Superior & Posterior Mediastinum Flashcards
What is the mediastinum?
The interval between pleural sacs, which extends from thoracic inlet to diaphragm
What line demarcates superior from inferior mediastinum? What does it pass through?
Imaginary line drawn between sternal angle and disc between TV4 and TV5
Passes through bifurcation of trachea, concavity of arch of aorta, and just above pulmonary trunk
What are the subdivisions of the inferior mediastinum and what do they contain?
Anterior mediastinum - posterior to body of sternum, but anterior to heart. Contains fat, remnants of thymus gland, and lymph nodes
Middle mediastinum - contains heart, beginning or termination of great vessels, and pericardium
Posterior mediastinum - posterior to heart and pericardium, anterior to T5-T12. Contains thoracic aorta, esophagus, thoracic duct, vagus + sympathetic nerves, veins, and lymphatics
What two mediastinum are in direct continuity with the superior mediastinum?
Anterior and posterior mediastinum
What is the location of the superior mediastinum and what are its principle contents?
Location: Behind manubrium, in front of TV1-4
Contents:
- Arch of aorta and the roots of its branches
- Brachiocephalic veins and SVC
- Vagus + phrenic nerves, left recurrent laryngeal (around aorta) and sympathetics.
- Trachea, esophagus, thoracic duct
- Thymus gland
What is the path of the aorta from the ascending to thoracic?
Ascending aorta emerges from pericardial sac, extends to sternal angle and continues as arch. The arch runs upward, to the left in front of the trachea, turns backward and down over the left main bronchus + pulmonary trunk.
Arch is in almost a sagittal plane behind the lower part of the manubrium, at it continues as thoracic aorta below TV4
What are the three branches of the arch of aorta?
- Brachiocephalic trunk (right, first branch)
- Left common carotid artery
- Left subclavian artery
What are the branches of the brachiocephalic artery?
- Inconstant thyroid imea
- Right subclavian
- Right common carotid - terminal
What are the brachiocephalic veins (BCVs) formed by? What do they form?
Formed by: Internal jugular vein + subclavian vein
Form: Superior vena cava, after the left brachiocephalic vein crosses to the right behind the manubrium and joins right BCV
What is the important relationship of the right vagus nerve + right recurrent laryngeal?
Goes anterior to first part of subclavian artery, but right recurrent laryngeal loops posterior to the subclavian. Vagus will ultimately go posterior to the root of the lung
What is the important relationship of the left vagus nerve + left recurrent laryngeal?
Descends anterior to arch of aorta and posterior to root of lung.
Left recurrent laryngeal: leaves vagus at arch of aorta and hugs behind the arch to the LEFT of ligamentum arteriosum, then ascends at the right side of the arch between trachea and esophagus
What is the path of the right and left phrenic nerves?
Right: lateral surface of SVC, anterior to roots of lungs between fibrous pericardium + mediastinal pleura, accompanied by pericardiacophrenic artery of ITA.
Left: Crosses arch of aorta, anterior to roots of lungs between fibrous pericardium + mediastinal pleura, accompanied by pericardiacophrenic artery of ITA
What is the function of the phrenic nerve, and where does it come from?
Comes from C3-C5
Function: motor to diaphragm, sensory to pericardium + diaphragmatic pleura
Where is the thymus and what happens to it? What is its blood supply?
It is a bilobar organ of the superior mediastinum which often extends into anterior mediastinum and neck. It is behind the sternum and anterior to arch of aorta + major vessels
Blood supply: Internal thoracic artery
It is important for developing immune system, but begins to regress after puberty
What are the main contents of the posterior mediastinum?
- Thoracic aorta + esophagus
- Vagal nerves + Sympathetic trunks + branches
- Thoracic duct + lymph nodes
- Azygos venous system
What is the path of the descending thoracic aorta until it terminates?
Starts to the left of midline at TV4, the slants to the right until reaching midline at TV12, where it passes through the aortic hiatus of the diaphragm
What are the parietal branches of the thoracic aorta?
- Lower 9 pairs of posterior intercostal arteries (First 2 are from costocervical trunk off subclavian)
- Subcostal arteries (last pair, under 12th rib)
- Superior phrenic artery - supplies diaphragm
Where does the artery of Adamkiewicz arise and what is its function?
Arises from one of the lower posterior intercostal arteries on the left side. It is the major blood supply of the lower spinal cord, damaging it can cause paraplegia
What are the visceral branches of the aorta?
The bronchial arteries from the upper aorta
Also has branches to esophagus, pericardium, and mediastinum
What is the aortic isthmus and why is it susceptible to injury?
Part of aorta between the left subclavian artery and ligamentum arteriosum. It is susceptible the LSA and the rest of the aortic arch will move forward during deceleration, while the thoracic aorta will be held back by ligamentum arteriosum, which can rupture it.
Where is the ligamentum arteriosum?
Connects to pulmonary trunk and arch of aorta immediately distal to the origin of the left subclavian artery
How might the left recurrent laryngeal be impinged and how will the patient present?
