Pericardiocentesis is a needle aspiration of pericardial fluid. This procedure is both therapeutic and diagnostic. It can be performed as an emergency measure to relieve cardiac tamponade and it can also provide a fluid sample to confirm and identify the cause of pericardial effusion (excess pericardial fluid) to help determine appropriate therapy.

Normally, small amounts of plasma-derived fluid within the pericardium lubricate the heart, reducing friction for the beating heart. Excess pericardial fluid may accumulate after inflammation, cardiac surgery, rupture, or trauma to the chest or pericardium. Rapidly forming effusions, such as those that develop after cardiac surgery or penetrating or blunt trauma, may induce cardiac tamponade, a potentially lethal syndrome marked by increased intrapericardial pressure, which prevents complete ventricular filling and thus reduces cardiac output. Slowly forming effusions, such as those that occur in pericarditis, typically pose less immediate danger because they allow the pericardium more time to adapt to the accumulating fluid.

The pericardium normally contains 10 to 50 mL of sterile fluid. Pericardial fluid is clear and straw colored, without evidence of pathogens, blood, or malignant cells. The white blood cell count in the fluid is usually less than 1,000/μl. Its glucose concentration should approximate the glucose levels in blood. Pericardial effusions are typically classified as transudates or exudates. (See Pericardial effusions: Transudates and exudates.)