The pleural space is defined as the area between the lungs and the chest wall. Normally there is no soft tissue or free air
present in this space. A very small amount of fluid (undetectable on radiographs or ultrasound) may be present within the
thoracic cavity. Clinical signs of pleural space disease include tachypnea or difficulty breathing. (Classically rapid/shallow
breathing). It is important to recall that pleural effusion is a sign rather than a specific diagnosis. Physical examination
findings may include respiratory distress and muffled heart/lung sounds. Occasionally, lung sounds may appear normal. It may
be possible to percuss a ventral dullness. Other findings may reflect the underlying disease process (ie gallop, murmur, fever,
trauma). Diagnosis of pleural effusion may be made either through thoracocentesis, or imaging (usually radiographs, but fluid
may also be visualized on ultrasound). Radiographic signs of pleural effusion include decreased detail ('white out"), scalloping
of the ventral lung borders, fissure lines between lung lobes and an obscured cardiac silhouette. Chronic pleural effusions
may result in the radiographic appearance of "rounded" lung margins. Therapy of pleural effusion is directed at both improving
respiratory status by removing fluid/air and at identifying the underlying cause.
Thoracocentesis is performed by clipping and sterilely preparing an area between the seventh and ninth rib at approximately
the costochondral junction. Occasionally, the site may be lower or higher based upon suspicion of fluid (lower) or air (higher).
The animal should be gently restrained in a sternal or standing position. Typically, in cats and small dogs a butterfly catheter,
stopcock and 5-30 ml syringe are used. In large cats and most dogs, a longer needle is required. Aseptic technique is recommended.
A distinct "pop" is felt upon entering the pleural cavity. Volumes of fluid and air retrieved should be recorded. Animals
with chronic effusions are MUCH more likely to develop a pneumothorax following thoracocentesis. Samples of fluid should be
saved for cytology and culture (if indicted). Fluid analysis is often very helpful in determining the cause. Fluid may be
characterized as a transudate, modified transudate, or exudate. A transudate is defined as s clear acellular fluid with a
low specific gravity. Pure transudates are relatively rare but may occur secondary to severe hypoalbuminemia or overhydration.
Modified transudates are the most common type of pleural effusion and may be caused by heart failure, neoplasia (lymphoma
or metastatic) or systemic inflammatory conditions (eg pancreatitis). Causes of exudates include pyothorax or feline infectious
peritonitis. Other effusions include chylothorax or hemothorax.
Pure transudate: Usually develops in hospitalized pets or animals with chronic protein-losing conditions. Fluid looks similar
to water and is relatively acellular. Treatment includes thoracocentesis, colloid support, ? diuretics as well as therapy
for the underlying condition.
Heart failure: Right-sided heart failure may result in significant accumulations of pleural fluid. Usually, physical examination
shows other signs of heart failure, including pericardial effusion, such as abnormal heart sounds, tachycardia or jugular
venous distension. Cytological findings may be dependant on duration but usually are mildly inflammatory (non-degenerate neutrophils,
macrophages, reactive mesothelial cells). Treatment is directed at therapy of heart failure and may include diuretics (furosemide,
spironolactone/hydrochlorothiazide) and vasodilators.
Neoplastic: Neoplasia is a common cause of pleural effusion. Some neoplasms exfoliate well (lymphoma, some carcinomas). Pleural
effusion from animals with neoplasia may contain neoplastic cells, although the lack of cytological criteria for malignancy
does NOT exclude neoplasia from the differential list. Usually, neoplastic effusions are modified transudates. Therapy for
neoplastic effusion includes periodic thoracocenteisis, intra-cavitary therapy (eg. Cisplatin, bleomycin, carboplat) or shunting.
Infectious: Pleural effusions may be infectious in origin. Affected animals usually have systemic signs of sepsis (lethargy,
fever, leukocytosis or leukopenia). Cytology from infectious effusions show degenerative neutrophils, and intra and extra-cellular
bacteria. Infections may be aerobic or anaerobic. Anaerobic infections may be particularly foul smelling. In cats, bite wounds
are considered the most common source of infection, while in dogs, bite wound and penetrating foreign bodies (sticks/plant
awns) are frequently responsible. Occasionally, pleural effusions are identified secondarily to bacterial pneumonias. Therapy
for pyothorax involves drainage (usually via chest tube) and antibiotics based upon culture and sensitivity. Some animals
may require surgical intervention for resection and drainage of affected tissues. The prognosis for pyothorax is frequently
good if the pet does not present in a moribund condition. Pleural effusions secondary to pneumonia may be sterile.