Thoracic Inlet

The thoracic inlet is limited by the body of the first thoracic vertebra posteriorly, the first pair of ribs and their costal cartilages anterolaterally, and the superior border of the manubrium anteriorly. Many important structures travel through the cervicothoracic junction. Resection of malignancies (e.g., Pancoast’s tumor) or repair of thoracic outlet syndrome requires intimate knowledge of the anatomic relationships in this area. In this kidney-shaped inlet, the trachea is midline and behind the great vessels, and the esophagus is posterior and slightly to its left. The innominate artery arises from the aortic arch and passes posterior to the manubrium and anterior to the trachea as it travels cephalad. The right and left internal jugular and subclavian veins join behind their respective sternoclavicular joints to form the left and right brachiocephalic veins. The left brachiocephalic vein travels right to join its counterpart to form the superior vena cava. The main muscles of this region are the sternocleidomastoid and scalene muscles, and the main nerves form the brachial plexus, but the phrenic and vagus nerves also pass through this aperture.

As the margin of the aperture slopes inferoanteriorly, the apex of each lung and its covering pleura (pleural cupola) project superiorly through the lateral parts of the thoracic inlet and are covered by a piece of cervical fascia, the suprapleural membrane (Sibson’s fascia).

Cervicoaxillary Canal

The cervicoaxillary canal is bounded by the first rib inferiorly, the clavicle superiorly, and the costoclavicular ligament medially (Fig. 1-2). The structures that pass through this space include the subclavian vein and artery and the brachial plexus.

Figure 1–2 Cervicoaxillary canal and the structures that traverse this opening.

The subclavian artery passes over the first rib and goes between the scalenus anterior and scalenus middle muscles. Before passing over the first rib on its way out the cervicoaxillary canal, it gives off branches, including the thyrocervical, internal mammary, and vertebral arteries. It becomes the axillary artery after it exits from behind the pectoralis minor muscle. Compression of this artery may lead to poststenotic dilation, stenosis, aneurysmal formation, or occlusion.

The subclavian vein travels from the arm and goes behind the pectoralis minor muscle before going between the first rib and clavicle. This space is bounded medially by the costoclavicular ligament and laterally by the scalenus anterior muscles as it inserts onto the first rib. Compression of the vein occurs in this area and can lead to stenosis and occlusion.

The brachial plexus is composed of nerve roots from the C5 through the T1 vertebral foramina. These nerve roots join to form the superior trunk (C5 and C6), the middle trunk (C7), and the inferior trunk (C8 and T1). These trunks separate into anterior and posterior divisions that fuse again to form lateral, medial, and posterior cords of the brachial plexus. The plexus travels through the tunnel formed by the anterior and middle scalene muscles. It travels between the first rib and the clavicle and finally under the pectoralis minor muscle to gain access to the arm.

In the normal anatomy, there is ample room for each of these structures. There are four major areas in which the vessels or nerves can be compressed (Table 1-1).This topic will be discussed in greater detail in Chapter 26.

Table 1–1 Sites and Structures Compressed in Thoracic Outlet Syndrome

Diaphragm

The diaphragm, at the inferior thoracic aperture, separates the thoracic cavity from the abdominal cavity. The inferior thoracic aperture, which slopes inferoposteriorly, is limited by the 12th thoracic vertebra posteriorly, the 12th pair of ribs and costal margins anterolaterally, and the xiphisternal joint anteriorly.

The diaphragm is a curved musculotendinous sheet that is mainly convex toward the thoracic cavity. It is a continuous sheet of muscle with low posterior and lateral attachments and high anterior attachments. The central tendon is a thin but strong aponeurosis. The domes of the diaphragm descend 2 cm during quiet respiration and may travel as much as 10 cm during heightened respiratory requirements. During expiration, the right diaphragm may rise as high as the level of the nipple, whereas the left rises to a level one rib space lower. With maximal inspiration, the diaphragm flattens against the abdominal contents. The right diaphragm may flatten to the level of the 11th rib, whereas the left may flatten to the level of the 12th rib. The right diaphragm has to work against the liver, whereas the left needs to push only against the stomach and spleen. Therefore, the right diaphragm has significantly stronger fibers than the left.

