The Mediastinum-The Pericardium

The Mediastinum

The mediastinum is defined as ‘the space which is sandwiched between the two pleural sacs’. For descriptive purposes the mediastinum is divided by a line drawn horizontally from the sternal angle to the lower border of T4 (angle of Louis) into superior and inferior mediastinum.

The inferior mediastinum is further subdivided into the anterior in front of the pericardium, a middle mediastinum containing the pericardium itself with the heart and great vessels, and posterior mediastinum between the pericardium and to lower eight thoracic vertebrae (Fig. 22).

The Pericardium

The heart and the roots of the great vessels are contained within the conical fibrous pericardium, the apex (TOP) of which is fused with the adventitia (THE OUTERMOST CONNECTIVE TISSUE COVERING OF ANY ORGAN OR VESSEL) of the great vessels and the base with the central tendon of the diaphragm.

Anteriorly it is related to the body of the sternum, to which it is attached by the sternopericardial ligament, the 3rd–6th costal cartilages and the anterior borders of the lungs. Posteriorly it is related to the oesophagus, descending aorta, and vertebra T5–T8, and on either side to the roots of the lungs, the mediastinal pleura and the phrenic nerves.

The inner aspect of the fibrous pericardium is lined by the parietal layer of serous pericardium. This is reflected around the roots of the great vessels to become continuous with the visceral layer or epicardium. The lines of pericardial reflexion are marked on the posterior surface of the heart by the oblique sinus. (Fig. 23)  

The oblique sinus is bound by the inferior vena cava and the four pulmonary veins, which form a recess between the left atrium and the pericardium, and the transverse sinus which is between the superior vena cava and left atrium posteriorly and the pulmonary trunk and aorta anteriorly.

The Thorax-Lower Respiratory Tract

The trachea (Figs 14, 15)

The trachea is about 11.5cm length and nearly 2.5cm diameter. It commences at the lower border of the cricoid cartilage (C6) and terminates by bifurcating at the level of the sternal angle of Louis (T4/5) to form the right and left main bronchi. (In the living subject, the level of bifurcation varies slightly with the phase of respiration; in deep inspiration it descends to T6 and in expiration it rises to T4.)

Relations

Lying partly in the neck and partly in the thorax, its relations are:

Cervical

•◊◊anteriorly— the isthmus of thyroid gland, inferior thyroid veins, sternohyoid and sternothyroid muscles;

•◊◊laterally—the lobes of thyroid gland and the common carotid artery;

•◊◊posteriorly—the oesophagus with the recurrent laryngeal nerve lying in the groove between oesophagus and trachea (Fig. 16).

Thoracic

In the superior mediastinum its relations are:

•◊◊anteriorly—commencement of the brachiocephalic (innominate) artery

and left carotid artery, both arising from the arch of the aorta, the left brachiocephalic

(innominate) vein, and the thymus;

•◊◊posteriorly—oesophagus and left recurrent laryngeal nerve;

•◊◊to the left— arch of the aorta, left common carotid and left subclavian

arteries, left recurrent laryngeal nerve and pleura;

•◊◊to the right—vagus, azygos vein and pleura (Fig. 17).

 Structure

The patency of the trachea is maintained by a series of 15–20 U-shaped cartilages. Posteriorly, where the cartilage is deficient, the trachea is flattened and its wall completed by fibrous tissue and a sheet of smooth muscle (the

trachealis). Within, it is lined by a ciliated columnar epithelium with many goblet cells.

Clinical features

Radiology

Since it contains air, the trachea is more radio-translucent than the neighbouring structures and is seen in posteroanterior and lateral radiographs as a dark area passing downwards, backwards and slightly to the right. In the elderly, calcification of the tracheal rings may be a source of radiological confusion.

Displacement

The trachea may be compressed or displaced by pathological enlargement of the neighbouring structures, particularly the thyroid gland and the arch of the aorta.

‘Tracheal-tug’

The intimate relationship between the arch of the aorta and the trachea and left bronchus is responsible for the physical sign known as ‘tracheal-tug’, characteristic of aneurysms of the aortic arch.

