Anatomy of the Lung PDF

2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Anatomy of the Lung Dean P. Schraufnagel, MD, Daniel Raymond, MD Key Points 1. Thorough understanding of the anatomic patterns of the airways, pulmonary arteries, veins, an...

2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Anatomy of the Lung Dean P. Schraufnagel, MD, Daniel Raymond, MD Key Points 1. Thorough understanding of the anatomic patterns of the airways, pulmonary arteries, veins, and lymphatics is essential for all forms of lung surgery. 2. Airways, pulmonary arteries, and bronchial arteries run together in the center of the bronchopulmonary segments; pulmonary veins run in the periphery and intersegmental planes. 3. Variations of both large and microscopic structures are common; the airway anatomy is the most constant and pulmonary venous anatomy is the most varied. 4. Pulmonary lymphatics are found in both the peripheral and central parts of the segments. Lymphatic drainage patterns from each lobe are generally predictable, and evaluation of lymph node metastases gives essential prognostic information for patients with non-small cell lung cancer. 5. Locations of pulmonary lymph nodes act as intraoperative landmarks, aiding in the identification of other pulmonary structures during lung resection. 6. Knowledge of anatomic relationships in the hilum is the key to successful pulmonary surgery. Historical Note Our current system of anatomical classification of the lung originated in the pre-antibiotic period of the 1800s, when drainage procedures of intrapulmonary infections relied upon external landmarks. An English pathologist, Ewart, proposed an early system, which classified the lung into functional units in 1889. In 1932 two American otorhinolaryngologists, Kramer and Glass, first described the bronchopulmonary segment. In 1943, Jackson and Huber formalized a widely adopted nomenclature— with 18 named bronchopulmonary segments, 10 on the right and 8 on the left— that continues to be used today Figure 1 Pulmonary Airway Branches, with Accompanying Lobes and Bronchopulmonary Segments There remains an alternate numbered system for the bronchopulmonary segments, known as the Boyden classification. This system denotes 10 segments on the right, and 10 segments on the left (Figure 1). The left upper lobe, apicoposterior segment in the Jackson-Huber classification system is split into the left-sided B1 and B2 segments in the numbered Boyden classification. Similarly, the named anteromedial segment of the left lower lobe in the Jackson Huber classification is split into the left B7 and B8 segments in the Boyden classification. Both of these segments share a common bronchopulmonary segmental airway trunk https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 1/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Lung Development The lungs originate as a groove in the embryonic ventral foregut. This groove elongates, and two bronchopulmonary buds develop at its ends at three weeks gestation. At four weeks, each bronchopulmonary bud begins to branch into primary bronchopulmonary buds, and secondary bronchopulmonary buds. By eight weeks, the major lobar bronchi have developed. By seventeen weeks the majority of the airways have been formed. Alveoli begin to form at week twenty of gestation, and continue to form until a child has reached the age of eight to ten years old. Anatomic Orientation With minimally invasive surgery, thoracoscopic cameras have a limited field of vision. The camera lens required for optimal visualization may be angulated, and the images are projected on to different screens to multiple viewers in the operating room. These variables create the need for a consistent vocabulary to communicate anatomic orientation during thoracic surgery. Superior refers to the direction of the patient’s head; inferior refers to the direction of the patient’s feet; anterior refers to the direction of the patient’s sternum; posterior refers to the direction of the patient’s spine; medial refers to the direction of the patient’s mediastinum; and lateral refers to the direction of the patient’s axilla (Figure 2). Figure 2 Anatomic Orientation General Anatomy The anatomical structure of the lungs contributes to their primary function, which is gas exchange. Gas exchange requires air movement. Air moves into and out of the lungs due to changes in the dimensions of the lung. The lungs are elastic organs whose size varies based upon the respiratory cycle. At peak inspiration, total lung capacity, can be up to 6 L. At the end of expiration, functional residual capacity, the lungs in the same sized adult will contain about 2.5 L Variability in density occurs throughout the lung. The lung is less dense in the apical and peripheral areas, which contain larger proportioned alveoli. The increased density in the basilar and central areas of the lung relates to increased blood containing vasculature. A normal human being has two lungs. The right lung is wider than the left lung, but shorter due to the dome of the liver, which projects upward into the right hemithorax. The apex of each lung can extend up to 3 cm above the medial third of the clavicle. The right lung has three lobes; upper, middle, and lower. The left lung has two lobes, upper and lower. A pulmonary lobe is defined as a portion of lung that has its own vascular and airway supply and is encased by pleura. The functional unit of the lung is the bronchopulmonary segment. This is defined as an anatomical portion of lung parenchyma with its own unique airway, pulmonary arterial supply, and pulmonary venous drainage. As was mentioned earlier, depending on which classification system is used, human lungs contain either 18 or 20 bronchopulmonary segments. https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 2/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Within each segment, the segmental pulmonary artery and bronchus run together. The segmental pulmonary vein runs in the intersegmental fissure, and marks the boundary of each bronchopulmonary segment. Bronchopulmonary segments are pyramidal shaped with their apex pointing towards the center of the lung, and the base pointing towards the periphery Fissures Interlobar fissures are depressions that extend from the outer surface of the lung and—in the case of a complete fissure—continue all the way to the hilum. The right lung has two fissures, the oblique, or major, and the horizontal, or minor, fissures. The left lung has one fissure, the