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===Introduction===
===Signalment===
The pericardium is a sac that surrounds the heart and serves to protect the heart as well as maintain its position and shape. The pericardium is comprised of an outer fibrous layer and an inner serous layer. The fibrous outer layer is composed of collagen and elastin. The serous layer is composed of a single layer of mesothelial cells. The serous pericardium lines the inside of the fibrous layer, where it is known as the parietal layer, and also overlies the heart, where it is referred to as the visceral layer of the serous pericardium. The pericardial cavity is the space between the visceral and parietal layers, which usually contains a very small amount of fluid.
Pericardial effusion is a pathological accumulation of fluid within the pericardial sac. The rate of this fluid accumulation within the pericardium determines the haemodynamic effects, and therefore clinical signs. A slow accumulation is initially well-tolerated, but can eventually lead to increased pressure causing compression of the right atrium and right ventricle (cardiac ''tamponade''). Rapid fluid accumulation results in acute tamponade.
Pericardial effusions may be idiopathic, neoplastic, infectious, traumatic, toxic or may result from atrial rupture in dogs with myxomatous mitral valve disease (MMVD).
Neoplasia affecting the heart, heart base or pericardium is the most common cause of pericardial effusion in dogs.
Idiopathic pericardial effusions are the second most common cause of pericardial effusion in the dog. Effusions are usually haemorrhagic and the cause is unknown. One study has demonstrated progression of idiopathic pericardial effusion to mesothelioma over time in a series of Golden Retrievers.
In cats, the majority of pericardial effusions are secondary to congestive heart failure. However, cardiac lymphoma, heart based tumours and a variety of metastatic tumours have been reported as causes of pericardial effusion.
===Signalment===
Haemangiosarcoma has a predilection for the right atrium, particularly in German Shepherd dogs.
Heart based tumours, such as chemodectomas, are most common in Boxers.
Mesotheliomas have a higher prevalence in small-medium breeds.
Idiopathic pericardial effusion is most common in middle-aged, male, large and giant breed dogs. Golden Retrievers, Labrador Retrievers, Great Danes, Newfoundlands and St. Bernards are overrepresented.