Mediastinum Flashcards
What purpose does the mediastinum serve?
The mediastinum separates not only the thoracic cavity into three compartments but also keeps the two pleural cavities separated.
How is the mediastinum divided?
The mediastinum is divided at the plane between the sternum/manubrium junction and the IV disk at level T4/T5 into the superior and inferior mediastinum. The inferior mediastinum is further divided.
What structures does the superior mediastinum contain?
The structures at the posterior aspect of the superior mediastinum often continue down into the posterior aspect of the inferior mediastinum as well.
From superficial --> deep, the superior mediastinum contains: - The thymus (if it is present) - The great veins (immediately behind the manubrium) Above the heart: - The aortic arch - The vagus and phrenic nerves Further back and behind the heart: - The trachea - The oesophagus - The thoracic duct - The L recurrent laryngeal nerve
What structures are found in the anterior inferior mediastinum?
Anterior Mediastinum
The anterior mediastinum contains the thymus; which involutes in adolescence and so anatomically, the ant. mediastinum generally only contains fat and several lymph nodes.
What structures are found in the middle inferior mediastinum?
Middle Mediastinum
The middle mediastinum contains the heart and the roots of the great vessels enclosed in the pericardial sack.
What structures are found in the posterior inferior mediastinum?
Posterior Mediastinum
Continuations from the superior posterior mediastinum. It also contains the azygous system of veins which is likely to be the most variable of anatomy in the body, running along the posterior thoracic wall.
Give a general explanation about the great veins in the superior mediastinum.
The Great Veins
Initially there is a symmetrical arrangement of veins however as it continues further inferiorly towards the heart, the veins tend more towards the right where the receiving chambers are and the arteries tend towards the left chambers.
Initially we have an internal jugular vein which drains the head and neck (R&L) and a subclavian vein draining the upper limb (R&L). These two veins meet behind the sternoclavicular joint. This junction of the two veins is now called the brachiocephalic vein (R&L). The left brachiocephalic vein has a lower, more horizontal course that runs behind the manubrium across the trachea. The right brachiocephalic vein has a shorter, more vertical course. These two brachiocephalic veins unite to form the SVC behind the first right coast cartilage which enters into the RA from above.
The SVC also receives the azygous vein which has collected blood from the thorax (deep). It empties into the posterior aspect of SVC at the level of the second right coastal cartilage.
It then enters the RA at the level of the third right costal cartilage.
If a vein needs to be inserted with a wide bore canula then it is usually done through the IJV or the subclavian. The IJV is preferred as it is further from the apex of the lung, and less likely to cause a pneumothorax and if it starts to bleed it is far easier to apply pressure. It is also done under ultrasound guidance and it is very difficult to screw up.
Give a general explanation about the great veins in the superior mediastinum.
The Great Veins
Initially there is a symmetrical arrangement of veins however as it continues further inferiorly towards the heart, the veins tend more towards the right where the receiving chambers are and the arteries tend towards the left chambers.
Initially we have an internal jugular vein which drains the head and neck (R&L) and a subclavian vein draining the upper limb (R&L). These two veins meet behind the sternoclavicular joint. This junction of the two veins is now called the brachiocephalic vein (R&L). The left brachiocephalic vein has a lower, more horizontal course that runs behind the manubrium across the trachea. The right brachiocephalic vein has a shorter, more vertical course. These two brachiocephalic veins unite to form the SVC behind the first right coast cartilage which enters into the RA from above.
The SVC also receives the azygous vein which has collected blood from the thorax (deep). It empties into the posterior aspect of SVC at the level of the second right coastal cartilage.
It then enters the RA at the level of the third right costal cartilage.
If a vein needs to be inserted with a wide bore canula then it is usually done through the IJV or the subclavian. The IJV is preferred as it is further from the apex of the lung, and less likely to cause a pneumothorax and if it starts to bleed it is far easier to apply pressure. It is also done under ultrasound guidance and it is very difficult to screw up.
Explain general features of the aorta?
The aorta arches upwards, backwards and to the left.
The ascending aorta is the first portion after the heart and is where the first branches originate (coronary arteries).
At the level of the plane of the manubriosternal junction, it changes into the arch of the aorta. The arch continues until it is adjacent to the the T4/T5 disk.
When it is on the left of this disk it become the Descending aorta.
