Pericardium Heart

Original Editor - Veena Jose Panicker Top Contributors - Veena Jose Panicker, Lucinda hampton, Kim Jackson and Vidya Acharya

Introduction

Layers: heart wall.

The pericardium is a protective, fibroserous sac that encloses the heart and its great blood vessels. It is located in the middle mediastinum, behind the body of the sternum and the 2nd–6th costal cartilages, and in front of the T5 to T8 thoracic vertebrae.

Functions of the Pericardium

1. Restricts Excessive Movement of the Heart: The pericardium helps keep the heart in place, preventing it from shifting excessively during movement or changes in body position.
2. Lubricates and Provides a Smooth Environment for Heart Movements: The pericardial cavity, between the two layers of the pericardium, contains a small amount of fluid that reduces friction as the heart beats, allowing it to contract and relax smoothly.
3. Limits Cardiac Distension: The pericardium prevents the heart from overstretching by providing a fixed boundary. This helps in maintaining optimal heart size and function, preventing excessive dilation, which could impair cardiac efficiency.

In short, the pericardium acts as a protective and functional barrier, ensuring the heart operates efficiently and safely within the chest cavity.

Embryological relations of Heart and Pericardium

• As the heart develops and begins to move, the fibrous pericardium (outer layer) forms from the somatic mesoderm (the outer layer of mesoderm). The heart and the great vessels grow into and push the serous pericardium from behind.

• The heart and great vessels invaginate (push into) the serous pericardium, which forms two layers:
  • The parietal layer lines the inside of the fibrous pericardium.
  • The visceral layer directly covers the heart and is also known as the epicardium. This developmental process ensures that the heart is fully enclosed and protected while still having enough space to function and move properly within the chest cavity.

Subdivisions

The pericardium consists of two main components, each with specific functions:

1. Fibrous Pericardium (Outer Layer):

• The fibrous pericardium is a tough, single-layered fibrous sac that surrounds the heart and the roots of the great blood vessels.
• Functions:
  • It provides structural support and anchors the heart within the chest.
  • It prevents the heart from over-expanding or over-distending by restricting excessive movement.
  • It protects the heart from infection and physical trauma.

2. Serous Pericardium (Inner Layer):

• The serous pericardium is a double-layered membrane that lies inside the fibrous pericardium. It consists of:
  • Parietal layer: The outer layer of the serous pericardium, which is attached to the fibrous pericardium.
  • Visceral layer (epicardium): The inner layer of the serous pericardium that is directly attached to the heart muscle (the epicardium). This layer is continuous with the parietal layer at the base of the great vessels.
• Between these two layers (parietal and visceral) is the pericardial cavity, which contains a small amount of pericardial fluid. This fluid serves as a lubricant to reduce friction as the heart beats, allowing smooth movement during contraction and relaxation.

To summarize the fibrous pericardium provides protection, structural support, and limits heart movement. The serous pericardium offers lubrication and reduces friction, allowing the heart to function smoothly. It is composed of two layers: the parietal layer and the visceral layer (epicardium). The space between these layers contains pericardial fluid to aid heart movement.

Fibrous pericardium

The fibrous pericardium is a tough, cone-shaped sac made of fibrous tissue that surrounds the heart. It has several key features:

1. Apex (top): The apex is rounded and located at the level of the sternal angle (where the sternum meets the ribs). It is connected to the large blood vessels at the heart's base and to a layer of tissue in front of the trachea.
2. Base (bottom): The base is wide and attached firmly to the central tendon of the diaphragm, which helps the heart stay in place.
3. Front connections: The fibrous pericardium is linked to the top and bottom parts of the sternum (breastbone) by weak ligaments.
4. Back connections: At the back, the fibrous pericardium is near the main branches of the bronchial tubes (bronchi), the esophagus (food pipe), the nerves around it, and the descending part of the aorta (large blood vessel).
5. Side connections: On each side, the fibrous pericardium is in contact with the pleura (membrane around the lungs), the lungs themselves, the phrenic nerve, and blood vessels supplying the diaphragm.

Serous pericardium

The serous pericardium is a thin, double-layered membrane that covers the heart. It has two layers:

1. Outer layer (parietal pericardium): This layer is attached to the fibrous pericardium, which is the tough outer sac around the heart.
2. Inner layer (visceral pericardium or epicardium): This layer is attached directly to the heart itself, except in areas where blood vessels are present, like in the grooves of the heart.

The two layers are connected at the base of the large blood vessels like the aorta, pulmonary trunk, the two vena cavae, and the pulmonary veins.

Between the two layers is the pericardial cavity, which is a small space filled with a tiny amount of fluid. This fluid helps reduce friction as the heart beats, allowing it to move smoothly inside the pericardium.

