1. Thoracic and Pulmonary Anatomy-Review of CT Surgery

Sarah Chen and Luis A. Godoy

This chapter is a revision and update of that included in the previous editions of the TSRA Review written by Simon R. Turner (2^nd edition), John C. Haney (1^st edition), and Jason A. Williams (1^st edition).

Thoracic cage

The bony thorax consists of 12 pairs of ribs (1-7 being “true,” 8-10 “false,” and 11-12 “floating” ribs), the thoracic vertebrae, and the sternum. The first rib is the shortest and flattest and on its superior surface features grooves for the subclavian vessels, separated by the anterior scalene muscle. The artery runs posterior to this muscle along with the brachial plexus, while the vein runs anterior. The head of each rib articulates with its corresponding vertebra and the one superior, with the exception of ribs 1, 11, and 12, which articulate with a single vertebra. The sternal angle of Louis, between the manubrium and body, provides an important landmark for the 2nd intercostal space, the aortic arch, the tracheal bifurcation, and T4.

The intercostal neurovascular bundle runs on the undersurface of each rib. The intercostal arteries supply the posterior and lateral chest wall, and arteries 1-9 combine with the internal mammary arteries to supply the chest wall anteriorly. Distally, arteries 10, 11, and the subcostal artery supply the anterior abdominal wall. Intercostal veins drain into the azygos and hemiazygos veins on the right and left, respectively. Intercostal nerves exit the spinal canal via the intervertebral foramen and provide the chest wall with sensory and motor innervation, and also convey fibers of the sympathetic chain. There are three layers of intercostal muscles: external (running inferomedially), internal (running superomedially), and innermost.

Overlying these layers are the muscles of the upper extremity, which may be divided during thoracotomy or used for thoracic reconstruction or buttressing. These muscles are described in Table 1-1. The pectoralis major, latissimus dorsi, and serratus anterior muscles are commonly used in thoracic reconstruction procedures.

Table 1-1.

Muscle	Origin	Insertion	Innervation	Vascular supply
Pectoralis major	Clavicular head: anterior surface of medial half of clavicle Sternocostal head: anterior surface of sternum, superior 6 costal cartilages, aponeurosis of external oblique muscle	Lateral lip of intertubercular sulcus of humerus	Lateral and medial pectoral nerves; clavicular head (C5, C6), sternocostal head (C7, C8, T1)	Pectoral branch of the thoracoacromial trunk
Latissimus dorsi	T7-12, thoraco-lumbar fascia, iliac crest	Floor of intertubercular sulcus of humerus	Thoracodorsal nerve (C6, C7, C8)	Thoracodorsal artery
Serratus anterior	Ribs 1-8	Anterior surface of medial border of scapula	Long thoracic nerve (C5, C6, C7)	Lateral thoracic artery

Other important muscles include the trapezius, which arises from the occiput and C7-T12 and inserts on the superior scapula and clavicle. Deep to the trapezius is the rhomboid (major and minor), arising from C7-T5 and inserting on the medial border of the scapula.

These muscles are not commonly used for reconstruction but may be divided during thoracotomy. The auscultatory triangle, which may be utilized in muscle-sparing thoracotomy, is bordered by the trapezius superomedially, the latissimus inferiorly, and the scapula laterally.

The floor of the thoracic cage is formed by the diaphragm, consisting of two crural pillars (arising from L1-3) and costal fibers (arising from the lower six ribs) inserting in a central tendon. Apertures allow the passage of major structures: IVC and right phrenic nerve branches (caval hiatus at T8 via the central tendon), esophagus and vagus nerves (esophageal hiatus at T10 between the right and left fibers of the right crus, which are also known as the surgical crura), and aorta, azygous vein and thoracic duct (aortic hiatus at T12, between the true anatomic right and left crura). The foramen of Morgagni lies posterior to the sternum and allows the passage of the superior epigastric branches of the internal mammary artery. The periphery of the diaphragm is innervated by intercostal nerves, and thus may be incised circumferentially without interrupting phrenic nerve supply.

