Superior Mediastinum and Great Vessels

The superior mediastinum is superior to the transverse thoracic plane, passing through the sternal angle and the junction (IV disc) of vertebrae T₄ and T₅. From anterior to posterior, the contents of the superior mediastinum are the:

• Thymus.

• Great vessels, with the veins (brachiocephalic veins and SVC) anterior to the arteries (arch of aorta and roots of its major branches - the brachiocephalic trunk, left common carotid artery, and left subclavian artery) and related nerves (vagus and phrenic nerves and the cardiac plexus of nerves).

• Inferior continuation of the cervical viscera (trachea anteriorly and esophagus posteriorly) and related nerves (left recurrent laryngeal nerve).

• Thoracic duct and lymphatic trunks.

To summarize systemically, the order of the major structures in the superior mediastinum, from anterior to posterior, is: (1) thymus, (2) veins, (3) arteries, (4) airway, (5) alimentary tract, and (6) lymphatic trunks.

Great Vessels

The right and left brachiocephalic veins are formed posterior to the sternoclavicular (SC) joints by the union of the internal jugular and subclavian veins. At the level of the inferior border of the 1^st right costal cartilage, the brachiocephalic veins unite to form the SVC. The left brachiocephalic vein is more than twice as long as the right brachiocephalic vein because it passes from the left to the right side, anterior to the roots of the three major branches of the arch of the aorta. The brachiocephalic veins shunt blood from the head, neck, and upper limbs to the right atrium.

The superior vena cava (SVC) returns blood from all structures superior to the diaphragm, except the lungs and heart. It passes inferiorly and ends at the level of the 3^rd costal cartilage, where it enters the right atrium of the heart. The SVC lies in the right side of the superior mediastinum, anterolateral to the trachea and posterolateral to the ascending aorta. The right phrenic nerve lies between the SVC and the mediastinal pleura. The terminal half of the SVC is in the middle mediastinum, where it lies beside the ascending aorta and forms the posterior boundary of the transverse pericardial sinus.

The ascending aorta, approximately 2.5 cm in diameter, begins at the aortic orifice. Its only branches are the coronary arteries, arising from the aortic sinuses. The ascending aorta is intrapericardial; for this reason, and because it lies inferior to the transverse thoracic plane, it is considered a content of the middle mediastinum (part of inferior mediastinum).

The arch of the aorta (aortic arch), the curved continuation of the ascending aorta, begins posterior to the 2^nd right sternocostal (SC) joint at the level of the sternal angle. It arches superiorly, posteriorly and to the left, and then inferiorly. The arch ascends anterior to the right pulmonary artery and the bifurcation of the trachea, reaching its apex at the left side of the trachea and esophagus as it passes over the root of the left lung. The arch descends posterior to the left root of the lung beside the T₄ vertebra. The arch ends by becoming the thoracic (descending) aorta posterior to the 2^nd left sternocostal joint.

The arch of the azygos vein occupies a position corresponding to the aorta on the right side of the trachea over the root of the right lung, although the blood is flowing in the opposite direction. The ligamentum arteriosum, the remnant of the fetal ductus arteriosus, passes from the root of the left pulmonary artery to the inferior surface of the arch of the aorta. The usual branches of the arch are the brachiocephalic trunk, left common carotid artery, and left subclavian artery.

The brachiocephalic trunk, the first and largest branch of the arch of the aorta, arises posterior to the manubrium, where it is anterior to the trachea and posterior to the left brachiocephalic vein. The trunk ascends superolaterally to reach the right side of the trachea and the right SC joint, where it divides into the right common carotid and right subclavian arteries.

The left common carotid artery, the second branch of the arch of the aorta, arises posterior to the manubrium, slightly posterior and to the left of the brachiocephalic trunk. It ascends anterior to the left subclavian artery and is at first anterior to the trachea and then to its left. It enters the neck by passing posterior to the left SC joint.

The left subclavian artery, the third branch of the arch of the aorta, arises from the posterior part of the arch, just posterior to the left common carotid artery. It ascends lateral to the trachea and left common carotid artery through the superior mediastinum; it has no branches in the mediastinum. As it leaves the thorax and enters the root of the neck, it passes posterior to the left SC joint.