Feature	Parietal Pericardium	Visceral Pericardium (Epicardium)
Location	Outer layer of the serous pericardium; attached to the fibrous pericardium	Inner layer of the serous pericardium; directly covers the heart
Attachment	Fused with the fibrous pericardium	Fused to the surface of the heart
Function	Protects the heart, forms part of the pericardial cavity	Reduces friction, provides a smooth surface for the heart's movement
Relation to Blood Vessels	Does not cover blood vessels	Covers the blood vessels at the heart's surface
Other Name	No alternate name	Also called the epicardium
Nerve Supply	Phrenic nerve (C3-C5) and vagus nerve	Autonomic nerve fibers (mostly sympathetic fibers from the cardiac plexus)
Response to Pain	Sensitive to pain; pain is referred to the neck, shoulders, and upper chest (due to the phrenic nerve)	Less sensitive to pain; pain is not typically felt here

Sinuses of the pericardium

The sinuses of the pericardium refer to specific anatomical spaces within the pericardial cavity that are formed during the development of the heart. These sinuses play important roles in facilitating the movement and pulsation of the heart structures.

1. Transverse Sinus of Pericardium:

• The transverse sinus is a horizontal passage that lies between the arterial and venous ends of the heart tube.

• It is located:
  • Anteriorly by the ascending aorta and pulmonary trunk.
  • Posteriorly by the superior vena cava.
  • Inferiorly by the left atrium.
  • It opens into the general pericardial cavity on each side.
• This sinus is clinically significant because it provides a surgical access point, allowing surgeons to clamp the great vessels during cardiac surgery without damaging surrounding structures.

Surgical Significance

The transverse pericardial sinus is an important anatomical feature in cardiac surgery, particularly during procedures involving the heart-lung machine or coronary artery bypass grafting (CABG). Here's a more detailed explanation of its surgical significance:

1. Access During Cardiac Surgery:
   • Once the pericardial sac is opened during cardiac surgery, a finger is often passed through the transverse pericardial sinus to gain access to the area behind the great vessels (the aorta and pulmonary trunk). This allows the surgeon to manipulate or control the vessels as needed during the operation.
2. Heart-Lung Machine Setup:
   • The transverse pericardial sinus plays a key role in cardiopulmonary bypass during heart surgeries. After gaining access to the sinus, a temporary ligature (a surgical tie) is passed through the sinus and around the ascending aorta and pulmonary trunk.
   • This ligature allows the surgeon to control blood flow in the great vessels while the heart-lung machine is set up. The heart-lung machine is responsible for oxygenating the blood and circulating it through the body during the surgery, especially when the heart is temporarily stopped or the blood supply is redirected.
3. Functionality of the Ligature:
   • After the ligature is passed through the transverse sinus, the tubes of the heart-lung machine are connected to the aorta and pulmonary trunk. The ligature is then tightened around the vessels to secure the tubing and ensure a stable connection, preventing blood from leaking and maintaining proper circulation during the operation.

Visual Representation

• The transverse pericardial sinus is typically depicted in surgical anatomy diagrams as a space that allows the manipulation and control of the great vessels without interfering with other structures in the pericardium.

2. Oblique Sinus of Pericardium:

The oblique sinus is a cul-de-sac located posterior to the left atrium of the heart.

• It is formed by the pericardial reflections around the veins, including the pulmonary veins and venae cavae.

• It is bounded:
  • Anteriorly by the left atrium.
  • Posteriorly by the parietal pericardium and esophagus.
  • On the sides by the pericardial reflections.
  • Inferiorly and to the left, it opens into the general pericardial cavity.
• This sinus permits the free pulsations of the left atrium and is important for the heart's ability to expand and contract within the pericardial sac.

Contents of the Pericardium

The pericardium, which is the sac surrounding the heart, contains several important structures:

1. Heart with its associated cardiac vessels and nerves.
2. Ascending aorta: The artery that carries oxygenated blood from the left ventricle to the body.
3. Pulmonary trunk: The artery that carries deoxygenated blood from the right ventricle to the lungs.
4. Lower half of the superior vena cava: The vein that returns deoxygenated blood from the upper half of the body to the right atrium.
5. Terminal part of the inferior vena cava: The vein that returns deoxygenated blood from the lower half of the body to the right atrium.
6. Terminal parts of the pulmonary veins: These veins return oxygenated blood from the lungs to the left atrium.

Blood Supply to the Pericardium

The blood supply to the pericardium (both fibrous and parietal layers) comes from the following arteries:

1. Internal thoracic artery: Branches of this artery supply the anterior aspects of the pericardium.
2. Musculophrenic artery: A branch of the internal thoracic artery, supplying the lower part of the pericardium.
3. Descending thoracic aorta: Supplies the posterior aspects of the pericardium.

The venous drainage of the pericardium follows the course of the arteries, draining into the corresponding veins.

Nerve Supply to the Pericardium

1. Fibrous and parietal pericardia are supplied by the phrenic nerves (C3-C5). These nerves carry sensory fibers and are sensitive to pain, which is why pain associated with pericarditis originates from these layers.
2. Epicardium (the visceral layer of the pericardium) is supplied by autonomic nerves from the heart. These nerves do not carry sensory fibers, so the epicardium itself is not sensitive to pain.
3. Pain in pericarditis originates specifically from the parietal pericardium.
4. Cardiac pain or angina originates in the cardiac muscle or the vessels of the heart, often due to ischemia (lack of oxygen). This pain is typically felt as a deep, crushing sensation.