oblique, or major fissure. The visceral pleura envelopes the outer surface of the lung, and continues within each interlobar fissure, thus creating a smooth surface between each lobe at the fissure. This allows lobes to move independently of one another. Fissures are considered complete if the lobe is only held together at the hilum, and incomplete if there is parenchymal fusion between lobes, 17.9% of left oblique, 30.4% of right oblique, and 62.3% of horizontal fissures are incomplete Figure 3 Relationship between fissures and rib cage The oblique fissure on the right begins posteriorly at the level of the fifth rib and runs anteriorly and inferiorly, following the course of the sixth rib. The horizontal fissure begins in the right oblique fissure at the mid-axillary line at the level of the sixth rib and runs anteriorly to the costochondral junction of the fourth rib. The horizontal fissure is the least well-developed of all fissures. The oblique fissure begins higher on the left, between the third and fifth ribs, and runs anteriorly and inferiorly to end at the sixth or seventh costochondral junction. The left oblique fissure is more vertical than the right (Figure 3). Variations in fissures commonly occur. When variations in fissures occur, there are likely to be variations in pulmonary vasculature as well. There may be fused, absent, or accessory fissures. Accessory fissures may form accessory lobes. A cardiac lobe exists when the medial basal segment of the left lower lobe is separated with an intersegmental fissure. A tracheal lobe exists if there is an intersegmental fissure separating the right upper lobe apical segment from the other segments of right upper lobe, and if that lobe has an accessory segmental bronchus coming directly off the trachea. The superior segment of either lower lobe and the lingula are commonly separated by accessory intersegmental fissures, and thus form accessory lobes. Another common variant is the azygos ‘lobe,’ which can be found in 0.5-1% of the population. This is not a true accessory lobe because it is created by an indentation in lung parenchyma from an aberrant loop of azygos vein and its associated mesentery, not a true accessory fissure. Because there is no unique airway, pulmonary arterial supply, and venous drainage, it cannot be removed as an independent segment. Intrapericardial Anatomy Intrapericardial exposure of central pulmonary structures is essential for lung transplantation, excision of central tumors, and proximal vascular control. Pulmonary Artery Trunk https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 3/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic The pulmonary arterial trunk arises from the base of the right ventricle. It continues for 4-6 cm superiorly and to the left. It bifurcates into the right and left main pulmonary arteries behind the aortic arch. Right Main Pulmonary Artery The right main pulmonary artery passes posterior to the aorta and the superior vena cava. There are two intrapericardial approaches to the right main pulmonary artery. The first is via the transverse sinus. This approach requires lateral retraction of the superior vena cava, and medial retraction of the aorta. The right main pulmonary artery is seen proximally where it runs behind the aorta (Figure 4). Figure 4 Right Main Pulmonary Artery, Intrapericardial View Through Transverse Sinus Figure 5 Right Main Pulmonary Artery, Intrapericardial View Through Postcaval Recess of Allison The second approach is through the postcaval recess of Allison. Its borders the superior pulmonary vein inferiorly, the superior vena cava medially, and the pericardial edge laterally. The right pulmonary artery can be controlled intrapericardially within this space by opening the pericardium behind the superior vena cava, and retracting it medially (Figure 5). https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 4/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Figure 6 Left Main Pulmonary Artery, Intrapericarial Approach Right Superior and Inferior Pulmonary Veins The pulmonary veins arise from the most posterior chamber of the heart, yet become the most anterior vessels in the hilum. The right inferior pulmonary vein courses along the posterior pericardial wall, and is only visible within the pericardium about 50% of the time. In 3% of patients, the right sided pulmonary veins form a common trunk before entering the left atrium. Left Main Pulmonary Artery The left main pulmonary artery comes off the pulmonary trunk and travels inferiorly and posteriorly. It exits the pericardium underneath the aortic arch. The left pulmonary artery can be controlled intrapericardially in the left pulmonary recess which is formed by the pericardium laterally, the left superior pulmonary vein inferiorly, and the ligament of Marshall—which is the remnant of the embryonic left superior vena cava—medially. Left Superior and Inferior Pulmonary Veins Similar to the right side, the left superior pulmonary vein arises from the most posterior aspect of the heart, and becomes the most anterior structure in the hilum. Inferior to it, lies the left inferior pulmonary vein. As opposed to the right side, on the left side, 25% of patients have fused superior and inferior pulmonary veins which form a common trunk that enters the left atrium. Hilar Anatomy The term pulmonary hilum refers to the root of the lung. There are no specific anatomic boundaries that define this area. The principle structures of the pulmonary hilum are the main pulmonary arteries, the superior and inferior pulmonary veins, and the mainstem bronchi. Although there can be significant anatomic variation of the bronchi, pulmonary arteries, and pulmonary veins distally, the relationships and positioning of structures in the hilum is usually well defined and predictable. Each lung is fixed in place by the hilum centrally. At the inferior base of each hilum, the mediastinal pleura of the anterior hilum and posterior hilum come together to form the inferior pulmonary ligament, which envelops the inferior pulmonary vein. Each hilum is bordered by a vascular arch; the aortic arch on the left and the azygos vein on the right. The phrenic nerve and vessels run anterior to each pulmonary hilum along the surface of the pericardium. Posterior to each hilum, the vagus nerves run along the esophagus. Right Hilum https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 5/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic In the right hilum, the most superior and