Tracheostomy

Tracheostomy may be required for laryngeal obstruction (diphtheria, tumours, inhaled foreign bodies), for the evacuation of excessive secretions (severe postoperative chest infection in a patient who is too weak to cough adequately), and for long-continued artificial respiration (poliomyelitis, severe chest injuries). It is important to note that respiration is further assisted by considerable reduction of the dead-space air. The neck is extended and the head held exactly in the midline by an assistant. A vertical incision is made downwards from the cricoid cartilage, passing between the anterior jugular veins.

Alternatively, a more cosmetic transverse skin crease incision, placed halfway between the cricoid and suprasternal notch, is employed. A hook is thrust under the lower border of the cricoid to steady the trachea and pull it forward. The pretracheal fascia is split longitudinally, the isthmus of the thyroid either pushed upwards or divided between clamps and the cartilage of the trachea clearly exposed. A circular opening is then made into the trachea to admit the tracheostomy tube. In children the neck is relatively short and the left brachiocephalic vein may come up above the suprasternal notch so that dissection is rather more difficult and dangerous. This difficulty is made greater because the child’s trachea is softer and more mobile than the adult’s and therefore not so readily identified and isolated. Its softness means that care must be taken, in incising the child’s trachea, not to let the scalpel plunge through and damage the underlying oesophagus.

In contrast, the trachea may be ossified in the elderly and small bone shears required to open into it. The golden rule of tracheostomy—based entirely on anatomical considerations— is ‘stick exactly to the midline’. If this is not done, major vessels are in jeopardy and it is possible, although the student may not credit it, to miss the trachea entirely.

The bronchi (Fig. 15)

The right main bronchus is wider, shorter and more vertical than the left. It is about 2.5cm long and passes directly to the root of the lung at T5. Before joining the lung it gives off its upper lobe branch, and then passes below the pulmonary artery to enter the hilum of the lung. It has two important relations: the azygos vein, which arches over it from behind to reach the superior vena cava, and the pulmonary artery which lies first below and then anterior to it. The left main bronchus is nearly 2.5cm long and passes downwards and outwards below the arch of the aorta, in front of the oesophagus and descending aorta. Unlike the right, it gives off no branches until it enters the hilum of the lung, which it reaches opposite T6. The pulmonary artery spirals over the bronchus, lying first anteriorly and then above it.

Clinical features

1◊◊The greater width and more vertical course of the right bronchus accounts for the greater tendency for foreign bodies and aspirated material to pass into the right bronchus (and thence especially into the middle and lower lobes of the right lung) rather than into the left.

2◊◊The inner aspect of the whole of the trachea, the main and lobar bronchi and the commencement of the first segmental divisions can be seen at bronchoscopy.

3◊◊Widening and distortion of the angle between the bronchi (the carina) as seen at bronchoscopy is a serious prognostic sign, since it usually indicates carcinomatous involvement of the tracheobronchial lymph nodes around the bifurcation of the trachea.

The lungs (Figs 18, 19)

Each lung is conical in shape, having a blunt apex which reaches above the sternal end of the 1st rib, a concave base overlying the diaphragm, an extensive costovertebral surface moulded to the form of the chest wall and a mediastinal surface which is concave to accommodate the pericardium.

The right lung is slightly larger than the left and is divided into three lobes—upper, middle and lower, by the oblique and horizontal fissures. The left lung has only an oblique fissure and hence only two lobes.

Blood supply

Mixed venous blood is returned to the lungs by the pulmonary arteries; the air passages are themselves supplied by the bronchial arteries, which are small branches of the descending aorta. The bronchial arteries, although small, are of great clinical importance. They maintain the blood supply to the lung parenchyma after pulmonary embolism, so that, if the patient recovers, lung function returns to normal. The superior and inferior pulmonary veins return oxygenated blood to the left atrium, while the bronchial veins drain into the azygos system.

Lymphatic drainage

The lymphatics of the lung drain centripetally (MOVING TOWARD THE CENTER) from the pleura towards the hilum. From the bronchopulmonary lymph nodes in the hilum, efferent lymph channels pass to the tracheobronchial nodes at the bifurcation of the trachea, thence to the paratracheal nodes and the mediastinal lymph trunks to drain usually directly into the brachiocephalic veins or, rarely, indirectly via the thoracic or right lymphatic duct.