The costal diaphragm receives its blood supply from the lower five intercostal and subcostal arteries, and the central portion is supplied by the phrenic arteries. The phrenic arteries may arise directly from the aorta, above the celiac axis, as a common trunk or individually. Occasionally, they arise from the celiac or renal arteries. The sole motor supply to the diaphragm is the phrenic nerve. It also supplies sensory fibers to the parietal and peritoneal pleura overlying the diaphragm. This accounts for the diaphragmatic irritation that is sometimes interpreted as ipsilateral shoulder pain.

The diaphragm has several openings through which structures can transverse from one cavity to another (see Fig. 1-1B). The inferior vena cava hiatus lies anteriorly and to the right of the midline, at the level of vertebra T8. The right phrenic nerve travels through this hiatus, and the left phrenic nerve penetrates laterally through its own opening at the same level. The esophageal aperture is at the level of the T10 vertebra, and the right and left vagal trunks that adhere to it enter the abdomen along with the esophagus.

The aortic aperture, at the T12 vertebral level, is formed by the interdigitating fibers (median arcuate ligament) of the right and left diaphragmatic crura. The azygos and hemiazygos veins and the thoracic duct also pass through this opening.

The greater and lesser splanchnic nerves gain access to the abdominal cavity via two small apertures in each of the crura, and musculophrenic branches of the internal mammary artery penetrate the diaphragm through small apertures near its connection with the costal cartilages of ribs seven to nine.

Diaphragmatic hernias can develop through known openings, such as a hiatal hernia through the esophageal hiatus, or through congenital defects. The most common congenital abnormality is a Bochdalek hernia, in which abdominal contents go through a posterolateral defect. This is seen more commonly on the left. A Morgagni hernia, on the other hand, occurs with a defect in the anterior aspect of the diaphragm just lateral to the xiphoid.

The Bony Thoracic Cage

Ribs

The 12 pairs of ribs are divided into the upper seven, which that are called true ribs or vertebrosternal ribs because they form complete loops between the vertebrae and the sternum, and the lower five ribs, which fail to reach the sternum and are considered false ribs. The eighth, ninth, and 10th ribs are called vertebrocostal because each of their costal cartilages articulates with the adjacent rib cartilage. Ribs 11 and 12 are free-floating or vertebral ribs because their only articulation is with their vertebrae.

Ribs three to nine are classified as typical ribs and have a head, neck, and a shaft. The head has an upper and a lower articular facet divided by a crest for articulation with two adjacent vertebrae in synovial costovertebral joints, the lower facet articulating with the upper border of its own vertebra. The neck is flattened, with the upper border curling up into a prominent ridge—the crest. A tubercle projects posteriorly from the end of the neck and marks the junction of the neck and the body (Fig. 1-3).

Figure 1–3 Typical rib.

The medial facet on the tubercle is covered with hyaline cartilage and makes a synovial joint with the transverse process of its own vertebra. The lateral facet receives the costotransverse ligament from the tip of its own transverse process. The shaft slopes downward and laterally for about 5 to 8 cm at an angle and then curves forward. Lateral to the angle, the lower border projects down as a sharp ridge, sheltering a costal groove. The angles of the ribs also correspond to the lateral extent of the erector spinae muscles of the back. The upper six ribs are bent into a tight curve so that the shaft has turned parallel with the neck of the rib. The lower six ribs have an opening out of the curve, which is completed by the long costal cartilages at the front of the chest. The fused cartilages of ribs seven to 10 course diagonally upward to the lower end of the sternum to form the subcostal angle. The anterior ends of the ribs have a concave fossa that is plugged by the costal cartilage in an immovable primary cartilaginous joint.

