Chapter 5: Other mediastinal structures The Large Arteries The Aorta The aorta is the main arterial trunk of the systemic circulation and in the healthy state its wall contain a large amount of yellow elastic tissue. It arises from the left ventricle and ascends for a short distance before arching backwards over the root of the left lung and descending through the thorax and abdomen. It is divided into an ascending part, an arch, a descending thoracic part and an abdominal part. Ascending aorta This is a wide vessel about 5 cm long. It begins at the aortic orifice and ascends to the right around the pulmonary trunk as far as the level of the sternal angle. Above each of the semilunar folds (valvules) of the aortic valve is a dilatation, an aortic sinus. It lies within the fibrous pericardium enclosed in a sheath of serous pericardium which is common to it and the pulmonary trunk. The lower part lies behind the infundibulum of the right ventricle and the beginning of the pulmonary trunk and above this it is related to the sternum. Posteriorly from below upwards are the left atrium, right pulmonary artery and the right main bronchus. To the left lie the left auricle and the pulmonary trunk; to the right, the right auricle and the superior vena cava. Right and left coronary arteries from the anterior and left posterior sinuses. Arch of the aorta This part of the aorta passes upwards behind the manubrium from the level of the sternal angle. It arches backwards and to the left over the root of the left lung and descends to reach the left side of the 4th thoracic vertebra. To its left lie the mediastinal pleura and lung, the left phrenic nerve in front and vagus behind. To its right lie the superior vena cava with the right phrenic nerve on its right side, the trachea and left recurrent laryngeal nerve, the oesophagus and thoracic duct and then the 4th thoracic vertebra, from before backwards. Inferiorly it crosses, from before backwards, the bifurcation of the pulmonary trunk, the ligamentum arteriosum, and the left main bronchus. Superiorly are its three branches, crossed anteriorly by the left brachiocephalic vein. The arch is connected inferiorly to the left pulmonary artery by the ligamentum arteriosum, 1
a fibrous remnant of the ductus arteriosus. The left recurrent laryngeal nerve passes posteriorly around the ligamentum and the arch. The superficial part of the cardiac plexus lies anterior to the ligamentum and the deep part of the plexus inferior to the arch of the aorta. Remnants of the thymus gland may be found in front of the arch. (i) the brachiocephalic artery is the first branch of the arch. It arises behind the manubrium and ascends, passing backwards and to the right as far as the right sternoclavicular joint where it divides into right subclavian and right common carotid arteries. : anteriorly the left brachiocephalic vein and thymus separate it from the manubrium. Posteriorly lies the trachea. To its right lies the right brachiocephalic vein and the superior vena cava and to its left the left common carotid artery. It has only terminal branches, the right subclavian and the right common carotid arteries. (ii) the left common carotid artery. (iii) the left subclavian artery. Descending aorta This descends from the left side of the 4th thoracic vertebra inclining medially to the front of the 12th vertebra where it passes through the diaphragm to become the abdominal aorta. Anteriorly, from above downwards, are the root of the left lung, the left atrium separated by pericardium, the oesophagus and the diaphragm. Posteriorly it lies on the vertebral column and hemiazygos veins. Its left side is in contact with the left pleura and lung and its right with the oesophagus above and the right lung and pleura below. The thoracic duct the azygos vein lie to its right side. (i) 3rd-11th posterior intercostal arteries and the subcostal artery. (ii) two or three bronchial arteries. (iii) four or five oesophageal arteries. (iv) mediastinal and diaphragmatic arteries. 2
The pulmonary trunk This wide vessel, about 5 cm long, begins at the pulmonary orifice, ascends posteriorly on the left side of the aortic trunk and ends by bifurcating into right and left pulmonary arteries in the concavity of the aortic arch. It lies in front of the transverse sinus and is contained within a common sleeve of serous pericardium with the ascending aorta. The right and left coronary arteries and the two auricles surround its base and it is covered anteriorly on its left by the left pleura and lung. (i) the right pulmonary artery passes to the hilus of the right lung behind the ascending aorta and superior vena cava, and in front of the oesophagus and right main bronchus. (ii) the left pulmonary artery passes in front of the left bronchus and the descending aorta to the hilus of the lung. It is connected by a short fibrous band, the ligamentum arteriosum, to the lower surface of the aortic arch. In the lung these arteries and their branches accompany the bronchi and bronchioles. The large veins The pulmonary veins are short wide vessels passing to the left atrium, usually two (upper and lower) from each lung. In the hilus of the lung they lie below and in front of the artery. On the right the upper passes behind the superior vena cava, the lower behind the right atrium. On the left both veins pass in front of