7th Lecture The Mediastinum 2014

The Thoracic Cavity &Mediastinum

Mediastinum present in the middle of the thoracic cavity & contains;
Esophagus
Trachea
Heart
Great Vessels
Pleural Cavities on either side of the mediastinum contain the lungs

The Mediastinum

Concept－all of organs between the left and right mediastinal pleurae is called mediastinum. It extends from the sternum in front to the vertebral column behind, and from the thoracic inlet above to the diaphragm below.

The mediastinum

Divisions of the mediastinum
There is a plane of division to which the whole topography (a study or detailed description of the various features of any object or entity and the relationships between them) of the mediastinum can be related, namely a plane passing horizontally through the sternal angle of Louis. This plane passes backwards to the lower border of the 4th thoracic vertebra.

Above the plain , between it and the thoracic inlet, lies the superior mediastinum. Below the plane, the inferior mediastinum is divided into three compartments by the fibrous pericardium: a part in front it , the anterior mediastinum; a part behind it , the posterior mediastinum; and the pericardium itself containing the heart and the roots of the great vessels forming the middle mediastinum.


Subdivisions of mediastinum
Superior mediastinum
Inferior mediastinum
Anterior mediastinum
Middle mediastinum
Posterior mediastinum

Left side of mediastnum

Root of lung

Pericardium

Phrenic n. &

pericardiacophrenic a.

Left vagus n.

Left recurrent n.

Thoracic aorta

Sympathetic trunk

Greater splanchnic n

Aortic arch

Thoracic duct

Left subclavian a.

Esophagus

Right side of mediastnum

Root of lung

Pericardium

Superior vena cava

Phrenic n. &
pericardiacophrenic a.

Left vagus n.

Azygos v.

Sympathetic trunk

Esophagus

Arch of azygos v.

Trachea

Inferior vena cava

Superior mediastinum
Locating－from inlet of thorax to plane extending from level of sternal angle anteriorly to lower border of T4 vertebra posterioly
Contents
Superficial layer
Thymus
Three veins
Left brachiocephelic v.
Right brachiocephelic v.
Superior vena cava

Middle layer

Aotic arch and its three branches
Phrenic n.
Vagus n

Posterior layer

Trachea
Esophagus
Thoracic duct

Subdivision of mediastinum as seen on cross section

anterior mediastinum (1)
middle mediastinum (2)
posterior mediastinum (3)

Subdivision of the Mediastinum According to Anatomists

Subdivision of mediastinum as seen on sagittal section superior mediastinum (1)

anterior mediastinum (2)
middle mediastinum (3)
posterior mediastinum (4)

superior mediastinum (1)

upper boundary -- plane of the thoracic inlet (jugular notch, first rib, first thoracic vertebra)
lower boundary -- plane of sternal angle (from sternal angle to lower border of fourth thoracic vertebra
lateral boundaries -- mediastinal pleurae
anterior boundary -- manubrium of sternum
posterior boundary -- bodies of thoracic vertebrae 1 thru 4
Subdivision of the Mediastinum According to Anatomists


inferior mediastinum (2)
anterior
anterior -- body and xiphoid of sternum
posterior -- pericardium
lateral -- mediastinal pleura
superior -- plane of sternal angle
inferior -- diaphragm
middle
anterior -- pericardium
posterior -- pericardium
lateral -- mediastinal pleura
superior -- plane of sternal angle
inferior -- diaphragm
posterior
anterior -- pericardium
posterior -- bodies of thoracic vertebrae 5 - 12
lateral -- mediastinal pleura
superior -- plane of sternal angle
inferior -- diaphragm

The superior mediastinum

Borders
The superior mediastinum is wedge-shaped. The anterior boundary is the manubrium. The posterior boundary is much longer, due to the obliquity of the thoracic inlet. It consists of the bodies of the first four thoracic vertebrae; this wall is concave towards the mediastinum.

Superior mediastinum

Locating－from inlet of thorax to plane extending from level of sternal angle anteriorly to lower border of T4 vertebra posterioly
Contents
Superficial layer
Thymus
Three veins
Left brachiocephelic v.
Right brachiocephelic v.
Superior vena cava

• Aortic arch

• Continuation of ascending aorta
• Curves upward, to the left and posteriorly, then downward, arching over left principal bronchus and pulmonary trunk to lower border of T4 level, to become descending aorta
• Branches (from right to left )
• Brachiocephalic trunk－extends to right sternoclavicular joint, bifurcates into right subclavian and right common carotid arteries
• Left common carotid artery
• Left subclavian artery

Structures at the thoracic inlet

At the thoracic inlet the esophagus lies against the body of the first thoracic vertebra. The trachea lies on the esophagus and itself touches the jugular notch. These two tubes thus wholly occupy the midline of the inlet. At the inlet the apices of the lungs lie laterally, separated by vessels and nerves passing between the superior mediastinum and the neck.