The first rib is exceptionally broad and short and very highly curved. The head is small and has a single facet for the synovial joint it makes with the upper part of the body of the T1 vertebra. The prominent tubercle is a fusion of the tubercle and the angle, and its medial facet forms a synovial joint with the first transverse process. The lateral prominent part of the tubercle receives the lateral costotransverse ligament and the costalis and longissimus parts of the erector spinae. Near the middle of the shaft, the sulci for the subclavian vessels are identified by the scalene tubercle, a spur that attaches the scalenus anterior muscle.

Intercostal Muscles

Fibers of the intercostalis externi muscle arise from the sharp lower border of the rib above and course inferomedially (i.e., in the direction of the fingers when the hands are put into the front pockets of trousers) to the smooth upper border of the rib below. Anteriorly, it is replaced by the anterior intercostal membrane. Between the bony ribs is muscle; between the costal cartilages is membrane. In the lower spaces, the muscle interdigitates with the fibers of the external oblique (Fig. 1-5).

Figure 1–5 Spatial organization of the thoracic wall on the right side.

The intercostalis interni muscle runs from the lower costal groove of ribs one to 11, to the upper surfaces of ribs two to 12 downward and backward. Anteriorly, they extend up to the sternum but posteriorly only up to the angles of the rib. Beyond that, they are replaced by the internal intercostal membrane, which attaches to the tubercle of each rib and vertebra.

The innermost or transversus layer is divided into three groups: the innermost intercostals (anterolateral), subcostal (posterior), and transversus thoracis (anteromedial) muscles. The fibers run downward and backward, as in the internal intercostal muscles. The subcostal muscles lie in the paravertebral gutter, are better developed inferiorly, and cross more than one space. The transversus thoracis was formerly called the sternocostalis, a more appropriate name. Digitations arise from the sternum bilaterally to the costal cartilages of ribs two to six.

The intercostalis intimi muscles also traverse more than one space and are better developed in the lower lateral spaces. In the plane between the innermost and outer two layers runs the neurovascular bundle. From above downward, the order is vein, artery, and nerve (mnemonic: VAN). Beyond the angle of the rib posteriorly, they are protected by the downward projection of the lower border of the rib. Hence, for a thoracotomy, the periosteum is stripped off the upper half of the rib, avoiding the lower border and the neurovascular bundle (Fig. 1-6).

Figure 1–6 Layers of the chest wall.

As in the rest of the body, the mixed spinal nerve is formed from a dorsal and a ventral root, the dorsal root being sensory and the ventral root containing somatic motor neurons. As it emerges from the intervertebral foramina, it branches into a dorsal and a ventral ramus. The dorsal ramus of the thoracic spinal nerve supplies the paravertebral back muscles and skin of the back. The ventral ramus communicates with the sympathetic chains via white rami communicantes (postganglionic fibers). Beyond this point, the true intercostal nerve lies just superficial to the parietal pleura in the endothoracic fascia. It gains the costal groove between the innermost intercostal and the internal intercostal muscles near the angle of the rib, where a collateral branch is given off. This small branch supplies the muscles of the space, the parietal pleura, and the periosteum of the rib. The main nerve itself has muscular branches, a lateral cutaneous branch, and a terminal anterior cutaneous branch. The lateral cutaneous branch given off along the midaxillary line gives off anterior and posterior branches to supply the skin over that space. Just lateral to the sternal margin, the terminal anterior cutaneous branches of the upper six nerves pierce the internal intercostal muscles, the external intercostal membrane, and the pectoralis major to reach skin. The lower five intercostal nerves slope downward behind the costal margin into the neurovascular plane of the abdominal wall. The subcostal or 12th thoracic nerve leaves the thorax by passing behind the lateral arcuate ligament and the subcostal artery and vein (Fig. 1-7). The cutaneous branches of each dermatome tend to overlap considerably; hence anesthesia after thoracic incisions is quite rare unless multiple intercostal nerves have been damaged. The first intercostal nerve is very small and supplies no skin, lacking both lateral and anterior cutaneous branches.

Figure 1–7 Organization of the intercostal space and the relationship of the arteries (A), veins (B), and nerves (C) to the muscle layers.

