Anatomy

Anatomy, Imaging and Practical Management of Selected Thoracic Surgical Procedures

Betty C. Tong
Thomas A. D’Amico

ANATOMY

Chest Wall and Surface Anatomy

The thoracic viscera are protected by the chest wall and sternum. The bony framework of the chest wall is composed of 12 thoracic vertebrae, intervertebral discs, 12 pairs of ribs with corresponding cartilages, and the sternum. The thoracic vertebrae and intervertebral discs are positioned in the posterior midline; the spinous processes are relatively easy to identify as landmarks. The scapula overlies a portion of the first seven pairs of ribs posteriorly. With the arm abducted, the (vertebral) medial border of the scapula is parallel to the oblique fissure of the underlying lung.¹

The sternum lies in the anterior midline and has three components: manubrium, body, and xiphoid process. The sternal notch, or superior border of the manubrium, is easily located between the clavicular heads. The junction of the manubrium and body of the sternum is called the sternal angle, or angle of Louis. This is an important landmark, corresponding to the level where the second costal cartilages articulate with the sternum. Since the first rib may be partially or completely obscured by the clavicle, accurate counting of ribs may commence at the sternal angle. This landmark is also important for deeper thoracic structures, marking the level of the tracheal bifurcation (carina) as well as the aortic arch.²

The upper seven pairs of ribs are considered to be true ribs because they form a complete circle between the sternum and vertebrae. The costal cartilages connect the ribs to the sternum anteriorly. In contrast to those of the first seven pairs of ribs, the costal cartilages of the 8th, 9th, and 10th ribs attach to the cartilage of the preceding rib. The 10th costal cartilage marks the most inferior point of the costal margin.¹ Aside from their vertebral attachments, the 11th and 12th ribs do not have other skeletal attachments and are considered to be “floating ribs.”

The blood supply to the chest wall comes from the subclavian artery and aorta. The subclavian artery gives rise to the internal thoracic artery, also known as the internal mammary artery, as well as the first two intercostal arteries. Together, these vessels supply the anterior chest wall. The lateral and posterior areas of the chest wall are supplied by the remaining intercostal arteries, which arise as direct branches from the aorta posteriorly. Importantly, the intercostal bundle, consisting of the intercostal artery, vein and nerve, runs along the inferior aspect of each rib and is subject to injury during procedures such as thoracotomy or even thoracostomy tube placement.

The extrathoracic muscles of the chest wall provide both anatomic landmarks as well as substrate for surgical reconstruction of chest wall defects. The latissimus dorsi provides a large and versatile myocutaneous flap. Supplied by the thoracodorsal artery, nerve and vein, it is used most frequently for reconstruction of anterior and lateral chest wall defects. The pectoralis major is often used for anterior and midline chest wall defects, and is especially useful for coverage of sternal wounds. The rectus abdominus, external oblique and trapezius muscles may also be used for reconstruction of chest wall defects. When intrathoracic muscle flaps are needed (eg, coverage of bronchial stumps, filling a post-pneumonectomy space), the intercostal muscles and serratus anterior muscle are most frequently utilized.

AIRWAY AND LUNGS

Trachea

The trachea, palpable in the anterior neck, enters the chest just posterior to the manubrium and serves as the ventilatory conduit between the larynx and mainstem bronchi. It spans from the inferior border of the cricoid cartilage, the only complete cartilaginous ring in the airway, to the carina. There, the airway divides into the right and left mainstem bronchi. The trachea is comprised of 14 to 19 C-shaped incomplete cartilaginous rings and elastic membranous tissue.³ During childhood, the cross-sectional area of the trachea is circular. With growth and development, it becomes elliptical in shape with its transverse length slightly longer than the anteroposterior length. However, normal anatomic variation among adults occurs frequently, and includes both circular and triangular cross-sectional shapes.⁴ The average tracheal length in an adult ranges from 10 to 15 cm; in general, tracheal dimensions are slightly larger in men than women.

The posterior membranous trachea is composed of smooth muscle and respiratory epithelium, with ciliated pseudostratified columnar epithelium and mucus-producing goblet cells. The intercartilaginous tissue between the tracheal rings also contains muscle. It is this muscular tissue that is responsible for dynamic changes in tracheal size and luminal diameter.

