Lungs, bronchi and trachea

Important anatomical relationships

The thyroid isthmus lies anteriorly over the region of the second tracheal ring. The two lobes of the thyroid gland are closely applied to the lateral aspect of the trachea and share a common blood supply from the inferior thyroid artery ( Ch. 18 ). The brachiocephalic (innominate) artery crosses over the mid-trachea obliquely and can be found at a higher level in children and some adults. The recurrent laryngeal nerves lie in the tracheo-oesophageal groove. The left has a longer course than the right, entering the tracheo-oesophageal groove after looping around the aorta. The right enters the tracheo-oesophageal groove after looping round the right subclavian artery (although occasionally it may be given off directly from the right vagus). At the level of the carina, the left main bronchus passes under the aortic arch and the right main bronchus passes beneath the azygos vein. The pulmonary artery lies anterior to the carina. For further reading about the surgical anatomy of the trachea, see reference .

Tracheal resection

Right bronchial tree

The right main bronchus is shorter than the left, starting at the trachea and finishing where the right upper lobe bronchus arises on its lateral wall about 1.2 cm from its origin. The right upper lobe bronchus gives off three segmental bronchi (apical, posterior and anterior segmental bronchus). Although usually a trifurcation, various combinations of branching can occur. The finding of just two orifices should prompt a search to make sure a tracheal bronchus is not present.

The intermediate bronchus then continues and lies posterior to the right pulmonary artery as it courses down the fissure. After 1.7–2 cm, the middle lobe bronchus arises and gives off a medial and a lateral segmental bronchus. The superior segmental bronchus of the lower lobe is usually given off posteriorly, slightly distal to the origin of the middle lobe bronchus; it may sometimes arise at the same level or above. Further distally, the basal stem bronchus gives off segmental bronchi to the medial, anterior, lateral and posterior basal segments.

Numerous anomalies of this typical arrangement occur. A tracheal bronchus arising from the lateral wall of the trachea can occur in up to 2% of individuals. This may supply the apical segment of the right upper lobe or actually represent a misplaced right upper lobe bronchus. More rarely, a bronchus can arise from the medial aspect of the bronchus intermedius; this is usually blind-ending but can be associated with some branching and a small amount of pulmonary parenchyma.

As the right main bronchus is more vertically orientated than the left main bronchus and is more in line with the trachea, it is more common for inhaled foreign bodies to be lodged in the right airway. However, it is uncommon for any foreign body to be lodged in the right upper lobe bronchus because this usually comes off the right main bronchus at an acute angle.

Left bronchial tree

The left main bronchus is longer than the right, being 4–6 cm in length. The left upper lobe bronchus divides into superior and lingular branches. The superior branch divides into apicoposterior and anterior segmental branches. Occasionally, the superior branch may trifurcate into apical, posterior and anterior segmental branches. The lingular branch divides into superior and inferior segmental branches. Approximately 0.5 cm distal to the upper lobe take-off, the lower lobe stem bronchus gives off the superior segmental bronchus of the lower lobe. The basal trunk continues for 1.5 cm before dividing into two branches, an anterior basal segmental branch and a common stem for the lateral and posterior basal segmental branches.

Variations occur in the way the upper and lingular divisions divide into their segments. A subsuperior bronchus can arise from the lower lobe bronchus. Very rarely, the right lower lobe (and sometimes a portion of the middle lobe) can be supplied by a bronchus arising from the left main stem bronchus that crosses the mediastinum (so-called bridging bronchus). Bronchial isomerism can occur with identical branching in both lungs and is frequently associated with other congenital abnormalities. Very rarely, a lobar bronchus may be absent.

Bronchoscopy

Bronchoscopy is a useful diagnostic and therapeutic procedure for airway conditions. For the surgeon, it is also an important preoperative tool for assessing the extent of resection required for lung tumours. While peripheral tumours may not be visible at bronchoscopy, more central tumours arising in the lobar bronchi will be visible. The surgeon decides whether the tumour and associated lobe can be resected with a margin of normal tissue or whether it involves another branch of the bronchial tree, meaning that either a sleeve resection, a bilobectomy or a pneumonectomy needs to be performed.

