**1. Introduction**

The thoracic cavity contains two pleural cavities and a third serous membranelined space, the pericardial cavity. The pericardium composed of fibrous and serous layers of the pericardial cavity is situated within the mediastinum. The parietal pericardium forms the superficial wall of the pericardium and is composed of an external fibrous layer and an internal serous layer. The pericardial cavity normally contains only a thin film of fluid positioned between parietal and visceral layers of serous pericardium [1]. These membranes provide the heart with a sac-like protective compartment. The normal pericardial sac contains 2–10 mL of clear, thin, serous fluid that acts primarily as a lubricant. If the fluid accumulates more than the limit in the pericardial sac, it is defined as pericardial effusion. The effusion usually accumulates slowly and progressively and presents with sign of chronic cardiac tamponade and right-sided heart failure with lethargy, exercise intolerance, or collapse [2].

Further, a rapid accumulation of fluid into the pericardial sac results in a rapid increase in intrapericardial pressure. If the intrapericardial pressure exceeds the pressure within the right atrium, acute compression of the heart (cardiac tamponade) and impairment of ventricular filling may develop. Compression of the heart by this pressure causes a drop in preload and reduces cardiac output [3]. Pericardial effusion usually occurs in middle to older dogs with a mean age of 6 years in large-breed dogs; however, it is found in age groups of 1–14 years. Neoplastic pericardial effusion is seen in brachycephalic breeds and short breeds.

German shepherds, golden retrievers, Great Danes, and Saint Bernards appear to be predisposed to idiopathic pericardial effusion. Both sexes are said to be affected. The most common causes of pericardial effusion in dogs are cardiac neoplasia, right-sided heart failure, cardiac rupture and idiopathic pericarditis and less commonly congenital pericardial disorders, trauma, or infectious processes [4].