Notes on the general topography of the superior mediastinum:

1. The superior mediastinum is in direct continuity with the anterior and posterior mediastinum and their separation from it is purely descriptive, not anatomical.
2. The plane of the sternal angle passes through the bifurcation of the trachea, the concavity of the arch of the aorta, and just above the bifurcation of the pulmonary trunk. On the plane the azygos vein enters the superior vena cava, the thoracic duct reaches the left side of the esophagus in its passage upwards from the abdomen. Also lying in the plane are the ligamentum arteriosum, and both superficial and deep cardiac plexuses.


3. The great veins and arteries of the superior mediastinum are asymmetrical. The veins are on the right, arteries on the left.
a midline Structures like trachea or bilateral like the apices of lungs or the phrenic and vagus nerves, thus have asymmetrical relationships on the right and left side.
4. Veins expand enormously, large arteries not at all, during increased blood flow. Thus there is much "dead space" on the right, none on the left, and it is into this space on the right side that tumors of the mediastinum or liquid collections tend to project.

5. The structures in the mediastinum form the medial relations of the lungs, being separated from them by the mediastinal pleura. Some of them make deep groove on the lungs. The left lung is intended by the left ventricle of the heart, the arch of the aorta, the subclavian artery and the left brachiocephalic vein, and perhaps lower part of the esophagus. Right lung carries impressions for the right atrium, subclavian artery and brachiocephalic vein, the superior vena cava, the azygos vein and the esophagus.

Contents

The superior mediastinum contains so many important structures that it is best to consider it in stages:.
Stage 1: the esophagus
The most posterior structure, closely related to the vertebrae (T1-T4), is the esophagus with the thoracic duct running up its left side. It is flattened anteroposteriorly. As it descends, it inclines slightly towards the left but is pushed back to the median plane by the arch of the aorta

Stage 2: the trachea

In front of the upper part of the esophagus is the trachea, which inclines slightly to the right and bifurcates at the level of the manubriosternal joint. Because of tracheal inclination, the right bronchus is more in line with the trachea than the left. The posterior surface of the trachea is flat where it is applied to the esophagus. It is kept patent by a series of C-shaped bars of cartilage.

Between the trachea and esophagus on the left side is the left recurrent laryngeal nerve, which comes from the vagus nerve and hooks under the ligamentum arteriosum. The arch of the aorta arches over the root of the left lung; the azygos vein arches over the root of the right lung.
In front of the tracheal bifurcation is the pulmonary trunk dividing into left and right pulmonary arteries. This has the appearance of a (T) with a sloping crosspiece.

The right pulmonary artery passes to the right lung behind the ascending aorta, superior vena cava and in front of the esophagus and right main bronchus. The left pulmonary artery goes to the left lung in front of the descending aorta and left main bronchus. The beginning of the left pulmonary artery is connected to the under surface of the aorta by the ligamentum arteriosum, a remnant of the fetal ductus arteriosus that short-circuited the functionless lungs by diverging most of the right ventricular outflow into the aorta. Also in front of the tracheal bifurcation are the tracheobronchial lymph nodes and the cardiac plexus.

The barchiocephalic trunk passes superolaterally to the right side of the trachea and the right sternoclavicular joint, where it divides into the right common carotid and right subclavian arteries. The arch of the aorta passes to the left of the trachea and esophagus, displacing the trachea to the right and constricting the esophagus.
The left phrenic nerve crosses the arch of the aorta in front of the vagus. The left superior intercostal vein crosses the arch from back to front, over the vagus and under the phrenic, relationships similar to those of the azygos vein on the right side, which is embryologically equivalent to it.


Since the arch is entirely behind the manubrium sterni, the left brachiocephalic vein is only just below the jugular (suprasternal) notch and is actually above it in children. The brachiocephalic veins, which arise posterior to sternoclavicular joints, unite to form the superior vena cava at the level of the inferior border of the first right costal cartilage.
The superior vena cava lies just to the right of the ascending aorta before opening into the right atrium at the level of the right 3rd costal cartilage. The only other tributary of the superior vena cava is the azygos vein. The brachiocephalic veins receive a number of tributaries including the left superior intercostal vein (into left brachiocephalic)
the inferior thyroid veins which come down from the neck in front of the trachea, the vertebral veins, and the internal thoracic veins.

