Superior Mediastinum Flashcards
Where does the superior mediastinum sit
Posterior to the manubrium of the sternum
Anterior to the first four thoracic vertebrae.
What is meant by the mediastinum
Is a thick midline partition that separates the two pleural cavities.
It extends from the superior thoracic aperture (inlet) to the inferior thoracic aperture and between the sternum anteriorly and the thoracic vertebrae posteriorly
It acts as a conduit for structures that pass through the thorax from one body region to another and for structures that connect thoracic organs to other body regions
List the contents of the mediastinum
trachea - from larynx to bifurcation into principal (right and left main) bronchi
oesophagus- from pharynx - muscular tube – pierces diaphragm at level of T10
heart and pericardium
thoracic duct - lymphatic drainage
nerves
great vessels
Describe the differences in the size of the heart using AP and PA X-rays
AP and PA X-rays are different views of the dimensions of the chest
If the structure is at the front of the chest, and the X-ray is at the back, X-rays will spread out before reaching the structure and X-rays close to the structure (not much difference in dispersion).
Results in different sizes
Hence heart appears larger on an AP X-ray
Why is the second rib not at T2 level
Ribs slope downwards- and so rib 2 articulates with T4/5.
What is the difference between the pharynx and larynx
Pharynx- G.I tract in throat
Larynx- airway in throat.
Ay which vertebral levels do structures pass through the diaphragm
T8- IVC
T10- Oesophagus
T12- Descending aorta
Describe the relevance of different vertebral levels in the thorax
T8,10,12- extent of pleural cavity
T6,8,10- extent of lungs
Describe the divisions of the mediastinum
Superior: above sternal angle
Inferior: below sternal angle to the inferior thoracic aperture (further divided):
Anterior: anterior to heart in pericardial sac
Middle: pericardial sac & heart
Posterior: posterior to pericardial sac and diaphragm
List the contents of the superior mediastinum
From anterior to posterior: Thymus Phrenic Nerves Great veins Main lymphatic trunks Vagus nerves Great arteries Trachea and main bronchi Upper oesophagus
Describe the boundaries of the mediastinum
Superior boundary- oblique plane passing from the jugular notch upward and posteriorly to the superior border of T1
Inferiorly- Transverse plane passing from the sternal angle to the intervertebral disc between T4/5
Laterally- mediastinal part of the parietal pleura.
Describe the great veins found in the superior mediastinum
The SVC is vertically orientated and begins posterior to the lower edge of the right first costal cartilage and terminates at the lower right third costal cartilage where it joins the right atrium.#
The lower half of the SVC is within the pericardial sac and therefore is contained in the middle mediastinum.
Inferior Vena Cava (IVC) enters right atrium from below, through central tendon of diaphragm- the heart sits on the central tendon.
Most of the IVC is present in the abdomen, once it passes through the central tendon it is in the right atrium.
Describe the formation of the SVC
formed by asymmetric union of right and left brachiocephalic veins, of which each forms an internal jugular (from head) and subclavian vein (from upper limbs); left brachiocephalic crosses posterior to the manubrium to join the right and form the SVC.
The union is on the right- not the centre.
Left subclavian- follows oblique course of sternoclavicular joint.
Where do the veins sit in relation to the arteries
More anteriorly
Describe the venous drainage of the anterior chest wall
Anterior intercostal veins drain into the internal thoracic vein which connect with the brachiocephalic veins in the neck.
Describe the venous drainage of the posterior chest wall
Drains the posterior wall of the thorax and abdomen
Arches over the right lung root
Drains into the SVC
has many branches (posterior intercostal veins) at the posterior of the chest next to vertebrae [asymmetric, with most on right side of the body - only accessory hemiazygous veins (above) and hemiazygous vein (below) on left)
Upper posterior regions may drain into left and right supreme intercostal veins.
left empties into left brachiocephalic
right empties into azygos.
What are the arteries of the superior mediastinum
Ascending aorta
Arch of aorta
Descending aorta
What are the branches of the ascending aorta
left/right coronary arteries branch from superior of the aortic valve to ensure heart gets best blood supply
Where do the left hemiazygos and accessory veins drain
Into the right atrium
Describe the branches of the aortic arch
Brachiocephalic trunk – divides into right common carotid and right subclavian arteries
Left common carotid artery
Left subclavian artery
Describe the course of the ascending aorta
Only the arch of the aorta is in the superior mediastinum.
It begins when the ascending aorta emerges from the pericardial sac and courses upward, backward and to the left as it passes through the superior mediastinum, ending at the vertebral level T4/5,
It can extend as high as the midline of the manubrium and sternum.
The arch is initially anterior and finally lateral to the trachea.
Describe the relations of the aorta and great arteries to the airway
Aortic arch arises anterior to trachea
Arches over the left main bronchus at the lung root
Trachea lies behind & between brachiocephalic and left common carotid arteries
trachea lies behind and between brachiocephalic and left common carotid arteries
Describe the common carotids
divide to external and internal carotids high in neck, supplying the head and neck alongside the vertebral arteries from the subclavian arteries
Describe the descending aorta
Aorta higher at front than back descending aorta (after the arch, running down back of chest to abdomen (exiting at T12))
Describe the pulmonary trunk
outflow of right ventricle, travelling backwards to get behind aortic arch and carrying deoxygenated blood to lungs via left/right pulmonary arteries
Describe the ligamentum arteriosum
Connects the pulmonary trunk with the arch of the aorta and allows blood to bypass the lungs during development of the foetus.
Closes soon after birth and forms the ligamentous connection seen in the adult.
Ligamentum arteriosum connects PT to aortic arch. Is remnant of the ductus arteriosus – bypasses lungs in foetal life.
Describe the phrenic nerves
Formed in the cervical plexus from C3, 4, 5
Motor to the diaphragm
Sensory to:
central tendon of the diaphragm
mediastinal pleura
pericardium (fibrous and parietal layer of serous pericardium)
peritoneum of central diaphragm
