Pleural Effusion

Chapter 164

Pleural Effusion

Pleural effusion is a pathologic accumulation of fluid within the pleural space. The volume of the effusion may be scant and mainly of diagnostic significance or so large as to create a life-threatening situation. Pleural effusions are a common cause of respiratory distress in dogs and cats. These disorders call for an organized diagnostic approach and often demand prompt therapeutic measures. This chapter begins with an overview of pleural effusions in dogs and cats that considers the general causes, differential diagnosis, diagnostic approach, and initial treatment. Specific types of pleural effusions are considered next. Finally, the diagnostic and management approaches appropriate for patients with pyothorax or chylothorax are discussed.

Overview of Pleural Effusions in Dogs and Cats

A wide variety of diseases result in accumulation of fluid in the pleural space and cause respiratory compromise as expansion of the lungs is diminished. The underlying cause generally determines the prognosis and management. But first, the condition must be recognized in the patient and a sample of pleural fluid obtained for analysis. At that point, the pleural effusion can be classified into one of several primary categories (Table 164-1) based on the gross appearance, protein content, specific gravity, total nucleated cell count, and cytologic characteristics of the cells. Additionally, aerobic and anaerobic cultures may be indicated. Analysis of biochemical characteristics of the fluid, especially the triglyceride and cholesterol levels, is indicated if the effusion is suspected to be chylous (these values should be compared with serum levels). By identifying the type of effusion present, potential causes and the differential diagnoses for the effusion can be generated (Box 164-1).

In dogs, common disease processes that cause pleural effusions include pyothorax, pericardial effusion, cranial mediastinal mass, chylothorax, metastatic neoplasia in the pulmonary parenchyma, and dilated cardiomyopathy. In cats, the most common underlying diseases associated with pleural effusion are pyothorax, mediastinal lymphoma, heart disease, and feline infectious peritonitis (FIP).

Bilateral pleural effusion is most common in dogs and cats because the mediastinum is functionally incomplete. Unilateral effusion may be encountered, is more common in cats than in dogs, and typically is associated with inflammatory processes. This point is clinically important because it is often assumed that drainage of one hemithorax also provides drainage of the other side, but this is not always the case.

Clinical Signs of Pleural Effusion

Respiratory signs caused by pleural effusion are all secondary to reduced lung expansion (and tidal volume), which initially results in rapid, shallow breathing. An asynchronous respiratory pattern also can be seen, and respiratory difficulty often is characterized by increased abdominal effort. Ultimately, the animal may exhibit an orthopneic position with the elbows abducted and neck extended to minimize resistance to breathing. If hypoxia becomes severe, mucous membranes can become pale or even cyanotic.

Thoracic auscultation reveals that lung and heart sounds diminish ventrally but are often still present dorsally. The degree of respiratory compromise is generally proportional to both the volume of fluid within the pleural cavity and the rate of accumulation. Fluid that accumulates gradually may reach a larger volume before producing clinical signs. Once a certain volume is exceeded, critical respiratory compromise exists, and rapid decompensation can occur if the patient is not handled extremely cautiously. Other signs and physical examination findings, depending on the cause and associated clinical complications, may include coughing, pyrexia, depression, anorexia, weight loss, arrhythmias, murmurs, jugular venous distension, and ascites.

Diagnostic Tests in Patients with Pleural Effusion

Patients with pleural effusion usually are affected by a serious to life-threatening disease and should be assessed carefully. A typical workup includes a complete blood count and serum biochemical profile as well as other tests relevant to the clinical situation. These may include urinalysis, viral testing in cats (feline leukemia virus, feline immunodeficiency virus), and specific tests for infectious diseases when appropriate. Advanced evaluations such as flow cytometry or thoracoscopy may be required to identify an elusive neoplastic process.

Diagnostic imaging is a critical part of the evaluation. Thoracic radiography should be delayed until the patient’s condition has been stabilized, usually after thoracocentesis. Additionally, the underlying cause of a pleural effusion may become obvious after thoracocentesis. Thoracic ultrasonography and computed tomography (CT) can be used to identify mass lesions in the pulmonary parenchyma or mediastinum when radiographs yield negative results. Echocardiography may be indicated to evaluate the heart and pericardial space. If metastatic or multicentric neoplasia is suspected, abdominal ultrasonographic examination of organs and regional lymph nodes should be considered.

