Normal pleural fluid is an ultrafiltrate of plasma secreted by the mesothelial cells. In dogs, the pleural fluid volume is small, about 0.1 to 0.2 ml/kg body weight. The pleural fluid protein concentration is 1.1 to 1.3 g/dl, whereas the total cell count is less than 3,000 cells/μl. The differential cell count includes mesothelial cells (70%), monocytes (28%), and lymphocytes (2%). Neutrophils are uncommonly present.

The main site of pleural fluid filtration is the parietal pleura. Fluid and solute filter from the parietal interstitium into the pleural cavity down a pressure gradient across the parietal pleura. Once formed, the pleural fluid drains into lymphatic stomata found along the dependant portions of the parietal pleura (mainly the diaphragmatic and mediastinal surfaces). Pleural fluid drainage is controlled, under normal conditions, by lymphatic drainage. No pulmonary lymphatics open or drain into the pleural space of dogs.

The most common causes of pleural effusions include diseases or disorders resulting in increased capillary hydrostatic pressure (right-sided heart failure, pericardial effusion, diaphragmatic hernia); decreased intravascular colloid osmotic pressure (hypoalbuminemia from chronic protein loss [renal disease] or lack of protein production [severe hepatic disease]); increased vascular capillary permeability (systemic inflammatory diseases, pneumonic processes); and decreased pleural fluid absorption or drainage as occurs secondary to pleural inflammation, neoplastic processes, or obstruction of the thoracic duct and draining lymph nodes.

Clinical signs of pleural effusion vary depending on the volume of fluid, the rapidity of its accumulation and the underlying disease process. Mild pleural effusions are difficult to detect and many dogs show no clinical signs of illness. When pulmonary parenchymal disease is absent, signs of serious respiratory distress are usually not evident until 60 ml/kg body weight of pleural fluid has accumulated. In dogs with signs of pleural fluid accumulation, dyspnea (with or without cyanosis) may be observed as well as open-mouth breathing, orthopnea, tachypnea, and reduced tidal volumes. Physical examination findings depend on the primary disease process. In most dogs with pleural effusion, respiratory sounds are decreased, especially ventrally, with concomitant muffling of heart sounds. A percussible fluid line may also be noted with ventral hyporesonance. Other nonspecific signs include ascites, dehydration, depression, fever, lymphadenopathy, heart murmurs, arrhythmias, inappetence, pale mucous membranes, pulse deficits, and weight loss.

Thoracic radiographs are usually necessary to confirm the presence of a pleural effusion. Radiographically, the effusion can be classified as free or encapsulated. Free fluid moves within the pleural cavity and is most characteristic of transudative effusions. Encapsulated fluid is trapped by adhesions and is often associated with chronic exudative effusions, such as pyothorax. The radiographic diagnosis of pleural effusion depends on finding one or more of the following radiographic signs: (1) the presence of interlobar fissure lines; (2) rounding of lung margins at the costophrenic angles; (3) separation of lung lobe borders away from the thoracic wall; (4) scalloping of the lung lobe margins dorsal to the sternum; (5) blurring of the cardiac silhouette; and (6) widening of the mediastinum. The radiographic views that are most helpful for recognizing small amounts (less than 100 ml) of pleural fluid are the lateral recumbent and ventrodorsal views. The earliest sign of pleural effusion is accumulation of fluid dorsal to the sternum in the lateral recumbent position, resulting in scalloping of the lung lobe borders as they retract from the thoracic walls. The ventrodorsal view is preferable to the dorsoventral view for dogs with small amounts of pleural fluid. Radiographs taken during expiration are also helpful when evaluating a dog with a small volume of pleural fluid because the volume of the lungs is less at this phase of breathing; therefore, the volume of effusion is relatively greater and is spread over a smaller area. Dramatic radiographic changes are associated with a large volume pleural effusion. Retraction and separation of the lung lobes from the thoracic wall should be present in all radiographic views. Thoracic width usually increases on the ventrodorsal and dorsoventral views. Lung lobes can collapse to half their original volumes. In the lateral and dorsoventral views, the heart, the mediastinum and the diaphragm may be totally obscured by fluid. Dorsal elevation of the trachea and caudal displacement of the liver and diaphragm are commonly present on the lateral view.

