CHAPTER 57 Pleura, lungs, trachea and bronchi
The lungs are the essential organs of respiration and are responsible for the uptake of oxygen into the blood and the removal of carbon dioxide. The functional design of the thorax facilitates this complex process. The muscles of respiration and the diaphragm, acting together, increase the intrathoracic volume, creating a negative pressure within the pleural space which surrounds the lung and causing expansion of the lung (see Ch. 58). The resultant reduction in intra-alveolar pressure prompts the conduction of air through the upper respiratory tract into the trachea and airways and thence into the alveoli, where gaseous exchange occurs. The process of breathing exposes the lung to noxious agents, including gases, dust particles, bacteria and viruses; the mucous barrier, mucociliary escalator, branching pattern of the airways and the cough reflex are all anatomical defences against these insults. Anatomical defects may compromise respiratory function; for example, chest wall abnormalities may cause restrictive lung disease. Similarly, ultrastructural abnormalities such as ciliary dysfunction (as seen in Kartagener’s syndrome) lead to recurrent respiratory infections and airway damage.
PLEURA
The right and left pleural sacs form separate compartments and touch only behind the upper half of the sternal body (see Fig. 53.6A,B), although they are also close to each other behind the oesophagus at the midthoracic level. The region between them is the mediastinum (interpleural space). The left pleural cavity is the smaller of the two, because the heart extends further to the left. The upper and lower limits of the pleurae are about the same on the two sides, but the left sometimes descends lower in the midaxillary line.
The interlobar fissures and posterior azygo-oesophageal and retrosternal pleural reflections are the only aspects of the normal pleura that can be visualized on a chest radiograph or CT scan (Fig. 57.1A–D; see Figs 55.16, 55.17). Demonstration of significant pleural shadowing in any other regions usually implies pathological abnormalities of the pleura. Thoracoscopy allows the direct inspection of both the parietal and visceral surfaces. The parietal pleura is translucent and at thoracoscopy the underlying muscles and blood vessels are visible. The visceral pleura is also translucent and has a grey variegated appearance due to the underlying lung and the vascular network in the subpleural layer.
PARIETAL PLEURA
Costovertebral pleura
Costovertebral pleura lines the sternum, ribs, transversus thoracis and intercostal muscles and the sides of the vertebral bodies; normally it is easily separated from these structures. External to the pleura is a thin layer of loose connective tissue, the endothoracic fascia, which corresponds to the transversalis fascia of the abdominal wall. Anteriorly, the costal pleura begins behind the sternum, where it is continuous with the mediastinal pleura along a junction extending from behind the sternoclavicular joint down and medially to the midline behind the sternal angle. From here, the right and left costal pleurae descend in contact with each other to the level of the fourth costal cartilages and then diverge. On the right side, the line descends to the back of the xiphisternal joint, while on the left the line diverges laterally and descends at a distance of 2–2.5 mm from the sternal margin to the sixth costal cartilage, forming the cardiac notch. On each side, the costal pleura sweeps laterally, lining the internal surfaces of the costal cartilages, ribs, transversus thoracis and intercostal muscles. Posteriorly, it passes over the sympathetic trunk and its branches to reach the sides of the vertebral bodies, where it is again continuous with the mediastinal pleura. The costovertebral pleura is continuous with the cervical pleura at the inner margin of the first rib and below it becomes continuous with the diaphragmatic pleura along a line which differs slightly on the two sides. On the right, this line of costodiaphragmatic reflection begins behind the xiphoid process, passes behind the seventh costal cartilage to reach the eighth rib in the midclavicular line, the tenth rib in the midaxillary line, and then ascends slightly to cross the twelfth rib level with the upper border of the twelfth thoracic spine (see Fig. 53.6A,B). On the left, the line initially follows the ascending part of the sixth costal cartilage, but then follows a course similar to that on the right, although it may be slightly lower.