The parietal pleura is attached to the inner chest wall by the endothoracic fascia, which allows elevation of the pleura in an avascular plane. The parietal (costal, mediastinal, and diaphragmatic) and visceral pleura transition at the pleural recesses, the hilum, and along the inferior pulmonary ligament. The diaphragmatic pleura is thin and inseparable from the diaphragm, while superiorly the pleura is supported by thickened endothoracic fascia, or Sibson’s fascia. This provides some protection as the pleural space may extend several centimeters above the clavicle. The inferior reach of the pleura is generally one interspace below that of the lung. While the lung reaches the 6^th, 8^th and 11^th spaces anteriorly, laterally, and posteriorly, the pleura generally extends to the 7^th, 9^th and 12^th spaces, respectively.

Lung

The lungs are divided into lobes and then further divided into bronchopulmonary segments. Bronchopulmonary segments are self-contained sections of lung with their own dedicated bronchial and vascular supply. The right lung has 10 segments:

Right upper lobe (RUL): apical, anterior and posterior
Right middle lobe (RML): medial and lateral
Right lower lobe (RLL): superior, anterior basal, posterior basal, medial basal, and lateral basal

The left lung is similar but with 3 major exceptions: the lingula takes the place of the middle lobe but is part of the upper lobe, the upper lobe apical and posterior segments are usually combined, and the anterior and medial basal segments of the lower lobe are usually combined. This leaves the left lung with 8 segments:

Left upper lobe (LUL): apicoposterior, anterior, superior lingular and inferior lingular
Left lower lobe (LLL): superior, anteromedial basal, posterior basal, and lateral basal

The bronchial anatomy of these segments is the most consistent, while the pulmonary arteries that run with them in the center of each segment are more variable. Veins run separately in the intersegmental plane.

The trachea is approximately 11 cm long with 17-21 rings, and bifurcates at the T4/5 level. The right bronchus is shorter, wider and more vertically oriented. The RUL bronchus typically arises within 2.5 cm of the carina, but can rarely arise directly from the trachea (“pig bronchus”). The bronchus intermedius gives rise to the RML and RLL bronchi. The RML bronchus arises across from the RLL superior segment bronchus. The bronchus intermedius then terminates into the basal branches. The left main bronchus, conversely, runs for 5-6 cm before it divides into the LUL branch anteriorly and the LLL branch posteriorly.

The right pulmonary artery runs anterior and inferior to the right bronchus, traversing the junction of the major and minor fissures. Since the right mainstem bronchus courses superior to the right pulmonary artery, it is said to be eparterial. The first branch of the right pulmonary artery, the truncus anterior, arises before it enters the lung to supply the RUL. The upper lobe may be further supplied by a posterior ascending branch entering the inferior surface of the lobe posterior to the superior vein, where it is at risk of injury during vein dissection. The superior segment artery arises posteriorly, also at the level of the middle lobe artery before the basal branches are finally given off.

The left pulmonary artery is a direct continuation of the main pulmonary artery, beginning anterior and superior to the left bronchus before wrapping around posteriorly. Since the left mainstem bronchus travels inferior to the left pulmonary artery, it is said to be hypoarterial. The LUL has the most variable arterial anatomy, and the anterior branches, which can be multiple, wide, short and hidden by the superior vein, are the most at risk for injury during lobectomy. The number of pulmonary arterial branches to the LUL vary from 2 to 7. Most commonly, the apicoposterior and anterior segments receive blood supply from an apicoanterior trunk. In some cases, the arteries for apical, anterior, and posterior segments originate separately. The LUL can also be supplied by posterior branches that arise from the inner curve of the ongoing pulmonary artery. In most cases, the lingula is supplied by one branch of the pulmonary artery, which gives off arteries for the superior and inferior segments. The continuation of the left pulmonary artery that supplies the LLL may be called the interlobar artery, from which branches to the superior segment (inferiorly), lingula (anteriorly), and basal segments are given off in succession.