Abnormal dilation or aneurysm of arch of aorta, or by enlarged lymph nodes in this area. Patient will present with hoarseness (supplies muscles of larynx)
What is the path of the esophagus?
Extends from cricoid cartilage (CV6) to cardiac orifice of stomach (TV10).
Descends on anterior surface of thoracic vertebrae to the right of the aorta, til it crosses anterior / to the left of the aorta in the lower part of the posterior mediastinum. Enters abdomen through esophageal hiatus of diaphragm at TV10
What structures are anterior to the esophagus as it descends in the superior through posterior mediastinum?
Left main bronchus, left atrium, diaphragm
What structures are posterior to the esophagus as it descends in the superior through posterior mediastinum?
Thoracic duct, right posterior intercostal arteries, aorta near diaphragm (where it crosses), veins
What is the arterial supply to the esophagus?
Superior to inferior: Inferior thyroid artery, thoracic aorta, left gastric artery
What is the venous drainage of the esophagus?
Superior to inferior: Inferior thyroid veins, azygos system, left gastric vein (coronary)
What does communication between the coronary vein (LGV) and the azygos system create? Why is this important?
Communication between portal system and systemic veins. Portal hypertension can cause blood to black up and distend the esophageal veins, which can bleed profusely
What is the nerve supply of the esophagus? Which nerves form it? What becomes of those nerves?
Esophageal plexuses of vagus nerve, which supplies muscle and glands (PANS to increase peristalsis and secretion)
Anterior plexus: Mostly left vagus
Posterior plexus: Mostly right vagus
They converge afterwards to form the anterior and posterior vagal trunks
Where are the three main sites where foreign material can get lodged in the esophagus?
- Upper esophageal sphincter - Beginning of esophagus formed by cricopharyngeus muscle
- Where arch of aorta + left main bronchus cross the esophagus
- Lower esophageal sphincter - esophageal opening in diaphragm
What is the upper esophageal sphincter?
High pressure zone that prevents air entering esophagus during breathing, also prevents reflux of esophageal contents into pharynx to guard against airway aspiration
What is the function of the cricopharyngeus muscle?
Major contributor to high pressure zone of upper esophageal sphincter. It is the lower belly of the inferior pharyngeal constrictor
What is the origin of the thoracic duct?
Begins as lymphatic sac between aorta and right crus of diaphragm before passing upward through aorta hiatus of diaphragm into the posterior mediastinum
Where does the thoracic duct go after entering the posterior mediastinum?
Ascends behind esophagus to TV5-6, where it cross to the left (it is the left lymphatic duct) and enters root of neck. It goes laterally behind the carotid sheath and enters the junction of the left subclavian and internal jugular veins.
What is the function of the thoracic duct?
Drains lymph of the whole body exp from the ride side of the head and neck, right upper limb, and right thorax (that is the right lymphatic duct’s job)
What three veins make up the azygos system of veins?
- Azygos vein (right)
- Hemiazygos vein (left)
- Accessory hemiazygos vein (left)
What is the course of the azygos vein? What does it receive blood from?
Begins in abdomen by junction of right subcostal and right ascending lumbar vein. Ascends posterolateral to the esophagus, arching superiorly (from behind) over the root of the right lung and entering superior vena cava.
Receives right posterior intercostal and hemiazygos veins
What is the course of the hemiazygos vein? What does it receive blood from?
Formed by union of left subcostal and left ascending lumbar veins. It crosses about TV10 to enter the azygos vein
Receives blood from left posterior intercostals and sometimes acessory hemiazygos
What is the course of the accessory hemiazygos vein and what does it receive blood from?
It joins the hemiazygos vein or cross to enter the azygos vein
Receives blood from the upper left-side posterior intercostal veins
What is the thoracic sympathetic trunk and where is it located?
It is a trunk of 12 ganglia connected to thoracic spinal nerves by grey and white rami.
Located on necks of ribs - most laterally placed structure in posterior mediastinum
What forms the stellate ganglion?
The inferior cervical ganglion fused to first thoracic ganglion
What nerves are most evident in the posterior mediastinum sympathetics dissection? How do they course
Splanchnic nerves - destined for abdomen and GI tract. Include the greater, lesser, and least, and they are preganglionic fibers which arise through
Splanchnic nerves course medial to sympathetic trunks on bodies of vertebrae and pierce the crura (plural of crus) of diaphragm to enter abdomen
Where are the lymph nodes of the mediastinum?
Prominent along ITA, hilum of lung, main bronchi, and bifurcation of trachea. This all drains into the thoracic duct or right lymphatic duct
What is the lower esophageal constrictor?
A specialized thickened region of circular muscle of distal esophagus, generates a tonic pressure higher than the intragastric pressure which stops reflux of gastric contents
What is the cysterna chyli?
The sac at the end of the thoracic duct
Contains yellowish fluid with chyle containing fatty lymphatics from the gut
Why do you place a central line on the right side?
To not risk interfering with the thoracic duct at the junction of IJV and subclavian vein