Explain general features of the aorta?
The aorta arches upwards, backwards and to the left.
The ascending aorta is the first portion after the heart and is where the first branches originate (coronary arteries).
At the level of the plane of the manubriosternal junction, it changes into the arch of the aorta. The arch continues until it is adjacent to the the T4/T5 disk.
When it is on the left of this disk it become the Descending aorta.
What are some features of the pulmonary vessels in the superior mediastinum?
The bifurcation of the pulmonary artery (R&L) lies within the concavity of the aorta. The left lung root is also situated in the concavity of the aortic arch.
There is a fibrous communication between the underside of the aorta and the bifurcation of the pulmonary trunk–> this is the ligamentum arteriosum. It is a remnant of what was a duct in the embryological development so as to avoid the pulmonary circulation.
There are three large branches of the aortic arch which supply the head, neck and upper limb on each side. The first branch is the brachiocephalic trunk heads to the right of the trachea and behind the right sternoclavicular joint and the brachiocephalic vein.
What are some of the branches of the aorta as it leaves the superior mediastinum?
This artery branches into the right common carotid and the right subclavian arteries.
The second and third branches are the Left common carotid and the left subclavian artery which branch separately. The left subclavian is deeper to the other branches.
This is the pattern for around 70% of aortas and there can be many variations –> R subclavian may branch last and go behind the oesophagus and cause difficulty swallowing etc.
What are some general features of the Phrenic nerve?
Phrenic Nerve formed from the ventral rami of C3,4,5 –> keep the diaphragm alive!
It travels through the neck and into the mediastinum. In the neck it descends onto a muscle called scalerus anterior and passes between the subclavian vein and artery on each side.
It will always pass anterior to the lung root structures and it is always the most lateral structure in the mediastinum.
The Right phrenic nerve is lateral to venous structures SVC, Atrium and IVC, peircing the diaphragm with the IVC . The left phrenic nerve is lateral to the arterial structures aortic arch and LV and peirces the diaphragm at the apex of the heart by itself.
The phrenic nerve also pierces the diaphragm because it supplies the underside of the diaphragm. The motor supply to the diaphragm is from the phrenic nerve on its abdominal surface. The phrenic nerve is also supplying sensory fibres to the mediastinum, diaphragmatic, pleural and pericardial surfaces.
What are some features of the vagus nerve?
Vagus Nerve
The vagus nerve is a large cranial nerve and as such it arises in the cranial cavity and travels into the neck and then into the thorax and then into the abdomen.
It runs posterolateral to the common carotid artery.
On each side of the neck, there is a neurovascular bundle to supply the head and neck (CCA, IJV and Vagus nerve).
The vagus nerve is aiming to run medially and along the midline. It enters the diaphragm with the oesophagus through the oesophageal hiatus in the muscular part of the diaphragm (T10).
The Right vagus nerve travels with the IJV and CCA and tucks in next to the trachea behind the lung root and then along the anterior oesophagus. On the front of the oesophagus, the nerve ramify and become an oesophageal plexus
–> contributes to pulmonary and cardiac plexuses along the way (parasympathetic activity)
The Left vagus nerve ideally would travel the same path but the aorta is in the way and so it cannot get in to the trachea. So it runs lateral to the aortic arch before traveling posterior to the phrenic nerve, crossing the superior intercostal veins and giving rise to the Left recurrent laryngeal nerve. it then travels behind the lung root and attaches to the anterior oesophagus.
The laryngeal nerves run in the tracheal oesophageal grooves.
Explain the features of the oesophagus and the trachea?
Trachea intended from the superior inlet, and it bifurcates (level T4/T5) in the superior mediastinum and doesn’t extend any further inferiorly into the mediastinum as the trachea. It has C shaped cartilages that are designed to keep it open.
The oesophagus is behind the trachea and is a flattened muscular tube, with the vertebral column behind. It travels all the way into the abdomen where it meets the stomach (it travels through superior and posterior mediastinum). The oesophagus moves with its decent slightly left of the midline and pierces the diaphragm to the left of the midline in front of the aorta.
Like any muscular tube, the oesophagus has narrowings. It has a superior narrowing where it enters the pharynx; inferiorly it has a narrowing where it passes through the diaphragm and an aortic narrowing (intrinsic) and a tracheal narrowing from compression of those structures.