posterior structure is the right mainstem bronchus. The right mainstem bronchus is in near continuity with the trajectory of the trachea, making a more direct line for aspirated oropharyngeal secretions or foreign bodies. The right main pulmonary artery exits the pericardium, and runs anterior and inferior to the right mainstem bronchus. It gives off its first branch, the truncus arteriosus, in the hilum, and the ongoing right pulmonary artery continues alongside the bronchus intermedius. The superior pulmonary vein is the most anterior structure of the right hilum, and the inferior pulmonary vein is the most inferior structure. Below the right hilum, the lowest portion of the thoracic duct lies between the azygous vein and aorta, with the esophagus anteriorly, and the spine posteriorly. Figure 7 Right Pulmonary Hilum: Anterior Approach Anterior exposure of the right hilum, allows access to the superior pulmonary vein, the right pulmonary artery, and the inferior pulmonary vein (Figure 7). A posterior exposure allows access to the right mainstem bronchus as it branches into the right upper lobe bronchus and the bronchus intermedius, and the inferior pulmonary vein. Superior exposure reveals the azygos vein and the right mainstem bronchus coming off the trachea. The inferior approach reveals the inferior pulmonary ligament, enveloping the inferior pulmonary vein (Figure 8). Figure 8 Right Pulmonary Hilum, Posterior Approach Table 1: Typical Number of Pulmonary Artery Branches to Each Lobe Lobe Typical Number of proximal Names of proximal pulmonary artery pulmonary artery branches branches (typical no.) Right upper 2-3 Truncus anterior (1), ascending (1-2) lobe Right 1-2 Middle lobe (1), accessory middle lobe (1) middle lobe Right lower 3-5 Superior segmental (1), basal segmental lobe (2-3) https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 6/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Left upper 2-7 Truncus anterior (2-3), posterior segmental lobe (2-4), lingular (1-2) Left lower 3-5 Superior segmental (1), basal segmental lobe (2-3) Left Hilum In the left pulmonary hilum, the left mainstem bronchus emerges from under the aortic arch. The position of the aortic arch over the left mainstem bronchus makes bronchoplastic resections on the left side more difficult than those on the right. The left pulmonary artery runs anteriorly and superiorly to the left mainstem bronchus. The most anterior structure of the left hilum is the left superior pulmonary vein, while its most inferior structure is the left inferior pulmonary vein. Right Upper Lobe The right upper lobe contains three segments, the apical (B1), anterior (B2), and posterior (B3) segments. Its outer surface borders the ribs, the medial surface borders the mediastinum, and the inferior surface borders the horizontal fissure. Airway The right mainstem bronchus is 1.2 to 1.5 cm in length from carina to right upper lobe takeoff–much shorter than the left mainstem bronchus. The right upper lobe bronchus is also known as the epiarterial bronchus because it arises from the right mainstem bronchus before the right pulmonary artery crosses over the bronchus intermedius. The right upper lobe bronchus comes off the right mainstem bronchus at close to a 90 degree angle. This angle makes it an easily recognizable orientation point during bronchoscopy. Figure 9 Bronchoscopic view of a ‘Pig Bronchus’ A common right upper lobe airway anomaly occurs when the right upper lobe bronchus comes directly off the trachea—an anomaly known as ‘pig bronchus’ or ‘bronchus suis (Figure 9).’ This occurs in 0.1 – 5.0% of the population. Additionally, the right upper lobe apical segmental bronchus alone can arise directly from the trachea. There can also be an absence of a true right upper lobe bronchus, so that the division of all three segmental bronchi come directly from the right mainstem bronchus. The right upper lobe bronchus is easily exposed from a posterior approach by dividing the pleura and reflecting the vagus nerve and esophagus posteriorly. Superiorly, the right upper lobe bronchus can be exposed by dividing the pleura between the azygos vein and the hilum. The truncus arteriosus branch of the right pulmonary artery is anterior to the right upper lobe bronchus with this approach. Arterial Supply https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 7/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic The right main pulmonary artery comes out of the pericardium behind the superior vena cava. The arterial supply to the right upper lobe consists of the truncus arteriosus branch of the right pulmonary artery, which comes off the main pulmonary artery in the hilum, and the ascending branches of the right pulmonary artery. The truncus arteriosus branch is usually the major vessel supplying deoxygenated blood to the right upper lobe, and the largest in size. It usually divides into two branches one centimeter after its takeoff from the right main pulmonary artery. However, in 3.6% of patients these two branches each come directly off the right main pulmonary artery In 10% of people, the truncus anterior branch is the only arterial supply to the right upper lobe. The other 90% of people have at least one ascending branch, arising from the interlobar continuation of the ongoing right pulmonary artery. These branches are much smaller than the truncus anterior. Usually, they supply the posterior segment of the right upper lobe and are known as posterior ascending arteries. In 60% of patients, there is one posterior ascending branch; in 29%, there are two; and in 1%, there are three. If, the ascending arteries supply the anterior segment of the right upper lobe, they are called anterior ascending branches. The truncus anterior can be exposed from the anterior hilum, and is best seen after division of the right upper lobe pulmonary vein, which conceals the inferior aspect of the truncus. The posterior ascending branches are seen further inferiorly, and care must be taken to make sure that the posterior ascending branch of the right upper lobe does not have a common trunk with superior segment of the right lower lobe, which occurs in 12% of patients. Venous Drainage The superior pulmonary vein forms from the confluence of the right upper lobe vein and the right middle lobe vein. These two are in close proximity, and care must be taken to avoid transecting the right middle lobe vein when performing a right upper lobectomy. The right upper lobe vein is formed from the confluence of the apical