Mediastinum

The heart, the arteries and veins leaving and entering the heart, the trachea and bronchi; the esophagus, and the vagus and phrenic nerves and sympathetic chains occupy the mediastinum, the area in the middle of the thoracic cavity between the two pleural sacs. The anteroposterior dimension of the thorax is narrowest in the mediastinum because of the presence of the thoracic vertebrae posteriorly. Laterally, the pleural sacs enclosing the lungs extend much further back alongside the vertebrae in the areas known as paravertebral gutters.

The mediastinum is divided, for descriptive convenience, into the superior and inferior mediastinum. An imaginary line joining the sternal angle and the intervertebral disc below T4 demarcates the boundaries of the superior and inferior of the mediastinum.

The superior mediastinum occupies the space between the thoracic inlet above and the imaginary horizontal plane. The inferior mediastinum lies below that line and extends as far as the diaphragm. The lateral borders of both subdivisions of the mediastinum are the parietal pleura covering the medial aspect of the lungs, the mediastinal pleura.

The inferior mediastinum is further divided into three parts;

  1. The anterior part between the sternum and fibrous perdicardium

  2. The large middle mediastinum containing the heart enclosed in the pericardium and the roots of the great vessels

  3. The posterior part between to the fibrous pericardium and thoracic vertebrae

Contents of Mediastinal Divisions:

The contents of the mediastinum can be summarized as:

  1. Aorta

  2. Superior and inferior vena cava

  3. Azygos and hemiazygos veins

  4. Lymphatics

  5. Vagus nerve

  6. Phrenic nerve

  7. Thymus

  8. Trachea

  9. Esophagus

Summary of Mediastinum:

The mediastinum is defined as the space between the pleural cavities in the thorax. It is bounded laterally by the pleural cavities, anteriorly by the sternum, and posteriorly by the vertebral column and is divided artificially into a superior and inferior division by a line from the sternal angle of Louis to the T4-T5 intervertebral disc. The inferior division is further divided into the anterior, middle, and posterior divisions.

  1. Superior mediastinum - the contents of the superior mediastinum include the trachea, esophagus, thymus, phrenic nerves, azygous vein, SVC, brachiocephalic artery and veins, aortic arch, left common carotid artery, left subclavian artery, and thoracic duct.

  2. Anterior mediastinum - the contents of the anterior mediastinum include the thymus, fat, lymph nodes, and connective tissue.

  3. Middle mediastinum - the contents of the middle mediastinum include the heart, pericardium, phrenic nerves, ascending aorta, SVC, and coronary arteries and veins.

  4. Posterior mediastinum - the contents of the posterior mediastinum include the descending aorta, esophagus, thoracic duct, azygos vein, splanchnic nerves, and vagus nerve.

The Pericardium

The following are some notes on the pericardium:

  1. The heart is enclosed in a tough fibrous sac, the fibrous pericardium to accommodate movement, as well as volume and pressure changes

  2. The pericardium is a membrane that covers the heart and the root of the great vessels, deep to the mediastinal pleura.

  3. Fibrous pericardium can be summarized as:

    • Forms the superficial layer which is continuous with the tunica adventicia of the great vessels

    • It is also continuous with the central diaphragm of to form the pericardiophrenic ligament

  4. Serous pericardium can be summarized as:

    • Has parietal and visceral layers

    • The visceral layer is also epicadium

    • The pericardial cavity between the parietal and visceral layers contains pericardial fluid which allows for frictionless movement during beating of the heart.

Pericardial Sinuses

The following is a summary of the pericardial sinuses:

  1. These sinuses form due to the reflection of the serous pericardium around the great vessels

  2. Transverse sinus

    • This is on the superior surface of the heart, posterior to the aorta and pulmonary artery, and anterior to the pulmonary veins. It allows the ligation of the pulmonary trunk nd ascending aorta during surgery.

  3. Oblique sinus

    • This is posterior to the base of the heart between the pulmonary veins and the inferior vena cava and a finger can be placed in this region inferiorly.