the aorta. The brachiocephalic veins are formed behind the sternoclavicular joints by the union of the corresponding internal jugular and subclavian veins. After a short course they unite to form the superior vena cava about the middle of the right border of the manubrium. The right brachiocephalic vein is about 3 cm long. It descends vertically behind the right border of the manubrium, anterolateral to the brachiocephalic artery. The right phrenic nerve descends along its lateral surface between it and the pleura. The left brachiocephalic vein is about 6 cm long. It descends obliquely behind the manubrium crossing above the arch of the aorta and in front of its three large branches and the trachea. Thymic remnants may lie anteriorly. Tributaries Corresponding to some branches of the subclavian artery, viz: (i) vertebral vein - drains neck muscles at the back and vertebral column. (ii) inferior thyroid veins - usually unite in front of the trachea, and their common trunk enters the left brachiocephalic vein. 3
(iii) internal thoracic vein - drains anterior chest wall and diaphragm. (iv) additionally, the thoracic duct enters the beginning of the left vein; the right lymph duct enters the beginning of the right vein. The left superior intercostal vein runs across the aortic arch to enter the left brachiocephalic vein. The superior vena cava is a wide vessel about 7 cm long. It is formed by the union of the two brachiocephalic veins at the middle of the right border of the manubrium and descends vertically behind the manubrium and body of the sternum to enter the right atrium at the level of the 3rd right costal cartilage. It possesses no valves. In its lower half it is covered by the fibrous and serous pericardium. Anterior are the right lung and pleura, and the manubrium. Posterior is the right lung root. Medal are the ascending aorta and the brachiocephalic artery. Lateral are the right phrenic nerve and the pleura and the lung. The azygos vein is its only tributary, entering it posteriorly about its middle. Determination of the right atrial pressure is important in the management of shocked patients' fluid needs. A catheter can be passed into the internal jugular or subclavian vein and thence into the superior vena cava and the right atrium. The azygos vein is formed in the abdomen in front of the 2nd lumbar vertebra either as a branch of the inferior vena cava or by the union of the right subcostal and ascending lumbar veins. It leaves the abdomen through the aortic opening of the diaphragm. In the thorax it lies to the right of the aorta and thoracic duct and ascends on the right of the vertebral bodies covered by pleura. At the level of the 4th thoracic vertebra, it arches forwards over the hilus of the right lung and enters the superior vena cava. Above the hilus it lies between the lung and pleura sac laterally and the oesophagus and trachea medially. (i) right ascending lumbar vein. Tributaries (ii) 4th-11th right posterior intercostal veins and subcostal vein. (iii) right superior intercostal vein. (iv) right bronchial veins. (v) accessory hemiazygos and hemiazygos veins - these lie on the left side of the vertebral column and receive the 4th-11th left intercostal veins. They pass to the right in front of the mid-thoracic vertebrae to open separately into the azygos vein. The left ascending lumbar vein continues as the hemiazygos vein. The inferior cava is a wide vessel which pierces the central tendon of the diaphragm and, after a short intrathoracic course, enters the lower part of the right atrium. 4
Other structures The phrenic nerves root. The right and left phrenic nerves descend through the thorax, in front of each lung The right phrenic nerve enters the thorax on the lateral side of the right brachiocephalic vein and in its descent it lies on the pericardium covering superior vena cava, the right atrium and the inferior vena cava. It passes through the caval opening in the diaphragm. Throughout its course it is covered laterally by mediastinal pleura. The left phrenic nerve enters the thorax in front of the left subclavian artery, behind the left brachiocephalic vein. In its descent it crosses the aortic arch and the pericardium covering the left ventricle and reaches and pierces the diaphragm. Throughout its course it is covered laterally by mediastinal pleura. Both nerves supply the diaphragm and also give sensory branches to the mediastinal and diaphragmatic pleura, to the pericardium, and to the peritoneum. Pain from inflammation of the diaphragmatic pleura or peritoneum may be referred to the deltoid region of the shoulder which is innervated by the supraclavicular nerves, mainly derived like the phrenic nerve, from the C4 segment of the spinal cord. The vagus nerves Right vagus nerve This enters the thorax posterolateral to the right brachiocephalic artery. It descends between the trachea medially, and the mediastinal pleura, the lung and the azygos vein laterally, to the back of the right main bronchus where it gives numerous branches to the pulmonary plexus before proceeding to the oesophageal plexus. Left vagus nerve This enters the thorax between the left common carotid and subclavian arteries, behind the left brachiocephalic vein. It descends inclining posteriorly across the left side of the aortic arch, giving off its recurrent laryngeal branch before passing medially behind the root of the lung to the oesophagus. The two vagi form the oesophageal plexus. From this plexus emerge anterior and posterior gastric nerves containing fibres of both vagi and sympathetic nerves. They descend through the oesophageal opening of the diaphragm. The anterior nerve supplies the front of the stomach, the duodenum, pancreas and liver; the posterior supplies the back of the stomach and gives branches which pass through the coeliac plexus to other abdominal viscera. (i) left recurrent laryngeal nerve - winds around the ligamentum arteriosum and aortic arch and then ascends in the groove between the trachea and oesophagus into the neck. 5
