The pericardium is a fibroserous sac that surrounds the heart and the roots of the great vessels. It provides mechanical protection, lubrication, and maintains the heart’s position within the mediastinum.
Layers of the Pericardium
Fibrous Pericardium
The fibrous pericardium is the tough, outer layer of the pericardial sac. It is composed of dense irregular connective tissue that provides structural support and prevents overdistension of the heart.
Characteristics:
- Thick and inelastic
- Continuous with the adventitia of the great vessels superiorly
- Attached to the central tendon of the diaphragm inferiorly
- Attached to the sternum via sternopericardial ligaments anteriorly
The fibrous pericardium anchors the heart within the mediastinum and protects it from overfilling and displacement.
Serous Pericardium
The serous pericardium is a thin, delicate membrane that lines the inner surface of the fibrous pericardium and covers the outer surface of the heart. It consists of two layers:
Parietal Layer: Lines the inner surface of the fibrous pericardium. It is separated from the fibrous layer by loose connective tissue.
Visceral Layer (Epicardium): Adherent to the surface of the heart. This layer is actually the outer layer of the heart wall and contains adipose tissue, particularly along the coronary sulcus and interventricular grooves.
Pericardial Cavity
Between the parietal and visceral layers is the pericardial cavity, a potential space that normally contains 15-50 mL of serous fluid. This fluid is an ultrafiltrate of plasma that provides lubrication, reducing friction between the moving heart and surrounding structures.
Pericardial Sinuses
The pericardial cavity contains two important recesses formed by the reflections of the serous pericardium around the great vessels:
Transverse Pericardial Sinus
Located posterior to the ascending aorta and pulmonary trunk, and anterior to the superior vena cava. This sinus is clinically significant because a surgeon can pass a ligature around the great vessels through this space during cardiac surgery.
Oblique Pericardial Sinus
A blind-ended recess located posterior to the heart, between the pulmonary veins. It is bounded by the reflections of the serous pericardium around the four pulmonary veins and the inferior vena cava.
Blood Supply and Innervation
Arterial Supply: The pericardium is supplied by the pericardiacophrenic artery (branch of the internal thoracic artery), musculophrenic artery, and small branches from the aorta and internal thoracic artery.
Venous Drainage: Via the pericardiacophrenic veins, which drain into the brachiocephalic veins.
Nerve Supply: The fibrous pericardium and parietal serous pericardium are innervated by the phrenic nerves (C3-C5). This explains why pericardial inflammation (pericarditis) often causes referred pain to the shoulder (C4 dermatome).
The visceral serous pericardium (epicardium) is innervated by autonomic fibers and is insensitive to pain.
Functions of the Pericardium
- Mechanical Protection: Protects the heart from trauma and infection
- Lubrication: Reduces friction between the heart and surrounding structures
- Anchoring: Maintains heart position within the mediastinum
- Barrier: Prevents overdistension of the heart chambers
- Barrier to Infection: Limits spread of infection from adjacent structures
Clinical Significance
Pericarditis
Inflammation of the pericardium, often caused by viral infections, myocardial infarction, uremia, or autoimmune conditions. Symptoms include sharp chest pain that worsens with inspiration and improves when leaning forward. Diagnosis is made by ECG (diffuse ST elevation), echocardiography, and clinical findings.
Pericardial Effusion
Accumulation of excess fluid in the pericardial cavity. Causes include pericarditis, heart failure, malignancy, and trauma. Small effusions may be asymptomatic, while large effusions can cause symptoms of compression.
Cardiac Tamponade
A life-threatening condition where rapid accumulation of fluid in the pericardial cavity compresses the heart chambers, impairing cardiac filling and reducing cardiac output. Beck triad (hypotension, muffled heart sounds, distended neck veins) is a classic presentation. Emergency pericardiocentesis is required.
Constrictive Pericarditis
Chronic thickening and scarring of the pericardium, restricting cardiac filling. Causes include tuberculosis, radiation therapy, and prior cardiac surgery. Treatment may require pericardiectomy.