Nerve supply

The pulmonary plexuses derive fibres from both the vagi and the sympathetic trunk. They supply efferents (refers to nerves that travel from the brain and spinal cord to the rest of the body) to the bronchial musculature (sympathetic bronchodilator fibres) and receive afferents (NERVE FIBRES) from the mucous membrane of the bronchioles and from the alveoli.

The bronchopulmonary segments of the lungs (Figs 20, 21) 

A knowledge of the finer arrangement of the bronchial tree is an essential prerequisite to intelligent appreciation of lung radiology, to interpretation of bronchoscopy and to the surgical resection of lung segments. Each lobe of the lung is subdivided into a number of bronchopulmonary segments, each of which is supplied by a segmental bronchus, artery and vein.

These segments are wedge-shaped with their apices at the hilum and bases at the lung surface; if excised accurately along their boundaries (which aremarked by intersegmental veins), there is little bleeding or alveolar air leakage from the raw lung surface.

The names and arrangements of the bronchi are given in Table 1; each bronchopulmonary segment takes its title from that of its supplying segmental bronchus (listed in the right-hand column of the table). The left upper lobe has a lingular segment, supplied by the lingular bronchus from the main upper lobe bronchus. This lobe is equivalent to the right middle lobe whose bronchus arises as a branch from the main bronchus.

Apart from this, differences between the two sides are very slight; on the left, the upper lobe bronchus gives off a combined apicoposterior segmental bronchus and an anterior branch, whereas all three branches are separate on the right side.

On the right also there is a small medial (or cardiac) lower lobe bronchus which is absent on the left, the lower lobes being otherwise mirror images of each other.

The Thoracic Cage-The Pleurae

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The two pleural cavities are totally separate from each other (Fig. 2). Each pleura consists of two layers: a visceral layer intimately related to the surface of the lung, and a parietal layer lining the inner aspect of the chest wall, the upper surface of the diaphragm and the sides of the pericardium and mediastinum.

The two layers are continuous in front and behind the root of the lung, but below this the pleura hangs down in a loose fold, the pulmonary ligament, which forms a ‘dead-space’ for distension of the pulmonary veins. The surface markings of the pleura and lungs have already been described in the section on surface anatomy.

Notice that the lungs do not occupy all the available space in the pleural cavity even in forced inspiration.

Clinical features

1◊◊Normally the two pleural layers are in close apposition (A POSITION OF CLOSENESS) and the space between them is only a potential one. It may, however, fill with air (pneumothorax),blood (haemothorax) or pus (empyema).

2◊◊Fluid can be drained from the pleural cavity by inserting a wide-bore needle through an intercostal space (usually the 7th posteriorly). The needle is passed along the superior border of the lower rib, thus avoiding the intercostal nerves and vessels (Fig. 8). Below the 7th intercostal space there is danger of penetrating the diaphragm.

3◊◊For emergency chest drainage—for example traumatic haemothorax or haemopneumothorax—the site of election is the 5th intercostal space in the mid-axillary line. An incision is made through skin and fat and blunt dissection carried out over the upper border of the 6th rib. The pleura is opened, a finger inserted to clear any adhesions and ensure the safety of the adjacent diaphragm before inserting a tube into the pleural space and connecting it to an under-water drain.

4◊◊Since the parietal pleura is segmentally innervated by the intercostal nerves, inflammation of the pleura results in pain referred to the cutaneous distribution of these nerves (i.e. to the thoracic wall or, in the case of the lower nerves, to the anterior abdominal wall, which may mimic an acute abdominal emergency).

The Thoracic Cage-The Diaphragm

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The diaphragm is the dome-shaped septum (DIVING WALL OR PARTITION) dividing the thoracic from the abdominal cavity. It comprises two portions: a peripheral (OF OR RELATING TO THE SURFACE OR OUTER PART OF A BODY OR ORGAN) muscular part

which arises from the margins of the thoracic outlet and a centrally placed aponeurosis (A SHEETLIKE FIBROUS MEMBRANE RESEMBLING A FLATTENED TENDON THAT SERVES AS A FASCIA TO BIND MUSCULES TOGETHER OR TO CONNECT MUSCLE TO BONE)(Fig. 10).