The blood supply to the trachea is segmental, and closely related to that of the esophagus. The inferior thyroid artery arises from the thyrocervical trunk of the subclavian artery and supplies blood to both the proximal trachea and esophagus. Branches of the bronchial arteries, which arise directly from the aorta, supply the lower trachea, carina and bronchi. These arterial branches enter the tracheoesophageal groove and further divide into primary tracheal and esophageal branches. The primary tracheal branches further divide into lateral longitudinal vessels, which join together and run parallel to the longitudinal axis of the trachea, and transverse intercartilaginous arteries, which supply blood in a circumferential manner. Extensive circumferential dissection of the trachea and airways is to be avoided in order to prevent bronchial stump dehiscence or anastomotic disruption.

Bronchopulmonary Anatomy

At the carina, the trachea divides into the right and left mainstem bronchi. The lungs are divided into lobes and segments. The first branch off the right main bronchus is the right upper lobe bronchus. The ongoing airway, called bronchus intermedius, then divides into the right middle and lower lobe bronchi. The three lobes of the right lung are further divided into 10 segments (Table 8–1). The right upper lobe bronchus gives rise to the apical, anterior and posterior segments, while the right middle lobe contains the medial and lateral segments. The right lower lobe is composed of the superior segment and four basilar segments: anterior, posterior, medial and lateral.

Table 8–1. Lobar and Segmental Anatomy of the Lungs

The left lung is slightly smaller than the right lung, and has only two lobes (upper and lower) and eight segments. The lingula, the inferior portion of the left upper lobe, is anatomically analogous to the right middle lobe. From the carina, the left main bronchus gives rise to the left upper and lower lobe bronchi. The left upper lobe contains the apicoposterior and anterior segments in addition to the superior and inferior lingular segments. The left lower lobe is comprised of the superior segment, in addition to three basilar segments: anteromedial, lateral, and posterior.

The arterial blood supply to the lungs follows the segmental bronchial anatomy. The main pulmonary artery arises from the heart and divides into the left and right pulmonary artery trunks. Branches of the right main pulmonary artery include the truncus arteriosus and posterior ascending artery, which supply the right upper lobe. The right middle lobe branch supplies its namesake, and the right lower lobe is supplied by a superior segmental artery branch as well as by branches of the basilar trunk. Similarly, the left main pulmonary artery gives rise to apical, anterior, posterior and lingular branches, which supply the left upper lobe. The superior segmental artery and common basal trunk supply the left lower lobe.

Paired pulmonary veins, superior and inferior, provide venous drainage from the lungs to the right atrium of the heart. The right superior pulmonary vein drains the right upper and middle lobes, and the right inferior pulmonary vein drains the right lower lobe. Similarly, on the left side, the superior pulmonary vein drains blood from the left upper lobe and the inferior pulmonary vein drains the left lower lobe. Rarely do the pulmonary vein branches join to form a single vessel or “common vein.” However, this must be recognized during pulmonary resection in order to avoid division of the entire venous drainage from the lung, which would necessitate pneumonectomy.

ESOPHAGUS

The esophagus serves as a conduit for food and drink between the hypopharynx and stomach, and traverses three anatomic regions: the neck, thorax, and abdomen. The cervical esophagus measures approximately 5 cm in length and is located between the trachea and vertebral column. Proximally, the esophagus begins in the neck at the cricopharyngeus, or upper esophageal sphincter. Of the three areas of normal anatomic narrowing of the esophagus, the cricopharyngeus is narrowest and the most common site of iatrogenic perforation. Upon swallowing, the cricopharyngeus relaxes to accommodate the bolus traveling from the pharynx to the esophagus. The right and left recurrent laryngeal nerves reside in their respective tracheo-esophageal grooves and are at risk for injury during dissection of either the trachea or esophagus in this region.

The thoracic esophagus measures approximately 20 cm in length. From its entry at the thoracic inlet to the tracheal bifurcation, the esophagus maintains its close anatomic relationship with the posterior tracheal wall and the prevertebral fascia. Another area of normal anatomic narrowing occurs at the level of the aortic arch, where an indentation in the left lateral esophageal wall is often seen on both endoscopic examination and contrast esophagography. From this point on, the esophagus continues anterior and often slightly left of the vertebral column until it reaches the diaphragmatic hiatus, the third site of normal anatomic narrowing.

