Overview of the Mediastinum and Cardiac anatomy
The mediastinum is the space located between the right and left pleural cavities,
and posterior to the sternum.
It is divided into a superior and inferior mediastinum.
inferior mediastinum is divided into an anterior, middle, and posterior mediastinum.
transverse thoracic plane divides the superior from the inferior mediastinum.
through the sternal angle and T4/T5 vertebral interspace.
The transverse thoracic plane
serves as a guide to the location of the arch of the aorta and the bifurcation of the trachea.
In a supine position, the arch of the aorta lies superior to the transverse thoracic plane,
and the bifurcation of the trachea is transected by the plane.
In a seated or standing
position, the arch of the aorta is transected by the transverse thoracic plane, and the
tracheal bifurcation lies inferior to the plane.
The superior mediastinum
contains the following structures:
arch of the aorta,
and the roots of its major branches (brachiocephalic trunk, left common carotid artery,
left subclavian artery); brachiocephalic veins, superior vena cava, vagus and phrenic
nerves, trachea, esophagus, thoracic duct, and thymus gland. The arch of the aorta begins
at the level of the sternal angle and passes up and back and to the left. The trachea lies
anterior to the esophagus, and divides into the 2 primary bronchi at the level of the sternal
angle (of Louis).
The R and L vagus nerves pass posterior to the root of the lung,
whereas the phrenic nerves pass anterior to the root of the lung.
The left vagus nerve
gives off the recurrent laryngeal nerve which hooks under the arch of the aorta just lateral
to the ligamentum arteriosum.
(The ligamentum arteriosum is the remnant of the fetal
ductus arteriosus and attaches between the aortic arch and the left pulmonary artery.)
The middle mediastinum contains the following structures:
ascending aorta, pulmonary trunk, superior vena cava, inferior vena cava, and bronchi at
the root of the lungs.
The posterior mediastinum contains the following structures:
aorta; thoracic duct; azygos and hemiazygos veins; esophagus, thoracic sympathetic
trunk, and posterior mediastinal lymph nodes.
The arrangement of the structures is
described as a “duck between 2 geese”, i.e. the thoracic duct lies between the azygos vein
on the right, and the esophagus on the left.
The azygos vein drains the back,
thoracoabdominal walls, and mediastinal viscera.
It serves as a collateral pathway for
venous drainage from the inferior vena cava to the superior vena cava into which it drains
after arching over the R root of the lung.
It communicates with the posterior intercostal
veins and the vertebral venous plexuses that drain the vertebra and vertebral canal.
azygos vein also receives venous blood from the hemiazygos vein on the inferior left side
of the thoracic cavity, and the accessory hemiazygos vein on the superior left side of the