The muscular fibres are arranged in three parts.

1◊◊Avertebral part from the crura (AN ELEONGATED PART OF AN ANATOMICAL STRUCTURE) and from the arcuate ligaments (REFERS TO AN ARC-SHAPED LIGAMENT).

The right crus (SEE CURA) arises from the front of the bodies of the upper three lumbar vertebrae (ONE OF THE FIVE HUMAN VERTEBRAE GROUPS) and intervertebral discs (A CARTLIGOUS DISC SERVING AS A CUSHION BETWEEN ALL OF THE VERTEBRAE IN THE SPINAL COLUMN EXCLUDING THE FIST TWO) the left crus is only attached to the first two vertebrae.

The arcuate ligaments are a series of fibrous arches, the medial (IN THE MEDIAN PLANE OF THE BODY OR THE MIDLINE F AN ORGAN) being a thickening of the fascia covering psoas major (A LARGE MUSCLE THAT RUNS FROM THE LUMBAR SPINE THROUGH THE GROIN ON EITHER SIDE) and the lateral(SIDE) of fascia overlying quadratus lumborum (A MUSCLE IN THE BACK WHICH ATTACHES TO THE TOP OF THE PELVIS AND THE SPINE IN THE UPPER LUMBAR AREA). The fibrous medial borders of the two crura form a median arcuate ligament over the front of the aorta.

2◊◊Acostal part is attached to the inner aspect of the lower six ribs and costal

cartilages.

3◊◊Asternal portion consists of two small slips from the deep surface of the

Xiphisternum. The central tendon, into which the muscular fibres are inserted, is trefoil

in shape and is partially fused with the under-surface of the pericardium.

The diaphragm receives its entire motor supply from the phrenic nerve (C3, 4, 5) whose long course from the neck follows the embryological migration of the muscle of the diaphragm from the cervical region (see below). Injury or operative division of this nerve results in paralysis and elevation of the corresponding half of the diaphragm.

Radiographically (IN X-RAYS), paralysis of the diaphragm is recognized by its elevation and paradoxical (SELF-CONTRADICTORY) movement; instead of descending on inspiration it is forced upwards by pressure from the abdominal viscera (INTERNAL ORGANS).

The sensory nerve fibres from the central part of the diaphragm also run in the phrenic nerve, hence irritation of the diaphragmatic pleura (PART OF THE PARIETAL PLEURA)(in pleurisy) or of the peritoneum on the undersurface of the diaphragm. Subphrenic collections of pus or blood produces referred pain in the corresponding cutaneous (AFFECTING OR RELATED TO THE SKIN) area, the shoulder-tip. The peripheral part of the diaphragm, including the crura, receives sensory fibres from the lower intercostal nerves.

Openings in the diaphragm

The three main openings in the diaphragm (Figs 10, 11) are:

1◊◊the aortic (at the level of T12) which transmits the abdominal aorta, the thoracic duct and often the azygos vein;

2◊◊the oesophageal (T10) which is situated between the muscular fibres of the right crus of the diaphragm and transmits, in addition to the oesophagus, branches of the left gastric artery and vein and the two vagi (EACH OF THE TENTH PAIR OF CRANIAL NERVES, SUPPLYING THE HEART, LUNGS, UPPER DIGESTIVE TRACT, AND OTHER ORGANS OF THE CHEST AND ABDOMEN);

3◊the opening for the inferior vena cava (T8) which is placed in the central tendon and also transmits the right phrenic nerve.

In addition to these structures, the greater and lesser splanchnic nerves pierce the crura and the sympathetic chain passes behind the diaphragm deep to the medial arcuate ligament.

The development of the diaphragm and the anatomy of diaphragmatic herniae (A CONDITION IN WHICH PART OF AN ORGAN IS DISPLACED AND PROTRUDES THROUGH THE WALL OF THE CAVITY CONTAINING IT).