Bronchoscopy can be performed using fibreoptic bronchoscopy or rigid bronchoscopy. Fibreoptic bronchoscopy is normally carried out under local anaesthetic with light sedation (if the patient can tolerate the procedure) or under general anaesthetic with airway protection (for example, laryngeal mask, single-lumen intubation or rigid bronchoscopy). When bronchoscopy is performed under local anaesthetic, the bronchoscope is usually passed through the nose and then under direct vision through the vocal cords into the trachea. Alternatively, this can be performed through the mouth with a mouth guard protecting the teeth and the bronchoscope. The fibreoptic bronchoscope is a useful tool for initial diagnosis and assessment of airway pathology. It has an operating channel through which endoluminal biopsies and therapeutic procedures such as airway cautery/lasering can be performed. It is also a useful tool for airway toileting and secretion clearance, and recently has been used for placement of endobronchial valves and coils. Using a specialized fibreoptic bronchoscope with an ultrasound probe at its tip, it is now possible to perform endobronchial ultrasound (EBUS), which is particularly useful for endobronchial biopsies and sampling of lymph nodes close to the airway.

In situations where a larger endoluminal operating field is needed and a secure airway is required during intervention, rigid bronch­oscopy should be used. Such situations include airway foreign body, airway bleeding and endoluminal tumour debulking/stenting. Rigid bronchoscopy is performed under general anaesthesia. The surgeon inserts the bronchoscope into the mouth and, while protecting the teeth with his/her fingers, passes the bronchoscope over the tongue until the epiglottis is seen. The bronchoscope is then pushed anteriorly behind the epiglottis, allowing the vocal cords to come into view; the bronchoscope is next rotated and the beak of the instrument is passed through the cords into the trachea. Usually, the surgeon then passes a straight telescope or flexible bronchoscope through the rigid bronchoscope to complete the examination.

Lobes and fissures

The right lung has three lobes (upper, middle and lower) and two fissures (oblique and horizontal). The oblique fissure separates the lower lobe from the middle and upper lobes; it begins posteriorly at the level of the fifth rib and runs forward along the course of the sixth rib to the diaphragm. The horizontal fissure separates the middle from the upper lobe; it begins in the oblique fissure in the mid-axillary line at the level of the sixth rib and runs anteriorly to the costochondral junction of the fourth rib. The fissures may be complete so that the lobes are completely separate, or incomplete where the lobes are not easily separable. In more than 50% of cases, the horizontal fissure is incomplete and the middle lobe is fused to the anterior part of the upper lobe.

The left lung has two lobes (upper and lower) separated by the oblique fissure, which runs obliquely downward from between the third and fifth ribs to end at the level of the sixth or seventh costochondral junction.

Additional fissures may delineate accessory lobes, the most common being the posterior accessory, inferior accessory, middle lobe of left lung (lingual) and azygos lobes.

When planning resection, surgeons look carefully for the fissures on CT scans; they are usually visible and will indicate which lobe contains a lesion and whether more than one lobe is involved.

For further reading about pulmonary lobes and fissures, see reference .

Bronchopulmonary segments

Each lobe of the lung is divided into a number of bronchopulmonary segments, which can be regarded as individual functional units ( Fig. 47.3 ). Each segment is served by a segmental bronchus lying centrally within the segment; the veins are situated at the junction between adjacent segments.

Historically, the nomenclature of some of the bronchopulmonary segments has been the subject of debate. It was not clear whether the upper lobe should be regarded as three segments or two (an apico­posterior and an anterior segment). The medial basal bronchus on the left was absent in one classification and part of the anteromedial basal bronchus in another. In 1950, the Thoracic Society appointed a subcommittee to review the terminology and agree an international terminology, which is still widely adopted today. The apical segment of the lower lobe is now more usually called the superior segment of the lower lobe.

Prediction of postoperative lung function, which is important for deciding on the extent of resection, can be estimated by establishing the number of segments that are non-functional as a consequence of the tumour and the number of functioning segments that would need to be resected to clear the tumour. Knowing the existing lung function of the patient, it is possible to calculate the percentage loss of lung function (making the assumption that the segments of the lung have equal volume) and therefore to decide whether the patient will be left with enough functioning lung to survive the operation and have a reasonable quality of life.

Pulmonary arterial system

The main pulmonary artery arises from the pulmonary anulus surrounding the subpulmonary infundibulum at the base of the right ventricle, and slopes posterosuperiorly, at first anterior to the ascending aorta and then to its left; at the level of the fifth thoracic vertebra, inferior to the aortic arch at the left of the midline, and lying above the left main bronchus, it divides into right and left pulmonary arteries. The right main pulmonary artery is longer than the left but the segment lying outside the pericardium before the first branch is given off is shorter than its counterpart on the left. The branches of the pulmonary artery are more variable than those of the bronchi. They tend to lie close to the segmental bronchi and their branches ( Figs 47.4 , 47.5 ). For further reading about the pulmonary vascular system, see reference .