Thoracocentesis and pleural fluid analysis are indicated when physical examination raises suspicion of a significant pleural effusion or when pleural fluid is identified following thoracic radiography. As mentioned previously, analysis of a fluid sample is pivotal to the differential diagnosis. The gross appearance of pleural effusions can vary, ranging from clear to opaque; from translucent to flocculent; and from straw colored to yellow, milky, sanguineous, or red. Effusions of different causes, including those due to chylothorax and pyothorax, can contain red blood cells that create a pink to red fluid coloration; the underlying cause cannot be determined without microscopic and biochemical evaluations. If the effusion is caused by a septic process, it is often thick and opaque and may be malodorous. Evaluation of fluid samples should include cell count, measurement of total protein level, cytologic examination, and culture. Cytologic evaluation should identify the predominant cell type as well as any inflammatory, reactive, or neoplastic cells. When neutrophils are present these should be inspected for degenerative or toxic changes as well as for intracellular and extracellular bacteria. Special stains may be needed to identify chylomicrons. Additional aspects of the diagnostic workup for pyothorax and chylothorax are considered later.

Initial Management of Pleural Effusion

Recognition of the patient in respiratory distress is essential. Oxygen supplementation should be provided while the physical examination is performed. Placement of an intravenous catheter is beneficial, if tolerated by the patient, to allow for sedation if needed (see later) and provide immediate vascular access if the patient experiences respiratory or cardiovascular arrest while being handled. If pleural effusion is strongly suspected based on auscultation of the chest, thoracocentesis should be performed for both diagnosis and stabilization. Thoracic radiography is not necessary and may pose a significant risk to the patient. If ultrasonography is available, a quick scan of the chest can confirm the presence and location of the effusion to facilitate thoracocentesis. Thoracocentesis helps stabilize the patient’s condition and provides a sample of the fluid for diagnostic evaluation (see the following section on the thoracocentesis procedure).

Radiography or ultrasonography can be used after thoracocentesis to document successful evacuation of the pleural cavity. Radiographs may show an underlying cause that would have been obscured by the presence of fluid and collapse of the lung lobes before thoracocentesis. Ultrasonographic examination may reveal mass lesions or evidence of pericardial or cardiac disease.

If an animal remains in significant respiratory distress after successful aspiration of pleural fluid, concurrent heart or lung disease should be suspected (e.g., neoplasia, pulmonary contusions, pulmonary edema, or pneumonia). Fibrosing pleuritis or iatrogenic pneumothorax also should be considered.

The patient’s cardiovascular system should be assessed to determine whether any treatment is necessary to stabilize cardiovascular parameters. This may include administration of a bolus of intravenous fluids if hypovolemic or septic shock is present, or administration of diuretics if the patient has volume overload from congestive heart failure.

Procedure for Thoracocentesis

Performing thoracocentesis is an essential skill because thoracocentesis can be both therapeutic and diagnostic for patients with pleural space disease. Equipment needed includes clippers, surgical scrub, large syringe (10 to 60 ml), three-way stopcock, extension tubing and needle or butterfly catheter, collection bowl, and tubes for samples, including one with ethylenediaminetetraacetic acid (EDTA) and one without anticoagulant. A short over-the-needle catheter can be used to reduce the risk of laceration of the lung or blood vessels, although these catheters can kink once the stylet is removed, which negates any benefits they may carry. Sedation that spares the cardiovascular system may be needed depending on the stability of the patient’s condition and the patient’s temperament. A combination of an opioid and a benzodiazepine (e.g., butorphanol 0.1 to 0.3 mg/kg IV and diazepam 0.1 to 0.3 mg/kg) often is sufficient. Oxygen supplementation should be provided if the patient is in respiratory distress. With an assistant restraining the animal (preferably in sternal recumbency or standing), the appropriate rib space should be clipped and aseptically prepared. When pleural fluid is expected, use of the seventh or eighth intercostal space is recommended, at approximately the costochondral junction. Sterile gloves should be worn for insertion of the appropriately sized needle or butterfly catheter. For medium to large dogs a 1- or 1.5-inch needle may be required to penetrate the chest wall, whereas a image-inch butterfly needle is sufficient for most cats and small dogs. The needle tip should be placed just cranial to the rib to avoid intercostal blood vessels and nerves that are located caudally and should be inserted gently into the thorax perpendicular to the chest wall with the bevel of the needle pointing up while the hub of the needle and extension tubing are carefully observed for any signs of fluid. In cats with pleural effusion caused by heart failure, care must be taken to avoid puncturing a dilated left auricle, which often extends toward the left thoracic wall. Once the pleural space has been punctured, the needle can be directed ventrally so that the needle is almost parallel to the chest wall.