The development of the diaphragm and the anatomy of diaphragmatic herniae

The diaphragm is formed (Fig. 12) by fusion in the embryo of:

1◊◊the septum transversum (forming the central tendon);

2◊◊the dorsal oesophageal mesentery;

3◊◊a peripheral rim derived from the body wall;

4◊◊the pleuroperitoneal membranes, which close the fetal communication between the pleural and peritoneal cavities.

The septum transversum is the mesoderm (THE MIDDLE LAYER OF AN EMBRYO IN EARLY DEVELOPMENT, BETWEEN THE ENDODERM AND ECTODERM) which, in early development, lies in front of the head end of the embryo. With the folding off of the head, this mesodermal mass is carried ventrally (NEAR THE ABDOMEN) and caudally (TOWARD THE POSTERIOR END OF THE BODY),  to lie in its definitive position at the anterior (TOWARD THE FRONT OF THE BODY) part of the diaphragm.

During this migration, the cervical myotomes (PART OF EMBRYONIC VERTEBRAE) and nerves contribute muscle and nerve supply respectively, thus accounting for the long course of the phrenic nerve (C3, 4 and 5) from the neck to the diaphragm. With such a complex embryological story, one may be surprised to know that congenital abnormalities of the diaphragm are unusual.

However, a number of defects may occur, giving rise to a variety of congenital

herniae through the diaphragm. These may be:

1◊◊through the foramen of Morgagni (UNCOMMON DIAPHRAGMATIC HERNIAE); anteriorly between the xiphoid and costal origins;

2◊◊through the foramen of Bochdalek—the pleuroperitoneal canal—lying posteriorly;

3◊◊through a deficiency of the whole central tendon (occasionally such a hernia may be traumatic in origin);

4◊◊through a congenitally large oesophageal hiatus (PAUSE, BREAK). Far more common are the acquired hiatus herniae (subdivided into sliding and rolling herniae). These are found in patients usually of middle age where weakening and widening of the oesophageal hiatus has occurred (Fig. 13).

In the sliding hernia the upper stomach and lower oesophagus slide upwards into the chest through the lax hiatus when the patient lies down or bends over; the competence of the cardia (THE UPPER OPENING OF THE STOMACH, WHERE THE ESOPHAGUS ENTERS) is often disturbed and peptic juice can therefore regurgitate into the gullet. This may be followed by oesophagitis with consequent heartburn, bleeding and, eventually, stricture formation.

In the rolling hernia (which is far less common) the cardia remains in its normal position and the cardio-oesophageal junction is intact, but the fundus (THE UPPER PART OF THE STOMACH, WHICH FORMS A BULGE HIGHER THAN THE OPENING OF THE ESOPHAGUS) of the stomach rolls up through the hiatus in front of the oesophagus, hence the alternative term of para-oesophageal hernia. In such a case there may be epigastric  discomfort, flatulence and even dysphagia, but no regurgitation because the cardiac mechanism is undisturbed.

The movements of respiration

During inspiration the movements of the chest wall and diaphragm result in an increase in all diameters of the thorax. This, in turn, brings about an increase in the negative intrapleural pressure and an expansion of the lung tissue. Conversely, in expiration the relaxation of the respiratory muscles and the elastic recoil of the lung reduce the thoracic capacity and force air out of the lungs.

In quiet inspiration the first rib remains relatively fixed, but contraction of the external and internal intercostals elevates and, at the same time, everts the succeeding ribs. In the case of the 2nd–7th ribs this principally increases the anteroposterior diameter of the thorax (by the forward thrust of the sternum), like a pump handle.

The corresponding movement of the lower ribs raises the costal margin and leads mainly to an increase in the transverse diameter of the thorax, like a bucket handle. The depth of the thorax is increased by the contraction of the diaphragm which draws down its central tendon. Normal quiet expiration, brought about by elastic recoil of the elevated ribs, is aided by the tone of the abdominal musculature which, acting through the contained viscera, forces the diaphragm upwards.

In deep and in forced inspiration additional muscles attached to the

chest wall are called into play (e.g. scalenus anterior, sternocleidomastoid,

serratus anterior and pectoralis major) to increase further the capacity of

the thorax. Similarly, in deep expiration, forced contraction of the abdominal

muscles aids the normal expulsive factors described above.

