Pleural Effusion
A pleural effusion is a collection of fluid in the space between the lungs and the body wall. This space is called the pleural space. When your cat is normal, there should be no fluid or air in it.
Causes
There are several important diseases that can cause fluid to collect in the pleural space. To diagnose a specific disease, it is necessary to determine the type of effuion present. The most important types of effusions are:
1) Chyle: Chyle is part of the body's immune system. It is carried in tiny ducts from one lymph node to another to the large vein that enters the heart (the posterior vena cava). The fluid of chylothorax is white in color.
2) Pus: Pus is formed when an infection occurs in the pleural space. The infection is usually due to bacteria, but fungal infections also occur. The fluid of pyothorax is usually yellow to yellow-green in color.
3) Blood: Blood reaches this space due to trauma to blood vessels trauma or due to an inability for the cat's blood to clot properly. The latter sometimes occurs in cats poisoned with rat poison. The fluid of hemothorax is red in color.
4) Modified Transudate: This is a watery-like substance that may be due to cancer (lymphoma), sterile inflammations, or viral infections (Feline Infectious Peritonitis). This fluid may be clear, pink, or amber in color.
5) Transudate: This is a very thin, watery fluid that is usually associated with heart disease. It is usually clear.
Diagnosis
Cats with pleural effusions have difficulty breathing because the lungs are compressed by the fluid. This is often the reason they are presented for medical care. The first step in diagnosis is to take a radiograph (x-ray) of the chest to document that the difficulty breathing is due to a pleural effusion. If fluid is present, a small amount is removed with a syringe and needle. Several important tests are performed that include specific gravity determination, cell counts, protein level, and cytology (inspection of the specific cells). These tests generally allow us to classify the type of effusion in one of the above 5 categories. With this information, a list of possible diseases can be developed.
Treatment
The immediate need of your cat is to restore proper breathing ability. This is done by removing the fluid from the pleural space. This procedure is called thoracocentesis. Sedation is usually needed since a needle is inserted between the ribs so the fluid can be withdrawn. Several sites are usually aspirated in order to remove most or all of the fluid. Many of these cats are critical; therefore, sedation will incur some degree of risk.
In many cats, we will remove up to 240 ml (8 ounces) of fluid. If at least 100 ml is removed, breathing is improved. The more that we can remove, the better your cat will be able to breathe. However, the amount removed is also limited by how much is there. Therefore, we generally make another radiograph after fluid removal to determine how much, if any, remains. This may also allow us to see other problems in the chest that the fluid was hiding on the initial radiograph.
The ultimate treatment is to correct the underlying disease. That requires a diagnosis of a treatable disease. Once the underlying disease is cured or controlled, fluid does not reform and breathing remains normal.
Some diseases may be diagnosed quickly but will not be controlled quickly. This results in refilling of the pleural space after thoracocentesis and requires either repeated thoracocentesis or placement of a thoracostomy tube. The latter is a rubber-like tube that enters the pleural space but is anchored outside the body. A syringe is attached at various intervals (daily, twice daily, etc.) so the fluid can be painlessly withdrawn without sedation. The thoracostomy tube is removed when fluid production stops.
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