anterior vein, the inferior vein, and the posterior vein of the right upper lobe. Exposure of the right upper lobe vein can be done through an anterior approach, with posterior retraction of the lung. The right upper lobe vein is the most anterior structure in the right pulmonary hilum through this approach. It lies anterior and slightly inferior to the truncus anterior branch to the right upper lobe, and care must be taken when dissecting the vein to avoid injury to the underlying artery. The posterior segmental vein crosses over the trunk of the right pulmonary artery, and at this point this vein is commonly injured, especially when there is an incomplete horizontal fissure. Right Middle Lobe The right middle lobe contains two segments, the lateral (B4) and medial (B5) segments. The right middle lobe only projects anteriorly. Its superior surface is formed by the horizontal fissure, and its inferior surface is formed by the oblique fissure posteriorly, and the right hemidiaphragm more anteriorly. Its anterior surface is formed by the ribs of the anterior chest wall. Airway The bronchus intermedius is the continuation of the right mainstem bronchus after the right upper lobe bronchus take off. The bronchus intermedius is 2-4 cm long and terminates at the bifurcation of the right middle lobe and right lower lobe bronchi. The right middle lobe bronchus is approximately 2 cm long. It divides into the lateral and medial segmental bronchi of the right middle lobe. The right middle lobe bronchus is relatively long and thin. It is susceptible to compression in pathologic states by nearby enlarged lymph nodes or masses. When compression of the right middle lobe occurs, this is known as ‘middle lobe syndrome,’ especially if it is recurrent. Exposure of the right middle lobe bronchus can be done in two ways. With the fissure approach, the pleura is opened in the oblique fissure. The pulmonary parenchyma is divided, and this exposes the pulmonary artery. Generally, the middle lobe bronchus is slightly anterior, and medial to the anterior aspect of the pulmonary artery (Figure 10). Alternatively, exposure of the right middle lobe bronchus can be done from an anterior approach to the hilum, although this first requires division of the middle lobe vein for circumferential control. https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 8/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Figure 10 Right Pulmonary Artery as seen through the Oblique Fissure Arterial Supply The right middle lobe artery comes off the continuation of the right main pulmonary artery, known as the pars intralobares, on its anterior and medial surface, at about the same level as the posterior ascending branches to the right upper lobe. In 51% of patients, the right middle lobe has a second artery known as the accessory right middle lobe artery. The accessory artery to the right middle lobe most often arises at the same level as the pulmonary artery branch to the superior segment of the right lower lobe. The right middle lobe artery can be exposed by opening the oblique fissure. The pulmonary artery is located at the juncture of the horizontal and oblique fissures—known as the confluence of fissures. When performing middle lobectomy from the anterior approach, the artery is generally the most posterior structure, and is divided last. A landmark for the pulmonary artery is an interlobar lymph node (R11) known commonly as the right sided, ‘sump node.’ Occasionally, an anterior ascending branch to the right upper lobe comes off from the artery to the right middle lobe, and this anomalous branch can be injured when completing the horizontal fissure during anatomic surgical resection. Venous Drainage The right middle lobe vein forms from confluence of the lateral and medial segmental veins. In 70% of people the middle lobe vein drains into the superior pulmonary vein, 26% of the time it drains directly into the atrium, and 3.3 percent of the time it drains into the inferior pulmonary vein The right middle lobe vein can be approached anteriorly, with the lung retracted posteriorly. By dissecting between the superior and inferior pulmonary veins in the right hilum, one can determine the inferior border of the right middle lobe vein, and this allows visualization of any venous drainage of the right middle lobe into the right inferior pulmonary vein. Right Lower Lobe The right lower lobe is inferior and posterior to the right oblique fissure, and occupies much of the posterior right hemithorax. The right lower lobe contains five segments, the superior segment (B6), and four basal segments, the medial basal (B7), anterior basal (B8), lateral basal (B9), and posterior basal (B10). The basal segments touch the diaphragm whereas the superior segment does not. The outer surface is the costal surface of the posterior and lateral right hemithorax. Airway The origin of the right lower lobe bronchus occurs at the same level as the right middle lobe bronchus where the bronchus intermedius ends. The superior segmental bronchus comes off laterally, almost at the same level that the right middle lobe bronchus comes off medially. Distal to the superior segmental https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 9/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic bronchus, the lower lobe bronchus continues as the basilar trunk giving off branches to the four right lower lobe basilar segments. Most often, the first branch off the basal trunk is the medial basal segmental branch, and the anterior, lateral, and posterior basal segmental bronchi arise at nearly the same point. The right lower lobe bronchus is difficult to approach without dividing other structures. It can be approached through the oblique fissure. The pulmonary artery is directly anterior to the bronchus, so it is difficult to circumferentially isolate it without dividing the pulmonary artery branches to the lower lobe first. The right lower lobe bronchus can also be approached inferiorly after dividing the inferior pulmonary ligament and inferior pulmonary vein. Arterial Supply The branching pattern of the pulmonary arterial supply to the right lower lobe is similar to the branching pattern of the airway. The superior segmental artery is the first branch off the continuation of the right pulmonary artery to the right lower lobe of the lung after the take-off of the middle lobe artery. Most often, the superior segment is supplied by one branch, but 20% of the time, it can be supplied by two branches. Variations in