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. If large enough, it can compress the heart, causing cardiac tamponade and obstructive shock. 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).