Thoracocentesis is used as a diagnostic and therapeutic procedure. An area of the skin between the seventh and ninth intercostal space, just below the level of the costochondral junction is aseptically prepared and an 18- or 20-gauge needle attached to a three-way stopcock is introduced at the cranial rib border to avoid the intercostal vessels and nerve that traverse the caudal rib margin. The needle is advanced through the skin, subcutaneous tissues, and parietal pleural membrane. Constant negative pressure should be maintained on the syringe so that advancement of the needle is discontinued at the first sight of fluid. This will minimize the likelihood of damaging underlying lung tissue. In most cases, the volume of effusion is large enough that laceration of lung tissue is unlikely. In some instances, the effusion may be highly viscous (pyothorax and chylothorax) and difficult to aspirate. In these animals, chest tube placement may be necessary to completely evacuate the pleural space. Samples of the effusion should be collected for cytologic evaluation, determination of physical and biochemical characteristics, and aerobic, anaerobic, and fungal culture.

Pure transudates are clear and have a low specific gravity (<1.018), low total protein content (<2.5 g/dl) and contain few cells (<1,000/μl). Small lymphocytes, well-preserved neutrophils, mesothelial cells, and monocytes are present. Disorders causing hypoproteinemia should be considered in the list of differential diagnoses.

Modified transudates have a total protein content of 2.5 to 3.5 g/dl and a total cell count between 500 and 10,000 cells/μl. Cell types are similar to transudates, but well-preserved neutrophils may be present in variable numbers depending on the degree of inflammation (as seen in resolving septic effusions or progression from a transudate to an exudate). Differential diagnoses are numerous and include neoplastic diseases (lymphoma, mesothelioma, thymoma), translocation of abdominal effusion (peritoneal dialysis, generalized abdominal inflammatory processes), selective causes of increased hydrostatic pressure (lung lobe torsion, diaphragmatic hernia, neoplasia), and causes of pleural or pulmonary vasculitis (systemic inflammation [sepsis, rickettsial infections], localized inflammation [pneumonia, pancreatitis]), immune-mediated diseases, and allergic or anaphylactic reactions (drug reactions, snake envenomation).

Hemorrhagic effusions are characterized by packed cell volume, nucleated cell counts, and total protein content that are at least 25% of the peripheral blood. Platelets are not present unless bleeding has occurred 60 minutes before sampling. Erythrophagocytosis by macrophages is commonly seen. Sanguineous effusions have at least 5,000 to 6,000 red cells/μl. Differential diagnoses include trauma, coagulopathy, lung lobe torsion, diaphragmatic hernia, pulmonary infarction or abscesses, heartworm disease, and neoplasia.

Inflammatory and septic effusions are characterized by large numbers of neutrophils. The fluid is cloudy and the color varies from red to brown to yellow. Total protein content is high (>3.0 g/dl) as well as the nucleated cell count (>5,000/μl). Septic effusions are differentiated from inflammatory effusions by the presence of intracellular bacterial (sometime fungal) organisms and degenerative neutrophils (Fig. 23-1). Anaerobic species alone or in combination with aerobes (multiple organism infections are the norm) are typically isolated from these exudative effusions. Common anaerobic bacteria include Bacteroides spp. and Fusobacterium spp. Gram-negative organisms Actinomyces spp., Pasteurella spp., and Escherichia coli are also common. Grampositive bacteria (Corynebacterium spp., Streptococcus spp., and Nocardia spp.) are less often isolated. Nonseptic inflammatory effusions include those listed as causing modified transudates from chronic vasculitis. It should be remembered that the nonseptic inflammatory effusions can progress to septic effusions if the microorganism gain access to the effusion.