take-off of the right lower lobe superior segmental branch of the pulmonary artery are numerous. The artery can come off a posterior descending branch to the right upper lobe or from the basal segmental arterial trunk of the lower lobe. Additionally, posterior ascending branches to the right upper lobe can take off from the superior segmental artery. These must be kept in mind when dividing the fissure. After the take-off of the right lower lobe superior segmental pulmonary artery, the right pulmonary artery continues as the common basal artery, which then splits into four branches for the remaining four basilar segments of the right lower lobe of the lung. The right lower lobe arterial supply can be approached through the oblique fissure. The right lower lobe arterial branches are superficial to the airway in the fissure. From the fissure, the space between the posterior ascending and superior segmental artery classically leads to the airway bifurcation and the “sump node”. Venous Drainage The right inferior pulmonary vein forms from confluence of the superior segmental vein, which drains the superior segment of the right lower lobe, and the common basal vein, which drains the four basilar segments of the right lower lobe. The common basal vein forms from the confluence of the superior basal vein and the inferior basal vein. Occasionally, the superior segmental vein may enter the pericardium separately from the common basal vein. The inferior pulmonary vein lies at the most superior portion of the inferior pulmonary ligament. Division of the inferior pulmonary ligament is best achieved by retracting the right lower lobe of the lung superiorly. The right middle lobe veins, or even the posterior segmental vein to the right upper lobe, can course posterior to the airway and drain into the inferior pulmonary vein. Care must be taken not to transect any of these anomalous pulmonary venous branches if they exist. Left Upper Lobe The left upper lobe contains four bronchopulmonary segments, the larger apicoposterior segment (B1 & 3), the anterior (B2), the superior lingular (B4), and the inferior lingular (B5) segments. The left upper lobe’s inferior border is the oblique fissure, and the anterior and superior borders are the anterior and lateral chest wall of the left hemithorax. The lingula (inferior division of the left upper lobe) is analogous to the right middle lobe; they each contain two small segments that project anteriorly, and they both have a long narrow airway. Airway The left mainstem bronchus is longer, and runs at a more oblique angle than the right mainstem bronchus. The left mainstem bronchus exits the mediastinum underneath the aortic arch. It bifurcates https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 10/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic into the left upper lobe and left lower lobe bronchi 4 to 6 cm from the carina. This bifurcation of left upper and lower lobes is known as the secondary carina. After division into the upper and lower lobe bronchi, the upper lobe bronchus bifurcates into the superior and inferior divisions. The superior division bifurcates into the apicoposterior and anterior segmental bronchi. The inferior division bifurcates into the superior and inferior lingular bronchi. Limited exposure to the left upper lobe bronchus can be achieved from both anterior and posterior approaches (Figures 11 and 12). The bifurcation of the left main bronchus may be exposed between the inferior and superior pulmonary veins anteriorly. Circumferential access to the left upper lobe bronchus often requires dividing the superior pulmonary vein for adequate exposure. Posteriorly, the membranous portion of the distal left mainstem bronchus can be exposed. However, the ongoing pulmonary artery creates a significant barrier to further dissection of the airway from the posterior approach. Figure 11 Figure 12 Left Pulmonary Hilum, Anterior Approach Left Hilar Anatomy, Posterior Approach Arterial Supply The left main pulmonary artery exits the pericardium underneath the aortic arch and is anchored to the aorta at the aortic isthmus by the ligamentum arteriosum. The left sided recurrent laryngeal nerve wraps under the aorta on its way back up to the neck at the ligamentum arteriosum. The arterial supply to the left upper lobe is the most varied of all lobes. It can have as few as two and as many as seven branches. The branches are generally classified into groups, branches from the truncus anterior, which come off proximally, the posterior segmental arterial branches, which come off more distally, and lingular branches. The truncus anterior is a short branch located posteriorly to the superior pulmonary vein. If there is a large truncus anterior branch, usually there will be fewer posterior segmemntal arterial branches. About 70% of patients have two branches off of the truncus anterior; 15% have one; and 15% have three. The majority of patients have their apical posterior and anterior segments branches or trunks supplied by the truncus anterior. Sometimes, instead of having an anterior segmental trunk off of the truncus anterior, individual segmental branches can come directly off the left main pulmonary artery separately. The posterior segmental branches come off the ongoing left pulmonary artery after the origin of the truncus anterior branch. These branches come off the inner curve of the ongoing left pulmonary artery as it wraps around the left upper lobe bronchus. There may be zero to five, but usually there are one to two posterior branches that come directly off the left pulmonary artery and supply the posterior portion of the apical posterior segments. In 35% of patients, posterior branches to the left upper lobe come off in common trunks. The most commonly seen final arterial branch to the left upper lobe is the lingular branch. This vessel usually comes from an interlobar portion of the left pulmonary artery after the branch to the superior segment of the left lower lobe. However, it can cross the fissure and come off the basal segmental branches to the left lower lobe. It is often found deep to the left upper lobe bronchus when approaching anteriorly. Exposure of the truncus anterior is best done by retracting the lung laterally and posteriorly. The truncus anterior lies posterior and superior to the superior pulmonary vein. Between the left pulmonary artery https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 11/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic and the superior