Pathologies

Pericarditis and Cardiac Tamponade:

• Pericarditis is the inflammation of the pericardium (the protective sac around the heart). This inflammation leads to the accumulation of fluid in the pericardial cavity, termed pericardial effusion.
• If this effusion becomes excessive, it can put pressure on the heart, particularly the atria. This compression interferes with the heart's ability to fill with blood during diastole (the relaxation phase), thereby reducing cardiac output. This condition is called cardiac tamponade, which is a medical emergency.

Uremia and Pericarditis:

• Pericarditis is often seen as a terminal complication of uremia, a condition where kidney failure leads to the buildup of waste products in the blood, which can then trigger inflammation in various tissues, including the pericardium.

Pericardiocentesis:

• Pericardiocentesis is the procedure used to remove excess fluid from the pericardial cavity to relieve pressure on the heart. It can be performed through two main approaches:
  • Sternal approach: A needle is inserted through the 5th or 6th intercostal space, just to the left of the sternum.
  • Subxiphoid approach: A needle is inserted at the left costoxiphoid angle (where the ribs meet the xiphoid process) and directed upwards and backward at an angle of 45° to the skin.

Pericardial Friction Rub:

• A pericardial friction rub occurs when the parietal and visceral layers of the inflamed pericardium rub against each other due to the inflammatory exudate. This can cause a characteristic friction sound, which can be heard through a stethoscope. It may also be palpable on the chest wall, providing another diagnostic clue for pericarditis.

An approach to exploring the pericardial cavity

This process involves performing incisions to explore the pericardial cavity and identify important anatomical features such as the transverse sinus, oblique sinus, and the fibrous and serous pericardium. It also provides a detailed view of the heart's structures, including its borders, surfaces, grooves, apex, and base. This understanding is essential in both anatomical study and surgical procedures involving the heart and pericardium.

1. Incisions in the Pericardium:

• Vertical cuts: Start by making a vertical incision on each side of the pericardium, just anterior to the phrenic nerve. The phrenic nerve typically runs along the pericardium, and these cuts will be on the outer side of the pericardial sac.
• Transverse cut: Next, join the lower ends of these vertical incisions with a transverse incision approximately 1 cm above the diaphragm. This creates a flap of pericardium.

2. Flap of Pericardium:

• After the cuts, turn the flap of pericardium upwards and sideways. This exposes the pericardial cavity, which houses the heart. The turned flap will include both the fibrous layer and the parietal layer of the visceral pericardium.
  • Fibrous pericardium: This is the outermost layer, a tough, protective sac around the heart.
  • Parietal layer of visceral pericardium: This layer is part of the serous pericardium and lines the inside of the fibrous pericardium.
  • The visceral pericardium directly surrounds the heart, forming the inner layer of the pericardium.

3. Exploring the Transverse Sinus:

• Probe behind the aorta and pulmonary trunk: Pass a probe from the right side of the pericardium, moving it behind the ascending aorta and pulmonary trunk. The probe should appear on the left side, just to the right of the left atrium.
• This area is the transverse pericardial sinus, which separates the arteries (aorta and pulmonary trunk) from the veins (superior vena cava and pulmonary veins). This space is important during heart surgery, especially for the placement of the heart-lung machine.

4. Exploring the Oblique Sinus:

• Lift the apex of the heart upwards: By doing this, you gain access to the oblique sinus, which is located at the back of the heart.
• Insert a finger behind the left atrium: The oblique sinus is a cul-de-sac that lies between the inferior vena cava (IVC) and the lower left pulmonary veins. The sinus is bordered:
  • Right and below: by the inferior vena cava.
  • Above and left: by the lower left pulmonary vein.

5. Defining Key Structures:

• Borders of the heart: The borders of the heart include:
  • Right border: formed by the right atrium.
  • Left border: formed by the left ventricle.
  • Inferior border: formed by the right ventricle.
  • Superior border: formed by the great vessels (ascending aorta, pulmonary trunk, superior vena cava).
• Surfaces of the heart:
  • Anterior surface: largely formed by the right ventricle.
  • Inferior surface: formed mainly by the right ventricle and a part of the left ventricle.
  • Left surface: formed by the left ventricle.
  • Right surface: formed by the right atrium.
• Grooves of the heart:
  • The coronary sulcus (or atrioventricular groove) separates the atria from the ventricles.
  • The anterior interventricular groove (or left anterior descending groove) separates the right and left ventricles.
  • The posterior interventricular groove marks the division between the right and left ventricles on the posterior surface of the heart.
• Apex: The apex of the heart is formed by the left ventricle and points downwards and to the left.
• Base: The base of the heart is the top part, formed mainly by the left atrium and part of the right atrium.