The Anatomy of the Pericardium

The pericardium is a fluid-filled sac that encases the muscular body of the heart and the roots of the great vessels (including the aorta, pulmonary trunk, pulmonary veins, and the inferior and superior vena cavae).

This fibroserous sac is comprised of a serous membrane supported by a firm layer of fibrous tissue. The serous membrane is smooth tissue, lined by mesothelium; it secretes a fluid (called serous fluid) that functions as lubrication to lower friction between the heart and structures that surround the heart.

In addition to reducing friction, the pericardium functions to prevent the heart from filling up to the point of over-distension.

Anatomy

The pericardium is one of several serous membranous cavities in the human body; it is comprised of two closely connected structures, including:

• The fibrous pericardium: A tough external layer comprised of fibrous connective tissue.
• The serous pericardium: A thin layer that coats the internal surface of the fibrous pericardium.

Between the fibrous and serous pericardium layers, there is pericardial fluid.

Structure

The layers of the pericardium (in order from outermost to innermost layer) include:

• Fibrous pericardium: The fibrous outer layer of the pericardium. It is continuous with the tendon of the diaphragm and is comprised of robust connective tissue that functions to keep the heart from overexpanding when blood volume (inside of the heart) increases.
• Serous pericardium: The inside surface of the pericardium that is further comprised of two layers, including the parietal layer (the outer layer that lines the inside surface of the fibrous pericardium) and the visceral layer (the internal surface that comprises the epicardium).

The pericardial cavity—located between the outer and inner serous layer—contains a small amount of lubricating fluid, called serous fluid, which helps to lower the level of friction generated as the muscle of the heart contracts. 

Location

The pericardium surrounds the heart, which is in the thorax (chest) behind the sternum (breastbone).

The pericardium also surrounds the great vessels (including the aorta, venae cavae, and pulmonary artery). The fibrous sac can fix the position of the heart by attaching to nearby structures, including the sternum (breastbone), the diaphragm, and the tunica adventitia (the outer layer of the great vessels). 

Anatomical Variations

The pericardium, which develops during week five of fetal development, normally consists of a two-layered fibroserous sac containing the heart.

A rare congenital (present at birth) defect involves the failure of the pleuropericardial membranes to properly fuse.  This leads to a very rare birth defect, called pericardial agenesis. The condition is more common in males, and it may not involve any symptoms at all, but in some instances, it causes impairment of heart function, or could even cause death.

Symptoms could include fatigue and stabbing chest pain. The condition may be associated with other heart defects or syndromes.

Function

The pericardium has several very important roles pertaining to the heart, including:

• Keeps the heart positioned in the mediastinum via its attachment to the sternum, the diaphragm, and the outer layer of the great vessels (called the tunica adventitia).
• Prevents the heart from overfilling as the firm, fibrous layer of the pericardium is not able to be overextended. 
• Provides lubrication in the form of a very thin film of fluid—located between the two layers of the serous pericardium. This lubrication reduces the level of friction that the heart creates as it continually beats within the thorax (chest).
• Protects the heart from infection by serving as a physical barrier between the heart and adjacent organs—such as the lungs, which are prone to infection.

Associated Conditions

There are several common medical conditions associated with the pericardium, including:

• Pericarditis: An inflammation of the pericardium that may be caused by an infection, trauma, a myocardial infarction (heart attack), or other causes (including idiopathic or unknown causes). Pericarditis can be acute (sudden and severe) or chronic (long-term).
• Chronic constrictive pericarditis: Long-term inflammation of the pericardium which constricts the heart’s movement; this condition may require surgery, particularly if the condition is severe. 
• Pericardial effusion: An excess build-up of fluid in the pericardial sac, caused by pericarditis or other conditions of the pericardium.
• Cardiac tamponade: A serious condition involving a build up of fluid in the sac which interferes with the normal functioning of the heart by causing constriction of the heart; it can be caused by tumors, bleeding into the pericardium, or an accumulation of waste products in the blood that is normally eliminated via the urine, called uremia.

Tests

Many tests are available that can help doctors diagnose conditions of the pericardium, including:

• Pericardial fluid analysis: Sometimes the fluid of the pericardium is removed using a procedure called paracentesis. The fluid is examined for signs of disease. The test is performed by aspirating fluid from the sac with a large needle. An ultrasound may be used to guide the needle into the correct position before aspirating the fluid.

Other tests that may be performed to diagnose conditions of the pericardium include:

• Electrocardiogram: Also referred to as an ECG or EKG, an electrocardiogram is a test that involves a recording of the electric signals of your heart. There’s a specific pattern that clues the diagnostician in that tamponade or other conditions of the pericardium may be present.
• Chest X-ray: This may be performed to check to see if the heart has become enlarged when there is an increase in fluid in the pericardium.
• CT scans: Imaging tests that may be performed to check for a tumor, excess fluid, or other signs of disease in and around the heart such as constrictive pericarditis.
• Echocardiogram: This test is performed to check for fluid or pericardial effusion as well as the classic signs of constrictive pericarditis which include a stiff, thick pericardium that limits the movement of the heart.
• Cardiac MRI: An imaging test that assesses for excess fluid, inflammation, or thickening of the pericardium as well as compression of the heart.
• Cardiac catheterization: Performed to evaluate filling pressures in the heart, used to confirm a diagnosis of constrictive pericarditis.  
• Blood tests: Several different blood tests can be performed to test the fluid in the pericardium and help to detect the underlying cause of pericarditis.