pulmonary vein is the ligament of Marshall, which can be divided for proximal control of the left main pulmonary artery. For branches to the lingula, the best approach is through the oblique fissure with the left upper lobe retracted superiorly, and the left lower lobe retracted inferiorly. The lingular branches are best seen when the parenchyma of the interlobar fissure has been divided. Alternatively, the lingular branch can be seen after division of the left upper lobe bronchus from an anterior approach. The left sided ‘sump node,’ which is an interlobar (L11) node lies anterior to the pulmonary artery at the bifurcation of the left mainstem bronchus. Venous Drainage The left upper lobe is drained by the superior pulmonary vein, which forms from the confluence of multiple segmental pulmonary veins, usually four. These tributary veins include the apicoposterior vein, which drains the apicoposterior segment, the anterior vein, which drains the anterior segment, and the superior and inferior lingular veins, which drain the lingula. Occasionally, the lingular veins may drain into the inferior pulmonary vein, as opposed to the superior pulmonary vein. The superior pulmonary vein lies anterior and inferior to the left pulmonary artery, and thus should be divided to completely expose the pulmonary artery anteriorly. Approaching the left superior pulmonary vein intrapericardially provides additional length for division. Left Lower Lobe The left lower lobe is similar in shape, size, and orientation to the right lower lobe. It projects posteriorly and inferiorly and contains four segments, the superior segment (B6), and three basal segments, the anteromedial (B7 and B8), lateral (B9), and posterior (B10) segments. The superior surface of the left lower lobe is the oblique fissure. The inferior surface is the left hemidiaphragm, and the outer surface is the posterior and lateral chest wall of the left hemithorax. Airway At the secondary carina, the left mainstem bronchus bifurcates into the upper and lower lobar bronchi. The first branch off of the left lower lobe bronchus is the superior segmental bronchus, which comes off posteriorly and laterally about 5 mm from the secondary carina. About 1.5 cm farther, the anteromedial basal segmental bronchus comes off anteriorly. The final portion of the left sided airway then bifurcates into the lateral basal and posterior basal segments. The left lower lobe bronchus can be approached through the oblique fissure with the lower lobe retracted inferiorly. It lies posterior to the inferior pulmonary vein, and is most easily approached after division of basilar pulmonary arterial branches (Figure 13). Figure 13 Left Pulmonary Hilum Approached from Fissure https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 12/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Arterial Supply The pulmonary arterial supply to the left lower lobe is similar to that of the right lower lobe. All of the pulmonary arterial branches to the left lower lobe arise in the oblique fissure. The first arterial branch to the left lower lobe is the superior segmental branch. Seventy two percent of the time, there is a single branch to the superior segment of the left lower lobe; 26% of the time there are two branches to the superior segment of the left lower lobe; and 2% of the time there are three. This comes off after the posterior branches to the left upper lobe, and before the lingular branches (Figure 13). It comes off posteriorly, with the bronchus to the superior segment to the left lower lobe in the interlobar fissure. The next branch is the lingular branch to the upper lobe, which comes off the superior aspect of the ongoing pulmonary artery. Following the lingular branch, the pulmonary artery becomes the common basilar trunk. It then bifurcates into an anterior branch, which supplies the anteromedial basal segment, and a posterior branch, which supplies the lateral basal and posterior basal segments. In 3.5% of cases, there is a branch from the posterior segmental artery, which supplies the superior segment. Pulmonary arterial branches to the left lower lobe are best approached through the oblique fissure. The left lower lobe pulmonary arteries lie anterior to the left lower lobe airway branches. All of the pulmonary artery branches can be divided together when performing a lobectomy, but often the superior segmental artery is divided separately. Venous Drainage The left lower lobe is drained by the left inferior pulmonary vein. This vein forms from the confluence of the superior segmental vein, which drains the superior segment of the left lower lobe, and the common basal veins, which drain the four basal segments of the left lower lobe. The left inferior pulmonary vein can be approached by dividing the left inferior pulmonary ligament. Occasionally, the superior segmental vein may enter the pericardium separately from the common basal vein. Bronchial Arteries and Lymphatics Bronchial Arteries Bronchial arteries are systemic arteries that arise from the aorta or its proximal branches to supply oxygenated blood to the central airways and large pulmonary vessels. They represent 3-5% of cardiac output. Although they are often divided—during lung transplantations, seemingly without clinical importance—there is evidence that bronchial arteries play a role in the pulmonary immune system and in fluid balance. Bronchial arteries usually number from one to four. The most common variation is two bronchial arteries to the left, and one to the right (Figure 14). This explains why bronchoplastic procedures on the right are more susceptible to ischemia than those on the left. The bronchial arteries originate from the descending aorta or its proximal branches between the T5 and T6 vertebrae. Bronchial arteries may have significant collateralization; both the left and right bronchial arteries can supply the airways on the opposite side. https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 13/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic Figure 14 Bronchial Artery Anatomical Variants 3 The right bronchial artery most commonly originates as a branch of the first or—less often second— right intercostal artery. This vessel is also known as the right intercostobronchial artery. It courses from its origin at the proximal intercostal artery behind to the esophagus to the right mainstem bronchus. During its course across the mediastinum, the right bronchial artery gives branches to the mid- esophagus, trachea, pericardium, left