The abdominal portion of the esophagus measures 1.25 to 2 cm in length and includes part of the lower esophageal sphincter. As the esophagus traverses the diaphragmatic hiatus, it is surrounded by the phrenoesophageal membrane, a fibroelastic ligament arising from the subdiaphragmatic fascia as a continuation of the transversalis fascia of the abdomen. The lower limit of this membrane blends with the serosa of the stomach; a prominent anterior fat pad marks its end, which also marks the approximate location of the gastroesophageal junction.

The blood supply to the esophagus varies by anatomic location. Branches of the inferior thyroid artery are responsible for blood supply to the cervical esophagus. Bronchial arteries supply the thoracic esophagus. Approximately 75% of individuals have one right-sided and one- or two left-sided branches, which arise directly from the aorta. The abdominal portion of the esophagus receives arterial blood supply from branches of the inferior phrenic and left gastric arteries. One unique feature of the esophagus is the extensive collateral network of vessels in the muscular and submucosal layers. Upon entry to the esophageal wall, the arteries divide to form longitudinal anastomoses. As a result, the esophagus can undergo extensive mobilization with nominal risk of devascularization or ischemic necrosis.

Venous drainage of the esophagus occurs through a submucosal venous plexus, which then flows into a periesophageal venous plexus. The esophageal veins originate from the periesophageal venous plexus. Further drainage of the esophageal veins varies by region: in the neck, the esophageal veins drain into the inferior thyroid vein; in the thorax, esophageal veins drain into the bronchial, hemiazygos and azygos veins; and in the abdomen, drainage is into the coronary vein.

MEDIASTINUM

The mediastinum is the area of the thorax that resides between the pleural spaces, extending from the thoracic inlet to the diaphragm. It is divided into the superior and inferior mediastinum by a plane passing through the sternal angle and the fourth thoracic vertebra. There are three anatomic subdivisions of the inferior mediastinal space: anterior, middle, and posterior. The anterior mediastinum contains the internal mammary arteries and veins, lymph nodes, thymus gland, connective tissue and fat. Ectopic parathyroid or thyroid gland tissue may also be found in the anterior mediastinal compartment.

The middle mediastinum is also called the visceral compartment. It contains the pericardium, heart, great vessels, trachea and proximal mainstem bronchi, and esophagus. In addition, lymphatics, the right and left vagus and phrenic nerves, thoracic duct, connective tissue and fat are also contained within the middle mediastinum.

The posterior mediastinum includes the paravertebral sulci, or potential spaces located along each side of the vertebral column. Neurogenic structures such as the ventral ramus, thoracic spinal ganglia, and sympathetic trunk are found in the paravertebral sulci. The proximal portions of the intercostal arteries and veins, connective tissue and lymphatics are also located within the posterior mediastinum.

DIAGNOSTIC IMAGING MODALITIES

Imaging of the Lungs

Several different imaging modalities are used in the evaluation of pulmonary disorders. Plain chest radiography, done in the primary care or emergency room setting, often provides the first indication of pulmonary pathology. A standard chest radiograph consists of two images: one taken in the posterioranterior (PA) projection and the other from the left lateral erect position (Figure 8–1). These images provide visualization of the pulmonary parenchyma, mediastinum, bony structures and diaphragm. Lateral decubitus films are often employed to evaluate the mobility of pleural effusions seen on plain films.

Figure 8–1. PA and lateral chest x-ray demonstrating left-sided pleural effusion. The presence of sternal wires indicates prior median sternotomy.