Two sets of intercostal arteries, the posterior and the anterior, are responsible for supplying the intercostal spaces. Their course and branching pattern closely conform to those of the intercostal nerves. The posterior intercostal arteries are branches of the descending thoracic aorta except in the first two spaces, where they are branches of the supreme intercostal artery, given off by the costocervical trunk of the second part of the subclavian artery. The aortic branches lie on the left side of the mediastinum; consequently, the right posterior intercostal arteries are longer, can be easily dissected in the left chest, and are seen best while operating on descending thoracic aortic aneurysms.

The roots of all 12 pairs of posterior intercostal arteries give rise to a dorsal branch that supplies the vertebrae, spinal cord, and deep muscles of the back. Like the nerve, the intercostal artery gives off a collateral branch, the largest muscular branch that runs along the upper border of the rib below the space. After gaining the costal groove, near the midaxillary line, it gives off the lateral cutaneous branch. The subcostal artery or the 12th thoracic posterior intercostal artery follows a similar course but has no collateral branch.

The anterior intercostal arteries are branches of the internal thoracic arteries from the first part of the subclavian artery. The internal thoracic artery runs anterior to the transversus thoracis and on the internal surface of the costal cartilages and the internal intercostal muscles. About a finger’s breadth from the border of the sternum running vertically downward, it gives off two anterior intercostal arteries in each space. At the costal margin, below the sixth costal cartilage, it divides into the superior epigastric and musculophrenic arteries. The anterior intercostal arteries are smaller than the posterior intercostal arteries with which they anastomose and run predominantly along the lower border of each costal cartilage in the same fascial neurovascular plane. In the lower spaces, they are branches of the musculophrenic artery. There are no true anterior intercostal arteries in the last two spaces.

The internal thoracic arteries also give branches to the mediastinum, the thymus, the pericardium, and the sternum, and especially large perforating branches in the second to fourth space, the predominant supply to the lactating breast in females.

In each space there are one posterior and two anterior intercostal veins, designated by names identical to the arteries that they accompany. They lie above the artery and the nerve throughout their course (VAN) in the intercostal space. The anterior veins drain into the musculophrenic and internal thoracic veins. The vein of the first space or the supreme intercostal vein, posteriorly, may be a tributary of the brachiocephalic, vertebral, or superior intercostal vein. The superior intercostal vein is formed by the posterior intercostal veins of the second, third, and sometimes the fourth spaces. This drains into the azygos vein on the right side, and on the left side it arches over the aorta, superficial to the vagus and deep to the phrenic to open into the left brachiocephalic veins. Subcostal veins join the ascending lumbar veins and ascend on the left side as the hemiazygos and on the right side as the azygos vein draining the lower eight spaces. The blood in the hemiazygos vein drains across the midline, through median anastomoses into the azygos vein.

The internal thoracic vessels are part of the anastomotic chain that links the subclavian artery and brachiocephalic veins to the external iliac vessels. The intercostal vessels in turn connect to the descending aorta and azygos system of veins. In the presence of obstruction to flow, these anastomoses provide alternative channels for arterial and venous blood flow.

Anatomy of Breathing

Simple respiratory effort is attained mostly by diaphragmatic motion. The bony and muscular components of the thorax remain a stable cavity while the diaphragm acts as a piston. It flattens during inspiration to create negative intrathoracic pressure, resulting in expansion of the lungs. Once inspiration is complete, the diaphragm relaxes and the lung recoils to its original position. The intercostals muscles are important to prevent paradoxical motion during inspiration.

The thoracic cavity can change to meet increased respiratory needs. The accessory muscles of the chest wall help to increase intrathoracic volume during inspiration and decrease volume during expiration. In addition to the piston action of the diaphragm, the accessory muscles elevate the sternal body and xiphoid process anteriorly and superiorly. The lower ribs attached to the sternum follow this movement and therefore increase the diameter of the lower chest cavity. The manubrium of the sternum is relatively fixed, and the chest cavity at this level does not contribute significantly to increased respiratory needs. This dynamic response of the chest cavity helps meet increasing demands for oxygenation and ventilation.