A pericardial effusion is an abnormal accumulation of fluid in the pericardial cavity. The pericardium is a two-part membrane surrounding the heart: the outer fibrous connective membrane and an inner two-layered serous membrane. The two layers of the serous membrane enclose the pericardial cavity (the potential space) between them. This pericardial space contains a small amount of pericardial fluid. The fluid is normally 15-50 mL in volume. The pericardium, specifically the pericardial fluid provides lubrication, maintains the anatomic position of the heart in the chest, and also serves as a barrier to protect the heart from infection and inflammation in adjacent tissues and organs.

By definition, a pericardial effusion occurs when the volume of fluid in the cavity exceeds the normal limit. Some of the presenting symptoms are shortness of breath, chest pressure/pain, and malaise. Important etiologies of pericardial effusions are inflammatory and infectious (pericarditis), neoplastic, traumatic, and metabolic causes. Echocardiogram, CT and MRI are the most common methods of diagnosis, although chest X-ray and EKG are also often performed. Pericardiocentesis may be diagnostic as well as therapeutic (form of treatment).