Computed tomography (CT) studies provide excellent anatomic detail and characterization of abnormalities or lesions detected on plain radiographs of the chest. Important features that may raise or lower the suspicion of malignancy include the following: location, appearance (cavitary vs solid, spiculated vs well-defined), involvement of adjacent structures such as chest wall or great vessels. Enlarged mediastinal lymph nodes (> 1 cm in long axis) may suggest the presence of metastatic disease and therefore warrant further evaluation. CT may also provide additional information regarding mediastinal or hilar lymphadenopathy, chest wall lesions, and diseases of the lung parenchyma such as bronchiectasis or pulmonary fibrosis (Figure 8–2). Currently, contrast-enhanced CT angiography is widely used for diagnosis of acute pulmonary embolism. With a sensitivity over 80%, it has advantages over traditional ventilation-perfusion scans, including speed, detection of venous thrombosis, and characterization of nonvascular structures.⁵ However, patients with impaired renal function may not be candidates for use of intravenous contrast and in these cases, the ventilation-perfusion scan may be preferred.

Figure 8–2. A. Diffuse mediastinal adenopathy demonstrated on chest CT. B. CT with coronal reconstruction demonstrating pleural thickening consistent with mesothelioma. C. Bone windows of CT scan demonstrating a lesion of the right 3rd rib (arrow). D. Non-contrast CT scan of the chest demonstrating thymic mass in anterior mediastinum (arrow). E. CT scan of the chest demonstrating the presence of left-sided giant bullous disease.

In addition to CT, positron emission tomography (PET) imaging is very useful in further characterizing potentially malignant lesions. The technique uses a radiolabeled glucose (18-fluorodeoxyglucose [¹⁸FDG]) to identify tissues with increased metabolic activity, such as malignant or infectious processes. In a recent meta-analysis, the sensitivity of¹⁸FDG-PET for identifying malignant lesions was 96.8% and the specificity was 77.8%.⁶ However, one limitation of PET is that its resolution is limited in tumors less than 1 cm in size, and accuracy is decreased in these smaller lesions.⁷

Integrated PET/CT (Figure 8–3) and PET/CT fusion (Figure 8–4) studies combine the advantages of both modalities, providing excellent anatomic detail in addition to information regarding metabolic activity. As compared to PET alone and CT alone, integrated and fusion PET/CT studies have greater sensitivity, specificity, negative predictive value and overall accuracy for staging of mediastinal lymph nodes.^8,9 PET/CT can also be used for re-staging of the mediastinal lymph nodes following neoadjuvant therapy; however, the sensitivity of both standalone PET and integrated PET/CT are both lower after induction therapy.¹⁰

Figure 8–3. A. Left lower lobe lesion demonstrated on CT images. B. PET images of left lower lobe mass (arrow). C. Integrated PET/CT images of hypermetabolic left lower lobe mass.

Figure 8–4. PET/CT fusion of hypermetabolic left hilar mass.

For evaluation of suspected superior sulcus (Pancoast) tumors, MRI is often used and preferred. It is superior to CT for evaluating tumor involvement of the brachial plexus, subclavian vessels and vertebral bodies.¹¹ Involvement of these structures may alter surgical management or even render the patient unresectable, thereby changing the course of recommended therapy.

The ventilation-perfusion (V/Q) scan has several uses. Historically, it was most often used in the diagnostic evaluation of suspected pulmonary embolus. More recently, however, CT angiography has supplanted the V/Q scan for patients with suspected pulmonary embolus. Nevertheless, the quantitative V/Q scan is often used to estimate the contribution of the upper, middle, and lower lung zones to overall lung perfusion. Patients with marginal pulmonary function may be able to tolerate resection of a lobe or segment in which there is nominal perfusion as demonstrated by the quantitative V/Q scan.

Imaging of the Mediastinum

The plain chest radiograph is often the most common initial diagnostic imaging study for mediastinal abnormalities. The posteroanterior and lateral radiograph can reveal the presence of a mediastinal mass and its location (anterior vs posterior), as well as mediastinal widening. Further evaluation is usually done with CT imaging, which provides excellent anatomic detail with regard to size and location as well as proximity to and involvement of surrounding structures and great vessels. At present, CT angiography is the test of choice for imaging the great vessels in a hemodynamically stable patient with suspected aortic dissection. In a recent study comparing helical CT and surgical findings, CT demonstrated 100% accuracy in diagnosing Type A aortic dissections and intramural hematomas.¹² For hemodynamically unstable patients, however, transesophageal echocardiography is preferred. MRI, while not used routinely, may provide improved characterization of soft tissues relative to CT and may also be used to characterize vascular structures without the need for intravenous contrast.