Cervical pleura
The cervical pleura is a continuation of the costovertebral pleura over the pulmonary apex (Fig. 57.2). It ascends medially from the internal border of the first rib to the apex of the lung, as high as the lower edge of the neck of the first rib, and then descends lateral to the trachea to become the mediastinal pleura. As a result of the obliquity of the first rib, the cervical pleura extends 3–4 cm above the first costal cartilage, but not above the neck of the first rib. The cervical pleura is strengthened by a fascial suprapleural membrane, which is attached in front to the internal border of the first rib, and behind to the anterior border of the transverse process of the seventh cervical vertebra. It contains a few muscular fibres, which spread from the scaleni. Scalenus minimus extends from the anterior border of the transverse process of the seventh cervical vertebra to the inner border of the first rib behind its subclavian groove, and also spreads into the pleural dome, which it therefore tenses: it has been suggested that the suprapleural membrane is the tendon of scalenus medius. The cervical pleura (like the pulmonary apex) reaches the level of the seventh cervical spine approximately 2.5 cm from the midline. Its projection is a curved line from the sternoclavicular joint to the junction of the medial and middle thirds of the clavicle, its summit being 2.5 cm above it. The subclavian artery ascends laterally in a furrow below the summit of the cervical pleura (Fig. 57.2).
Mediastinal pleura
The mediastinal pleura is the lateral boundary of the mediastinum and forms a continuous surface above the hilum of the lung from sternum to vertebral column. On the right it covers the right brachiocephalic vein, the upper part of the superior vena cava, the terminal part of the azygos vein, the right phrenic and vagus nerves, the trachea and oesophagus. On the left it covers the aortic arch, left phrenic and vagus nerves, left brachiocephalic and superior intercostal veins, left common carotid and subclavian arteries, thoracic duct and oesophagus. At the hilum of the lung it turns laterally to form a tube that encloses the hilar structures and is continuous with the pulmonary pleura.
VISCERAL PLEURA
The pulmonary pleura is inseparably adherent to the lung over all its surfaces, including those in the fissures, except at the root or hilum of the lung and along a line descending from this, which marks the attachment of the pulmonary ligament (Figs 57.3, 57.4).
INFERIOR PULMONARY LIGAMENTS
Below the hilum the mediastinal pleura extends as a double layer, the pulmonary ligament, from the lateral surface of the oesophagus to the mediastinal surface of the lung, where it is continuous with the parietal pleura (Figs 57.3, 57.4). It is continuous above with the pleura around the hilar structures and below it ends in a free sickle-shaped border.
PLEURAL RECESSES
The inferior border of the right costodiaphragmatic recess is an important consideration in the surgical posterior approach to the kidney. Usually the pleura crosses the twelfth rib at the lateral border of erector spinae, so that the medial region of the kidney is above the pleural reflection (see Fig. 53.6B). However, if the twelfth rib does not project beyond the muscle, the eleventh rib may be mistaken for the twelfth in palpation, and an incision prolonged to this level will damage the pleura. Whether the lowest palpable rib is the eleventh or twelfth can be ascertained by counting from the second rib (identified at its junction with the sternal angle).
PNEUMOTHORAX
Any breach of the chest wall and parietal pleura or visceral pleura leads to the accumulation of air within the pleural cavity (pneumothorax). Fluid (hydrothorax), blood (haemothorax) and rarely lymph (chylothorax) can also accumulate in this space. Pneumothoraces may occur spontaneously or following trauma (e.g. rib fractures, penetrating injuries from sharp instruments, iatrogenic injury). Significant air in the pleural space is visible on a chest radiograph: there is separation of the parietal and visceral pleurae and an absence of pulmonary vascular markings in the corresponding area. Occasionally, a ball valve-like effect occurs, so that air enters the pleural space during inspiration but cannot escape in expiration. This tension pneumothorax can be life-threatening and should be suspected whenever there are unilateral decreased breath sounds and hyperresonance on percussion, hypotension, jugular venous distension and contralateral tracheal deviation. A tension pneumothorax requires immediate decompression by the insertion of an intercostal drain or wide bore catheter. Fluid collection in the pleural space may be due to congestive cardiac failure, hypoalbuminaemia, inflammatory, infective or neoplastic conditions. A pleural effusion causes obliteration of the costophrenic angle and the diaphragm, and a lateral meniscus is visible on a frontal chest radiograph. Drainage of the fluid and subsequent analysis is required for diagnostic purposes. Where there is a collection of pus (empyema) or blood, pleural drainage is essential for therapeutic purposes. Ultrasonography is useful in assessing the size and characteristics of an effusion, such as the presence of loculation and debris. It may even demonstrate underlying consolidated lung. Computed tomography is utilized to assess the underlying hidden lung parenchyma and mediastinal glands. For further review of the radiology of the pleurae and lungs, see Armstrong (2000).