If a small amount of frank blood is suddenly and unexpectedly aspirated from the thorax, or if the lungs can be felt rubbing against the tip of the needle, the procedure should be stopped and the needle removed and replaced at a slightly different location. If a large amount of blood is withdrawn from the thorax, 1 to 2 ml should be placed in a red-topped tube to see if the blood clots. Blood from a hemothorax should not clot within the tube, whereas blood from the heart or a blood vessel will clot normally, provided there is no significant concurrent coagulopathy. If any other type of fluid is seen in the hub of the needle, aspiration should continue until no more fluid can be removed. If the patient will tolerate it, the needle can be directed ventrally while the patient is rolled slightly to the side on which thoracocentesis is being performed, and reaspirating from a more ventral location can facilitate removal of as much fluid as possible.

Complications of Thoracocentesis

Potential complications of thoracocentesis include iatrogenic pneumothorax from lung laceration, cardiac tamponade from cardiac laceration and bleeding, intrathoracic hemorrhage from laceration of blood vessels, and reexpansion pulmonary edema in situations of chronic pleural effusion. Severe pneumothorax after thoracocentesis is much more common in patients with severe chronic pulmonary disease or inflammatory pleural effusions. Acute death from the stress of restraint is also possible. Appropriate sedation may reduce these risks, but care should be taken to choose drugs with minimal respiratory suppression, as mentioned earlier. After thoracocentesis, the patient should be monitored for recurrence of respiratory distress, which might indicate return of fluid to the pleural space or development of an iatrogenic pneumothorax or hemothorax (less common).

Types of Pleural Effusions

Once a pleural effusion has been identified and sampled, the fluid typically is classified based on physical, cytologic, and chemical characteristics. Although there is some overlap among these categorizations, this organizational approach facilitates diagnosis.


Transudates are the result of a disturbance in the overall balance of capillary hydrostatic pressure, intravascular colloid osmotic pressure, and vascular endothelial permeability, which are responsible for the formation of fluid in the pleural space and the rate of pleural fluid absorption. A decrease in absorption can occur secondary to inflammation of the pleura, obstruction of the pleural lymphatics with neoplasia or emboli, or lymphatic hypertension.

A pure transudate can be seen in severe hypoalbuminemia, which results in decreased intravascular oncotic pressure. Usually the albumin level must be less than 1.5 g/dl to result in pure transudate formation. Hypoalbuminemia may be caused by a protein-losing enteropathy or nephropathy, hepatic disease, or marked loss of inflammatory fluid through a wound or into a body cavity. Animals with hypoalbuminemia also may have ascites or peripheral edema.

Cardiac disease can affect the formation and absorption of fluid in the pleural space by increasing pulmonary and systemic capillary hydrostatic pressures. Systemic venous hypertension resulting from right-sided and bilateral congestive heart failure may result in accumulation of pleural transudate and is accompanied by other signs of cardiac disease. Cats can develop pleural effusion with right- or left-sided congestive heart failure. The fluid that is seen with heart disease typically is a modified transudate (slightly higher protein level than a pure transudate), but in some cats a chylous effusion is identified.

Lung lobe torsion or a diaphragmatic rupture can obstruct venous and lymphatic drainage, resulting in leakage of variable types of fluid through the organ capsule into the pleural space; one such fluid is a modified transudate, sometimes with a prominent number of red blood cells. Lung lobe torsion is not common in dogs, although Afghan hounds appear to be predisposed. Lung lobe torsion develops as a primary entity, particularly in deep-chested dogs, but also occurs as a consequence of pleural effusion in any breed. It is often difficult to determine whether pleural effusion or lung lobe torsion occurred first, and underlying causes for pleural effusion should be ruled out. Lung lobe torsion has been associated with trauma, chylothorax, pulmonary neoplasia, respiratory disease, and thoracic surgery. The right middle lung lobe followed by the right cranial lung lobe are most frequently affected. Radiography after pleural drainage usually reveals diffuse consolidation of a lung lobe, a vesicular gas pattern in the affected lung lobe, and possibly altered orientation or abrupt termination of the lobar bronchi. Lung lobe torsion requires immediate surgical treatment to remove the affected lung lobe.

Neoplasia can cause a modified transudate, particularly if it is not exfoliating into the effusion. Obstruction of lymphatic drainage is the most common way that neoplasia leads to this type of effusion. Systemic inflammatory response syndrome can cause a modified transudate pleural effusion due to increased endothelial permeability.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Pleural Effusion

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