The superior pulmonary veins drain the upper and middle lobes on the right and upper lobe and lingular segment on the left, and are the most anterior structures in either hilum. The inferior veins can be found at the superior limit of the inferior pulmonary ligament, and drain the lower lobes. It is important to recognize that the pulmonary veins can join together and enter the left atrium as a single trunk, which occurs more commonly on the left (25%) than on the right (3%). Failure to do so during lobectomy may inadvertently lead to the need for a pneumonectomy.

One to three bronchial arteries supply each lung, most commonly two on the left and one on the right and arise posterior to the hilum from either the anterolateral surface of the aorta or one of the intercostal arteries. These bronchial arteries may give off branches to the esophagus before running along the membranous airway to deliver systemic blood to the lungs.

Lymph nodes

For staging purposes, thoracic lymph nodes are divided into pulmonary and mediastinal regions, though in reality these nodal stations are in some physiologic continuity and only separated by short distances.

Mediastinal nodes (contained within the mediastinal pleura) are noted with single digits (levels 1 to 9) and include:

Highest mediastinal nodes (1 R/L): above the innominate vein; they can be low cervical, supraclavicular, or in the sternal notch
Upper paratracheal nodes (2 R/L): above the aortic arch and below the superior margin of the innominate vein
Prevascular and retrotracheal nodes (3 a/p): prevascular nodes (anterior to the SVC) are denoted 3a, retrotracheal nodes are denoted 3p
Lower paratracheal nodes (4 R/L): below the superior margin of the aortic arch and above the level of the carina
Subaortic/aortopulmonary window nodes (5): lateral to the ligamentum arteriosum, between the aorta and the left pulmonary artery
Para-aortic nodes (6): anterior and lateral to the ascending aorta and aortic arch
Subcarinal nodes (7): below the tracheal carina within the mediastinum
Paraesophageal (8 R/L): adjacent to the esophagus below the carina, excluding subcarinal nodes
Inferior pulmonary ligament nodes (9 R/L): nodes lying within the pulmonary ligament

Pulmonary nodes (located within the reflection of the visceral pleura) are noted with higher digits (levels 10 to 14) and include: hilar (10), interlobar (11), lobar (12), intersegmental (13) and subsegmental (14). N1 nodal involvement includes ipsilateral pulmonary nodes (within the ipsilateral visceral pleura). If the involved mediastinal nodes are ipsilateral to the tumor, the nodal staging is N2, whereas contralateral involvement is designated as N3 disease. Supraclavicular involvement also constitutes N3 disease. The path of lung cancer metastases through the intrathoracic nodes is not always predictable but is thought to often follow a progression from N1 to N2 to N3.

Other structures

The thoracic duct arises from the cisterna chyli in the abdomen and passes through the aortic hiatus. In the chest, it rises posterior to the esophagus in the aortoazygous groove until the T4/5 level where it crosses to the left to run behind the innominate vein and drain into the left subclavian-internal jugular vein confluence.

The phrenic nerves (C3, 4, 5) provide motor and sensory innervation to the diaphragm. They descend over the anterior scalene within its fascial sheath and enter the thoracic inlet between the subclavian arteries and the brachiocephalic veins. They then run on the lateral surface of the pericardium, 1.5 cm anterior to the hilum.

The vagal nerves, conversely, run posterior to the hilum. The recurrent laryngeal nerves branch from the vagi to ascend along the tracheoesophageal groove. The left nerve, which runs around the ligamentum arteriosum under the aortic arch, is at higher risk of injury during thoracic operations than the right, which runs around the right subclavian artery. During mediastinoscopy, the left recurrent laryngeal nerve is particularly at risk and the use of cautery should therefore be avoided when dissecting on the left side of the airway.

Suggested Readings

Kandathil A, Chamarthy M. Pulmonary vascular anatomy & anatomical variants. Cardiovasc Diagn Ther. 2018;8(3):201-207.