LUNGS
The lungs are the essential organs of respiration. They are situated on either side of the heart and other mediastinal contents (Figs 57.3, 57.4). Each lung is free in its pleural cavity, except for its attachment to the heart and trachea at the hilum and pulmonary ligament respectively. When removed from the thorax, a fresh lung is spongy, can float in water, and crepitates when handled, because of the air within its alveoli. It is also highly elastic and so it retracts on removal from the thorax. Its surface is smooth and shiny and is separated by fine, dark lines into numerous small polyhedral domains, each crossed by numerous finer lines, indicating the areas of contact between its most peripheral lobules and the pleural surface.
PULMONARY SURFACE FEATURES
Each lung has an apex, base, three borders and two surfaces (Figs 57.3, 57.4). In shape, each lung approximates to half a cone.
Apex
The apex, the rounded upper extremity, protrudes above the thoracic inlet where it contacts the cervical pleura, and is covered in turn by the suprapleural membrane. As a consequence of the obliquity of the inlet, the apex rises 3–4 cm above the level of the first costal cartilage; it is level posteriorly with the neck of the first rib. Its summit is 2.5 cm above the medial third of the clavicle. The apex is therefore in the root of the neck (see Fig. 35.5A). It has been claimed that, because the apex does not rise above the neck of the first rib, it is really intrathoracic, and that it is the anterior surface that ascends highest in inspiration. The subclavian artery arches up and laterally over the suprapleural membrane, grooves the anterior surface of the apex near its summit and separates it from scalenus anterior. The cervicothoracic (stellate) sympathetic ganglion, the ventral ramus of the first thoracic spinal nerve and the superior intercostal artery all lie posterior to the apex. Scalenus medius is lateral, the brachiocephalic artery, right brachiocephalic vein and trachea are adjacent to the right medial surface of the lung, and the left subclavian artery and left brachiocephalic vein are adjacent to the left medial surface of the apex of the lung.
Other impressions on the lung surface
In addition to these pulmonary features, cadaveric lungs that have been preserved in situ can show a number of other impressions that indicate their relations with surrounding structures (Figs 57.3, 57.4). On the right lung the cardiac impression is related to the anterior surface of the right auricle, the anterolateral surface of the right atrium and partially to the anterior surface of the right ventricle. The impression ascends anterior to the hilum as a wide groove for the superior vena cava and the terminal portion of the right brachiocephalic vein. Posteriorly this groove is joined by a deep sulcus which arches forwards above the hilum and is occupied by the azygos vein. The right side of the oesophagus makes a shallow vertical groove behind the hilum and the pulmonary ligament. Towards the diaphragm it inclines left and leaves the right lung, and therefore does not reach the lower limit of this surface. Posteroinferiorly the cardiac impression is confluent with a short wide groove adapted to the inferior vena cava. Between the apex and the groove for the azygos, the trachea and right vagus are close to the lung, but do not mark it.
On the left lung (Fig. 57.4) the cardiac impression is related to the anterior and lateral surfaces of the left ventricle and auricle. The anterior infundibular surface and adjoining part of the right ventricle is also related to the lung as it ascends in front of the hilum to accommodate the pulmonary trunk. A large groove arches over the hilum, and descends behind it and the pulmonary ligament, corresponding to the aortic arch and descending aorta. From its summit a narrower groove ascends to the apex for the left subclavian artery. Behind this, above the aortic groove, the lung is in contact with the thoracic duct and oesophagus. In front of the subclavian groove there is a faint linear depression for the left brachiocephalic vein. Inferiorly, the oesophagus may mould the surface in front of the lower end of the pulmonary ligament.