Thoracic Cage-Intercostal Spaces

The thoracic cage is formed by the vertebral column behind, the ribs and intercostal spaces on either side and the sternum and costal cartilages in front. Above, it communicates through the ‘thoracic inlet’ with the root (BASE) of the neck; below, it is separated from the abdominal cavity by the diaphragm (Fig. 1).

The intercostal spaces

There are slight variations between the different intercostal spaces, but typically each space contains three muscles, comparable to those of the abdominal wall, and an associated neurovascular bundle (Fig. 8).

 The muscles are:

1◊◊the external intercostal, the fibres of which pass downwards and forwards from the rib above to the rib below and reach from the vertebrae behind to the costochondral junction (RIB JOINT) in front, where muscle is replaced by the anterior intercostal membrane;

2◊◊the internal intercostal, which runs downwards and backwards from the sternum to the angles of the ribs where it becomes the posterior intercostal membrane;

3◊◊the innermost intercostal, which is only incompletely separated from the internal intercostal muscle by the neurovascular bundle. The fibres of this sheet cross more than one intercostal space and it may be incomplete. Anteriorly it has a more distinct portion which is fan-like in shape, termed the transversus thoracis (or sternocostalis), which spreads

upwards from the posterior aspect of the lower sternum to insert onto the inner surfaces of the second to the sixth costal cartilages. Just as in the abdomen, the nerves and vessels of the thoracic wall lie between the middle and innermost layers of muscles. This  eurovascular

bundle consists, from above downwards, of vein, artery and nerve, the vein lying in a groove on the undersurface of the corresponding rib (remember—v, a, n—vein, nerve, artery).

The vessels comprise the posterior and anterior intercostals. The posterior intercostal arteries of the lower nine spaces are branches of the thoracic aorta, while the first two are derived from the superior intercostal branch of the costocervical trunk, the only branch of the second part of the subclavian artery. Each runs forward in the subcostal groove to anastomose (COMMUNICATION BETWEEN VESSELS BY COLLATERAL CHANNELS )with the anterior intercostal artery.

Each has a number of branches to adjacent muscles, to the skin and to the spinal cord. The corresponding veins are mostly tributaries of the azygos and hemiazygos veins. The first posterior intercostal vein drains into the rachiocephalic or vertebral vein. On the left, the 2nd and 3rd veins often join to form a superior intercostal vein, which crosses the aortic arch to drain into the left brachiocephalic vein.

The anterior intercostal arteries are branches of the internal thoracic artery (1st–6th space) or of its musculophrenic branch (7th–9th spaces). The lowest two spaces have only posterior arteries. Perforating branches pierce the upper five or six intercostal spaces; those of the 2nd–4th spaces are large in the female and supply the breast.

The intercostal nerves are the anterior primary rami of the thoracic nerves, each of which gives off a collateral muscular branch and lateral and anterior cutaneous branches for the innervation of the thoracic and abdominal walls (Fig. 9).

Clinical features

1◊◊Local irritation of the intercostal nerves by such conditions as Pott’s disease of the thoracic vertebrae (tuberculosis) may give rise to pain which is referred to the front of the chest or abdomen in the region of the peripheral termination of the nerves.

2◊◊Local anaesthesia of an intercostal space is easily produced by infiltration around the intercostal nerve trunk and its collateral branch—a procedure known as intercostal nerve block.

3◊◊In a conventional posterolateral thoracotomy (e.g. for a pulmonary lobectomy) an incision is made along the line of the 5th or 6th rib; the periosteum over a segment of the rib is elevated, thus protecting the neurovascular bundle, and the rib is excised (REMOVED). Access to the lung or mediastinum is then gained though the intercostal space, which can be opened out considerably owing to the elasticity of the thoracic cage.
4◊◊Pus from the region of the vertebral column tends to track around the thorax along the course of the neurovascular bundle and to ‘point’ to the three sites of exit of the cutaneous branches of the intercostal nerves, which are lateral to erector spinae (sacrospinalis), in the midaxillary line and just lateral to the sternum ((Fig. 9)above).