Pericardial effusion is the accumulation of fluid within the pericardium. The pericardium is a fibrous sac that surrounds the heart and normally contains a very small amount of fluid. When an abnormal amount of fluid accumulates within the pericardium, the sac becomes maximally stretched and the fluid/blood begins to compress the heart. This leads to signs of shock (low blood pressure, elevated heart rate, weakness, pale gums, cold extremities, severe weakness or collapse, and even death). Pericardial effusion can also cause a dog to develop fluid in the chest cavity (pleural effusion) or abdomen (peritoneal effusion/ascites). Significant pericardial effusion is a life-threatening emergency that is treated with a procedure called a pericardiocentesis (see below).

Most cases of pericardial effusion in dogs are caused by a tumor/cancer associated with the heart, which can often be seen on an echocardiogram. However, certain types of cancer like mesothelioma and lymphoma may not form discrete tumors and must be diagnosed on pericardial fluid analysis and/or biopsy of the pericardium. Additionally, small tumors may not be detectable on echocardiogram initially. Other potential causes of pericardial effusion in dogs include idiopathic pericardial effusion (cause cannot be determined), right-sided congestive heart failure, atrial splitting, infection, inflammation and bleeding abnormalities. If an obvious cause for pericardial effusion is not seen on echo, submission of a sample of the effusion for analysis is recommended.

Signs of significant pericardial effusion include weakness (difficulty rising, stumbling), collapse, loss of consciousness, cold limbs, pale gums, labored breathing, increased resting breathing rate, exercise intolerance, vomiting, or abdominal distension. If any of these signs are seen, your pet should be evaluated by a veterinarian immediately.

Significant pericardial effusion requires immediate stabilization via a procedure called pericardiocentesis. Ideally an intravenous catheter is first placed to allow potentially life-saving medications and fluids to be administered as needed. A pericardiocentesis typically requires mild sedation to allow the dog to relax for the procedure. A small amount of a numbing medication is placed at the pericardiocentesis site, and a needle is then passed between the ribs into the pericardial sac. A syringe is used to remove as much of the pericardial effusion as possible, and samples of the pericardial fluid are saved for later analysis if necessary. An ECG is recorded continuously throughout the procedure to monitor for the development of significant cardiac arrhythmias. Pericardiocentesis is generally a safe procedure, although potential complications include arrhythmias, cardiac puncture and, in rare cases, death. If significant pleural effusion (fluid in the chest cavity) is present, this may also be removed to ease breathing. Unless severe, typically abdominal effusion (fluid in the abdomen) is not removed as this should slowly be reabsorbed after the pericardial effusion is resolved. Furosemide and other diuretics are not indicated for the management of pleural or abdominal effusion solely due to pericardial effusion and may cause harmful dehydration.

Although definitive identification of a cardiac tumor type is not possible without obtaining a biopsy sample of the mass (which is difficult to safely perform), a presumptive diagnosis can be made based on the location and echocardiographic appearance of the tumor. Hemangiosarcoma (HSA) is a tumor of blood vessels that is often found in the right atrium, in the spleen, in the liver, or on the skin. HSA affecting internal organs tends to be a very aggressive cancer and has often metastasized by the time it is diagnosed. HSA tumors have a robust blood supply and the blood vessels within the tumor are usually quite fragile. Because of this, the tumors are prone to spontaneous bleeding. When the tumor is associated with the heart, the blood fills the space between the pericardial sac and the heart itself (pericardial effusion).

Unfortunately, the prognosis for hemangiosarcoma of the heart is poor. With pericardiocentesis, some dogs may survive up to 1 month, although they often require multiple pericardiocentesis procedures as the pericardial effusion recurs. More aggressive treatment (chemotherapy or surgical removal of the sac surrounding the heart) only results in an average survival time of 6 months.