atrium, mediastinal lymph nodes, and the vagus nerve. When the right bronchial artery reaches the right mainstem bronchus it sits on the posterior membranous portion, and then divides into three branches--one to the carina, one to the right upper lobe, and one to the bronchus intermedius. There are usually either one or two left bronchial arteries, and these most often come directly from the aorta. When there are two left bronchial arteries, each one accompanies the left upper and left lower lobe bronchus. Like the right bronchial artery, the left bronchial arteries lie on the posterior membranous portion of the airways. The most proximal branches of bronchial arteries, along with direct branches from the aorta, supply the mid-thoracic esophagus. Once they reach the airways, the bronchial arteries course along the membranous portion of the mainstem bronchi. Bronchial arteries supplying the large hilar structures drain into systemic bronchial veins, which then empty into the azygos and hemiazygos veins. However, the majority of bronchial arteries supplying blood to more distal structures directly drain into pulmonary veins bypassing the systemic venous system altogether. Pulmonary Lymphatics The thoracic lymphatic system assists with drainage and transportation of fluid from the lungs, and transportation of chyle from the alimentary tract into the venous circulation. The lungs contain a superficial plexus of lymphatics located just underneath the visceral pleura and a deep plexus of lymphatics surrounding the airways, pulmonary arteries, and pulmonary veins. These two plexuses anastomose with each other at the hilum. Occasionally the superficial, subpleural lymphatics directly connect to mediastinal lymph nodes without anastomosing with the deep plexus. In this situation, patients with lung cancer may have what are known as, ‘skip metastases’ where a tumor metastasizes to mediastinal lymph nodes, without first metastasizing to lymph node stations that are more proximal to the primary tumor. Lung lymphatics originate in the lung interstitium between alveoli as partially endothelialized channels. As these channels fuse, they eventually become tubular, and are then considered lymphatic capillaries. Lymphatic vessels have valves every 2 to 10 mm that allow flux of fluid from, but not back to, the lung interstitium to keep it dry. Lymph nodes are interspersed along the trajectory of the lymphatic tree. The entire lymphatic system including its regional nodes can greatly expand in pathologic states such as cancer or heart failure. There are three principal lymphatic collectors in the chest. The posterior parietal chain travels along the spine and drains the posterior chest wall, parietal pleura, and posterior diaphragm. The anterior parietal chain travels with the internal thoracic vessels and drains the anterior chest wall, the anterior https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 14/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic diaphragm, and the medial breast. The medial visceral chain travels with airways and drains the lung parenchyma. All three chains eventually join and empty into the thoracic duct. Each pulmonary lobe has a characteristic pattern of lymphatic drainage. The right upper lobe drains first through the right paratracheal nodes. The right middle and lower lobes drain first through the subcarinal nodes. The left upper lobe usually drains first through the paratracheal nodes, although a third of the time it will drain through the subaortic and periaortic lymph nodes. The left lower lobe first drains through the subcarinal nodes. On both sides of the chest, there is a lymph node station, 11R and 11L, at the take-off of each upper lobe bronchus known as the ‘sump node.’ This node is an important surgical landmark during pulmonary resection. However, it also is an important prognostic indicator when it contains metastatic invasion because it is a nodal station that drains all lobes of each lung and thus has the name, ‘sump node’. Figure 15 Regional Lymph Nodes: 1, highest mediastinal; 2R and 2L right and left paratracheal; 3p paratracheal; 4R and 4L right and left tracheobronchial; 5 aortopulmonary; 6 para-aortic; 7, subcarinal; 8, paraesophageal; 9, pulmonary ligament; 10R and 10L right and left hilar; 11R and 11L, right and left interlobar; 12R and 12L right and left lobar; 13R and 13L right and left segmental; 14R and 14L, right and left subsegmental; Ao aorta; PA, pulmonary artery Mapping of the regional lymph nodes of the lung was first proposed by Narake and colleagues in 1978, and then revised by Mountain and Dressler in 1997, (Figure 15). Investigation into lymph node spread of non-small cell lung cancers is an essential component of staging. Regional lymph nodes of the lungs have been mapped into fourteen stations. Stations 1-9, which are easily recalled because they are all the single digit lymph node stations, are the mediastinal lymph nodes. Station 1 lymph nodes are the high cervical nodes, and these are rarely involved in lung cancer. Stations 2 and 4 lymph nodes are the upper and lower paratracheal nodes. Station 3 nodes are located in the prevascular anterior space (3a) or the retrotracheal space (3p). Station 5 nodes are underneath the aortic arch. Station 6 nodes are near the ascending aorta. Station 7 nodes are subcarinal. Station 8 nodes are paraesophageal. Station 9 nodes are in the inferior pulmonary ligament. If a patient has lung cancer that has metastasized to an ipsilateral mediastinal lymph node, this would be classified as at least N2 disease in the TNM (tumor, nodes, metastases) classification system. Contralateral mediastinal lymph node involvement of a lung cancer denotes at least N3 disease. Station 10 lymph nodes are hilar lymph nodes. Stations 11 to 14 nodes are intrapulmonary nodes; station 11 nodes are interlobar, station 12 nodes are lobar, station 13 nodes are segmental, and station 14 nodes are subsegmental. Metastases to ipsilateral station 10 to 14 lymph nodes of a non- small cell lung cancer are classified as at least stage N1 disease. Recent advances in minimally invasive cancer staging have brought many new ways to sample the regional lymph nodes of the lung (Table 2). Table 2: Pulmonary Lymph Node Stations and Their Accessibility by Various Techniques 2R 2L 3a 3p 4R 4L 5 6 7 8 9 10R 10L 11R 11L EBUS + + - + + + - - + - - + + + + https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 15/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic EUS +/- +/- - + +/- + +/- +/- + + + +/- +/- - - EBUS + EUS + + - + + + +/- - + + + + + + + EMN + + - + + + - - + - - + + + + TTNA + + + + + + + + + + + + + + + Cervical mediastinoscopy + + - - + + - - + - - + - - - Extended mediastinoscopy + + - - + + + + + - - - - - - Chamberlain - - - - - - + + - - - - - - - Left VATS - - - - - + + + + + + - + - + Right VATS + - + + + - - - + + + + - + - VAMLA + + - - + + - - + + - - - - - TEMLA + + + + + + + + + + + + - - - +: Accessible; -: Inaccessible; EUBS: Endobronchial ultrasound; EMN: Electromagnetic navigation bronchoscopy; EUS: Endoscopic ultrasound; TEMLA: Transcervical extended mediastinal lymphadenectomy; TTNA: Transthoracic needle aspiration; VAMLA: Video-assisted mediastinal lymphadenectomy; VATS: Video-assisted thoracis surgery Pulmonary Nerves The lungs are innervated by branches from the vagus nerves, and the sympathetic ganglia. The vagus nerves supply the vast majority of parasympathetic and sensory fibers to the airway. Postganglionic sympathetic nerve fibers innervate bronchial blood vessels. The right vagus nerve runs along the right side of the trachea. It passes posterior and medial to the superior vena cava and right hilum and splits into posterior pulmonary branches. These posterior pulmonary branches of the vagus nerve join with branches of the thoracic sympathetic ganglia to form the pulmonary plexus on the right side. The left vagus nerve courses anterior to the aortic arch, and gives off the left recurrent laryngeal nerve, which wraps under the aorta and then travels superiorly. After the recurrent laryngeal nerve take off, the left vagus nerve contributes branches to the pulmonary plexus on the left side. The anterior pulmonary plexus on both sides is poorly developed and made up of branches from the vagus nerves, and cervical sympathetic branches. The posterior pulmonary plexus is well-formed, and contains branches of the vagus nerves and thoracic sympathetic ganglia. After nerves exit the pulmonary plexus, they travel distally to form periarterial and peribronchial plexuses in each lung. Division of the pulmonary plexus, such as during bronchoplastic procedures or lung transplantation, does not seem to have much clinical significance. Commentary Drs. Schraufnagel and Raymond provide an excellent summary and illustrations of the key anatomy of the airways, pulmonary arteries, veins, and lymphatics. This information is essential for any thoracic surgeon. The authors also highlight the common anomalies of the bronchovascular structures. An important point made by the authors is the correct use of descriptors for anatomic orientation during conduct of the thoracic operative procedure. In addition, there are highly relevant descriptors of approaches to vessels, particularly when performing an intrapericardial dissection. Furthermore, the tables provided in the chapter allow for easy identification and retrieval of the common pulmonary arterial anomalies associated with each lobe. In summary, this is an excellent chapter that all thoracic surgeons should essentially have committed to memory. As the number of pulmonary segmentectomies is likely to increase, particularly from a minimally invasive approach, a thorough understanding of pulmonary arterial anomalies is quite important, and therefore this is another reason to be very familiar with the information contained within this chapter. https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 16/17 2/17/2021 Anatomy of the Lung | Pearson's General Thoracic References 1. Ugalde P, Camargo Jde J, Deslauriers J. Lobes, fissures, and bronchopulmonary segments. Thorac Surg Clin. 2007;17(4):587-99. [PMID:18271171] 2. BOYDEN EA, HAMRE CJ. An analysis of variations in the bronchovascular patterns of the middle lobe in fifty dissected and twenty injected lungs. J Thorac Surg. 1951;21(2):172-80. [PMID:14814715] 3. Sealy WC, Connally SR, Dalton ML. Naming the bronchopulmonary segments and the development of pulmonary surgery. Ann Thorac Surg. 1993;55(1):184-8. [PMID:8417676] 4. Broaddus VC, Mason RC, Ernst JD, et al. Murray & nadel’s textbook of respiratory medicine. Elsevier Health Sciences;2015. 5. MEDLAR EM. Variations in interlobar fissures. Am J Roentgenol Radium Ther. 1947;57(6):723-5. [PMID:20249164] 6. Milloy F, ANSON B, WRAGG L. PULMONARY ARTERIAL SUPPLY TO RIGHT UPPER LOBE OF LUNG BASED UPON A STUDY OF 300 LABORATORY AND SURGICAL SPECIMENS. SURGERY GYNECOLOGY & OBSTETRICS. 1963;116(1):35-&. 7. Barat M, Konrad HR. Tracheal bronchus. Am J Otolaryngol. 1987;8(2):118-22. [PMID:3592078] 8. Wragg LE, Milloy FJ, Anson BJ. Surgical aspects of the pulmonary arterial supply to the middle and lower lobes of the lungs. Surg Gynecol Obstet. 1968;127(3):531-7. [PMID:5663822] 9. Yazar F, Ozdogmus O, Tuccar E, et al. Drainage patterns of middle lobe vein of right lung: an anatomical study. Eur J Cardiothorac Surg. 2002;22(5):717-20. [PMID:12414036] 10. Milloy FJ, Wragg LE, Anson BJ. The pulmonary arterial supply to the upper lobe of the left lung. Surg Gynecol Obstet. 1968;126(4):811-24. [PMID:5643163] 11. Tong MZ, Johnston DR, Pettersson GB. The role of bronchial artery revascularization in lung transplantation. Thorac Surg Clin. 2015;25(1):77-85. [PMID:25430431] 12. Riquet M. Bronchial arteries and lymphatics of the lung. Thorac Surg Clin. 2007;17(4):619-38, viii. [PMID:18271173] 13. Brotons ML, Bolca C, Fréchette E, et al. Anatomy and physiology of the thoracic lymphatic system. Thorac Surg Clin. 2012;22(2):139-53. [PMID:22520281] 14. Riquet M, Hidden G, Debesse B. Direct lymphatic drainage of lung segments to the mediastinal nodes. An anatomic study on 260 adults. J Thorac Cardiovasc Surg. 1989;97(4):623-32. [PMID:2927166] 15. Naruke T, Suemasu K, Ishikawa S. Lymph node mapping and curability at various levels of metastasis in resected lung cancer. J Thorac Cardiovasc Surg. 1978;76(6):832-9. [PMID:713589] 16. Mountain CF, Dresler CM. Regional lymph node classification for lung cancer staging. Chest. 1997;111(6):1718-23. [PMID:9187199] 17. Dhillon SS, Dhillon JK, Yendamuri S. Mediastinal staging of non-small-cell lung cancer. Expert Rev Respir Med. 2011;5(6):835-50; quiz 851. [PMID:22082168] 18. Wang J, Li J, Liu G, et al. Nerves of the mediastinum. Thorac Surg Clin. 2011;21(2):239-49, ix. [PMID:21477774] 19. Fréchette E, Deslauriers J. Surgical anatomy of the bronchial tree and pulmonary artery. Semin Thorac Cardiovasc Surg. 2006;18(2):77-84. [PMID:17157224] https://ebook.sts.org/sts/view/Pearsons-General-Thoracic/1418453/all/Anatomy_of_the_Lung 17/17