Pulmonary borders
The inferior border is thin and sharp where it separates the base from the costal surface and extends into the costodiaphragmatic recess, and is more rounded medially where it divides the base from the mediastinal surface. It corresponds, in quiet respiration, to a line drawn from the lowest point of the anterior border which passes to the sixth rib at about the midclavicular line, then to the eighth rib in the midaxillary line (usually 10 cm above the costal margin), and then continues posteriorly, medially and slightly upwards to a point 2 cm lateral to the tenth thoracic spine (see Fig. 53.5A,B). The diaphragm rises higher on the right to accommodate the liver, and so the right lung is vertically shorter (by approximately 2.5 cm) than the left. However, cardiac asymmetry means that the right lung is broader, and has a greater capacity and weight than the left. The posterior border separates the costal surface from the mediastinal surface, and corresponds to the heads of the ribs. It has no recognizable markings and is really a rounded junction of costal and vertebral (medial) surfaces. The thin, sharp, anterior border overlaps the pericardium. On the right it corresponds closely to the costomediastinal line of pleural reflection, and is almost vertical. On the left it approaches the same line above the fourth costal cartilage; below this point it shows a variable cardiac notch, the edge of which passes laterally for 3.5 cm before curving down and medially to the sixth costal cartilage 4 cm from the midline. It therefore does not reach the line of pleural reflection here (see Fig. 53.5A) and so the pericardium is covered only by a double layer of pleura (area of superficial cardiac dullness). However, surgical experience suggests that the line of pleural reflection, the anterior pulmonary margin and the costomediastinal pleural recess are all variable.
PULMONARY FISSURES AND LOBES
Right lung
The right lung is divided into superior, middle and inferior lobes by an oblique and a horizontal fissure (Fig. 57.3). The upper, oblique fissure separates the inferior from the middle and upper lobes, and corresponds closely to the left oblique fissure, although it is less vertical, and crosses the inferior border of the lung approximately 7.5 cm behind its anterior end. On the posterior border it is either level with the spine of the fourth thoracic vertebra or slightly lower. It descends across the fifth intercostal space and follows the sixth rib to the sixth costochondral junction. The short horizontal fissure separates the superior and middle lobes. It passes from the oblique fissure, near the midaxillary line, horizontally forwards to the anterior border of the lung, level with the sternal end of the fourth costal cartilage, then passes backwards to the hilum on the mediastinal surface. The horizontal fissure is usually visible on a frontal chest radiograph. The oblique fissure is usually visible on a lateral radiograph and on a high resolution CT scan as a curvilinear band from the lateral aspect to the hilum (Fig. 57.1A–D). The small middle lobe is cuneiform and includes some of the costal surface, the lower part of the anterior border and the anterior part of the base of the lung. Sometimes the medial part of the upper lobe is partially separated by a fissure of variable depth which contains the terminal part of the azygos vein, enclosed in the free margin of a mesentery derived from the mediastinal pleura, and forming the ‘lobe of the azygos vein’. This varies in size, and sometimes includes the apex of the lung. It is always supplied by one or more branches of the apical bronchus. Radiographically, a pleural effusion may be limited to the azygos fissure. Less common variations are the presence of an inferior accessory fissure, which separates the medial basal segment from the remainder of the lower lobe, and a superior accessory fissure, which separates the apical segment of the lower lobe from the basal segments. The identification of the completeness of the fissures is important prior to lobectomy, because individuals with incomplete fissures are more prone to develop postoperative air leaks, and may require further procedures such as stapling and pericardial sleeves (see Venuta et al 1998).
Left lung
The left lung is divided into a superior and an inferior lobe by an oblique fissure (Fig. 57.4) which extends from the costal to the medial surfaces of the lung both above and below the hilum. Superficially this fissure begins on the medial surface at the posterosuperior part of the hilum. It ascends obliquely backwards to cross the posterior border of the lung 6 cm below the apex, then descends forwards across the costal surface, to reach the lower border almost at its anterior end. It finally ascends on the medial surface to the lower part of the hilum. At the posterior border of the lung the fissure usually lies opposite a surface point 2 cm lateral to the midline between the spines of the third and fourth thoracic vertebrae, but it may be above or below this level. Traced around the chest, the fissure reaches the fifth intercostal space (at or near the midaxillary line) and follows this to intersect the inferior border of the lung close to, or just below, the sixth costochondral junction (7.5 cm from the midline). The left oblique fissure is usually more vertical than the right, and is indicated approximately by the medial border of the scapula when the arm is fully abducted above the shoulder. A left horizontal fissure is a normal variant found occasionally.
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