(ii) cardiac branches - to the cardiac plexus. (iii) branches to the pulmonary and oesophageal plexuses. The thymus This bilobed mass of lymphoid tissues lies in front of the trachea in the root of the neck and upper thorax. It is large at birth but atrophies after puberty. variable in size, it may extend down beyond the aortic arch lying in front of the brachiocephalic veins and the left common carotid artery in the mediastinum. It is supplied by branches of the inferior thyroid and internal thoracic arteries and its veins drain to the internal thoracic and left brachiocephalic veins. Histology and embryology It is lobulated and formed of numerous cells embedded in a reticular network of fine collagen fibres. In the outer denser cortex, lymphocytes are predominant, the inner medulla contains paler round cells, thymocytes, and nests of concentrically arranged cells - Hassall's corpuscles. It is derived from the 3rd pair of pharyngeal pouches, and parathyroid tissue may be embedded in it. The oesophagus The oesophagus descends through the thorax mainly to the left of the midline, curves forwards in its lower part away from the vertebral column and pierces the diaphragm at the level of the 10th thoracic vertebra. In the upper part of the mediastinum it lies between the vertebral column posteriorly and the trachea anteriorly; the left recurrent laryngeal nerve lies in the groove between it and the trachea. Below, it is separated from the vertebral column, from above downwards, by the thoracic duct, the hemiazygos veins, the right posterior intercostal arteries and the descending thoracic aorta. Anteriorly below the trachea, it is crossed by the left bronchus, and below this it is separated by pericardium from the left atrium. On the right it is covered with mediastinal pleura and the azygos vein arches forwards above the lung root. On the left it is separated from the mediastinal pleura above the left subclavian artery, the thoracic duct, the aortic arch and descending aorta. Its lower part is in contact with the pleura. Radiological examination of the oesophagus can be achieved by having the patient swallow a radiopaque fluid. It will be seen to be constricted by the aorta and, in some patients with heart failure, by the left atrium. A more direct examination of any abnormality thus revealed can be achieved by a flexible light-carrying endoscope. The trachea and bronchi The trachea lies in the midline anterior to the oesophagus, the left recurrent laryngeal nerve ascending in the groove between them. Anteriorly it is crossed by the brachiocephalic 6
artery and then by the left brachiocephalic vein. On its left lie the common carotid and subclavian arteries above and the aortic arch below. On its right are the mediastinal pleura, the right vagus nerve and the azygos vein. The tracheal bifurcation, the carina, is at the level of the sternal angle and the lower border of the 4th thoracic vertebra. It lies anterior to the oesophagus, behind and to the right of the bifurcation of the pulmonary trunk. It is separated from the right pulmonary artery by the deep part of the cardiac plexus and tracheobronchial lymph nodes. The trachea lengthens slightly on inspiration and recovers on expiration because of the contained elastic tissue. The extrapulmonary bronchi: the main bronchus on each side arises at the bifurcation and descends passing laterally, enters the hilus of the lung where it divides, forming the intrapulmonary bronchial tree. The right bronchus is about 3 cm long. It is wider and somewhat more vertical than the left and foreign bodies therefore tend to enter it more frequently. The right upper lobe bronchus arises from it just before it enters the hilus. : anteriorly to the right pulmonary artery separates it from the pericardium and the superior vena cava. The arch of the azygos vein is above it and posteriorly lies the pulmonary plexus and the bronchial vessels. The left bronchus is about 5 cm long. : anterior to the left pulmonary artery separates it from the left atrium. The arch of the aorta is above it, and posteriorly lies the pulmonary plexus and bronchial vessels which separate it from the oesophagus and descending thoracic aorta. The structure of the extrapulmonary bronchi is similar to that of the trachea. Direct viewing of the trachea and proximal main bronchi is possible with a bronchoscope. Instillation of a radiopaque contrast medium, Lipiodol, into the bronchi, permits radiographs to show the arrangement of the distal smaller bronchi. Bronchial cancer is one of the commonest malignant diseases and readily spreads to the tracheobronchial lymph nodes. It can produce early symptoms of bronchial destruction. 7