Stage 3: the great arteries

The arch of the aorta passes backwards as well as to the left. The junction between the ascending aorta and the arch is at the level of the lower border of T4. Thus the whole arch is in the superior mediastinum. The major branches of the arch spiral around the trachea and esophagus (at first anterior then on either side); these are the brachiocephalic trunk (innominate), the left common carotid and the left subclavian arteries respectively.
The bronchial arteries to the lungs and the thyroidea ima artery to the thyroid gland may arise from the aortic arch.

Stage 4: the great veins

In the embryo the venous system is, at first, symmetrical but cross-connections drain most of the blood across the midline to the right. In the thorax the cross-channel is the left brachiocephalic vein. Hence both superior and inferior venae cavae are on the right and open into the right atrium.
Each brachiocephalic vein is formed by the junction of the corresponding subclavian and internal jugular veins; the left brachiocephalic crosses the midline just above the arch of the aorta.

Stage 5: The thymus gland

This important component of the lymphatic system lies behind the manubrium sterni but may extend up into the neck or down in the anterior mediastinum. It is molded around the great vessels and trachea but you may not be able to recognize it in the dissecting room since in adult life it is gradually replaced by fat. Because of the deposition of fat after puberty the pink color of the infant’s thymus changes to yellow. It reaches its largest size just before puberty but, relative to the adjacent structures, it appears at its largest about the time of birth. The rich arterial supply is derived mainly from the anterior intercostal and branches of the internal thoracic arteries. The veins end in the left brachiocephalic, internal thoracic, and inferior thyroid veins.

Inferior mediastinum

• Anterior mediastinum
Location－posterior to body of sternum and attached costal cartilages, anterior to heart and pericardium
Contents－fat, remnants of thymus gland, anterior mediastinal lymph nodes

The anterior mediastinum

The anterior mediastinum is very narrow and lies between the body of the sternum anteriorly and the fibrous pericardium posteriorly. It is continuous with the superior mediastinum at the sternal angle. It is limited inferiorly by the diaphragm.
The anterior mediastinum contains loose areolar tissue, fat, lymph vessels, two or three lymph nodes, sternopericardial ligaments, and a few branches of the internal thoracic artery. In infants and children, it may also contain the thymus gland.

• Middle mediastinum

Location－between anterior mediastinum and posterior mediastinum
Contents: heart and pericardium, beginning or termination of great vessels, phrenic nerves, pericardiacophrenic vessels , lymph nodes,

The posterior mediastinum

This contains esophagus, the descending aorta, the azygos venous system and the thoracic duct. It may be regarded as a duct leading from the neck and superior mediastinum to the abdomen so that all the structures mentioned above (in addition to the inferior vena cava) have to pass through the diaphragm.

• Posterior mediastinum

Location－posterior to heart and pericardium, anterior to vertebrae T5－T12
Contents: esophagus, vagus n., thoracic aorta, azygos system of veins, thoracic duct, thoracic sympathetic trunk, posterior mediastinal lymph nodes



Posteriorly－posterior esophageal plexus, thoracic aorta, thoracic duct, azygos v., hemiazygos .,accessory hemiazygos v., right posterior intercostal v.

Relations of thoracic aorta

Anteriorly－left root of lung, pericardium and esophagus
Posterior－ hemiazygos v., accessory hemiazygos v.,
Right－azygos v. and thoracic duct
Left－mediastinal pleura

Mediastinal spaces

Retrosternal space lies beween sternum and endothoracic fascia
Pretracheal space －lies within superior mediastinum, between trachea, bifurcation of trachea and aortic arch
Retroesophagus space － lies within superior mediastinum, beween esophagus and endothoracic fascia

Relations of esophagus

Anteriorly－trachea, bifurcation of trachea, left principal branchus, left recurrent n., right pulmonary a., anterior esophageal plexus, pericardium, left atrium, diaphragm

Relations of aortic arch

Anteriorly and to the left －pleura, lung，phrenic n., pericardiacophrenic vessels and vagus n.
Posteriorly and to the right－trachea, esophagus, left recurrent n., thoracic duct, deep cardiac plexus
Superiorly－its three branches, left brachiocephalic v. and thymus
Inferiorly－pulmonary a., arterial ligament, left recurrent n., left principal bronchus and superficial cardiac plexus

• Posterior mediastinum

Location－posterior to heart and pericardium, anterior to vertebrae T5－T12
Contents: esophagus, vagus n., thoracic aorta, azygos system of veins, thoracic duct, thoracic sympathetic trunk, posterior mediastinal lymph nodes



Left－left common carotid a., left subclavian a., aortic arch, thoracic aorta, superior part of thoracic duct
Right－arch of azygos v.