
    • Chronic bronchitis
    • Emphysema
    • Near drowning
    • Smoke inhalation
  • Recommended tests - Thoracic radiographs, thoracic ultrasonography, CBC, biochemistry profile, urinalysis and fluid analysis
  • Summary of diagnostic criteria - Thoracic radiographs confirm the presence of pleural effusion. Fluid analysis helps determine the class of fluid present and may provide important information as to the underlying cause of the effusion.
  • Prognosis - The prognosis varies depending on the underlying cause of the effusion. Dogs with diaphragmatic hernia, lung lobe torsion or hemothorax due to trauma have a fair to good prognosis. Animals with right-sided or bilateral congestive heart failure, heartworm disease, pulmonary thromboembolic disease, lymphoid granulomatosis, and neoplasia have a poor to guarded prognosis. Dogs with pleural effusion associated with pulmonary bacterial or mycotic infections, pyothorax, chylothorax, uremia, pancreatitis, and immune-mediated diseases have a fair to poor prognosis. The prognosis is often poor with cases of severe hypoalbuminemia; however, some are responsive to treatment of the primary disease and supportive measures.



The principles of therapy for pleural effusion involve relief of any respiratory distress by evacuation of pleural fluid and institution of emergency procedures, as well as treatment of the underlying cause of the effusion.


  • Emergency therapy - Emergency therapy involves the following:
    • Oxygen therapy
    • Thoracentesis - If continued production of pleural fluid is anticipated, then an indwelling chest tube is inserted so that either intermittent or continuous evacuation is possible.
    • Sedation and intubation in severe cases
    • Radiographs at least every 48 hours to monitor response to therapy.
  • Specific therapy - Definitive therapy requires the establishment of an etiologic diagnosis. A brief overview of therapy is discussed below. For more specific details regarding therapy, see the in-depth content articles pertaining to each specific disease.

    For animals with congestive heart failure, furosemide, oxygen and other heart medications are often recommended.

    For animals with pyothorax, broad-spectrum antibiotics are administered until culture and sensitivity results return. Then, antibiotic therapy is guided by sensitivity results. Intravenous fluids are often required to treat dehydration. Lavage of the chest through bilateral chest tubes or surgical exploratory thoracotomy is also usually performed.

    For animals with hemorrhagic effusions, blood transfusions or autotransfusion may be necessary. A thoracostomy tube may also be required.

    For dogs with chylothorax, an indwelling thoracostomy tube may be inserted. Surgical ligation of the thoracic duct and/or surgical shunting procedures are often performed. Dietary modification and medical therapy with the benzopyrones may also be instituted.

    Lung lobe torsion and diaphragmatic hernias are corrected by thoracotomy. The hernia is surgically repaired, and the twisted lung lobe is excised.


Long-term therapy depends on the underlying cause of the pleural effusion. It may include treatment such as antibiotics, furosemide for heart disease, and repeated thoracocenteses.



Frequent rechecks are recommended, at least initially. Radiographs are initially taken every 48 to 72 hours to monitor response to therapy.

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