Pertinent surgical anatomy of the thorax and mediastinum

Chest wall

The muscular, tendinous, and bony structures of the chest serve several functions. The chest wall must be rigid enough to protect the thoracic viscera and serve as a fixation point against which the muscles of the upper extremity and abdomen can work yet flexible enough to expand and contract with vigorous respirations.

With gentle respirations, the chest wall is a cylinder with the diaphragm as its piston. With inspiration, the diaphragm contracts, its dome is flattened, and like a piston, it descends in the chest. This motion increases the volume of the thorax and actively expands the lungs by drawing in air through the trachea. The lungs are very elastic and tend to collapse without outward forces keeping them expanded. With exhalation, the diaphragm relaxes, the elasticity of the lungs causes lung volume to decrease, and air is expelled. Ultimately, the tendency of the lung to collapse is countered by the outward force/rigidity of the chest wall. With vigorous respirations, the intercostal muscles, scalenes, and other accessory muscles of respiration elevate the ribs and increase the thoracic volume much more than usual. With vigorous respirations, the chest wall and diaphragm act in concert like a bellows increasing thoracic volume and then relaxing and allowing the elasticity of the lung to decrease thoracic volume.

The bony structures of the chest wall include 12 ribs, 12 thoracic vertebrae, and the sternum. All ribs articulate posteriorly with the transverse processes and vertebral bodies of their respective thoracic vertebrae and the vertebral body directly superior ( Fig. 1 ). Ribs 1 through 7 are called true ribs because they articulate anteriorly directly with the sternum through their own costal cartilage. Ribs 8, 9, and 10 are called false ribs because they articulate anteriorly to the costal cartilage of the rib above. This creates a construct of stair-stepping costal cartilages, which ultimately articulates with the sternum and creates the costal arch or costal margin. Ribs 11 and 12 are called floating ribs because they do not articulate with any structure anteriorly ( Fig. 2 ). Rather, they attach to the abdominal wall musculature, primarily the internal oblique muscle.

Because ribs 1 through 10 are fixed anteriorly and posteriorly, they function much like a bucket handle ( Fig. 3A ). When performing a tube thoracostomy, as you approach the sternum anteriorly and the transverse processes posteriorly, the size of the interspace becomes fixed and narrow. Laterally, away from these points of attachment, the ribs separate, and the interspace opens. The widest portion of the interspaces can be found at the lateral apogee or “keystone” of the rib. Tube thoracostomies placed laterally will be easier to place through the interspace and more comfortable for the patient ( Fig. 3B ). Also, when creating a thoracotomy, division of the intercostal muscles far anterior and posterior will create a larger working space without tearing the intercostal muscle or fracturing a rib with placement of the rib spreader. The skin need only be divided over the working space, not over the entire intercostal incision.

The sternum has three parts, the manubrium, the body, and the xiphoid process. The manubrium is thick and broad, articulating with the clavicle, first rib, and sharing the second rib articulation with the body of the sternum. The sternoclavicular articulation is the only bony articulation of the thorax to the shoulder girdle (see Fig. 2 ). Understanding the angle of the clavicle, manubrium, and first rib is important in safe placement of central venous catheters into the subclavian vein. The subclavian vein and artery leave the arm and enter the thoracic inlet over the top of the first rib and under the clavicle. Once under the clavicle, a needle directed parallel to the clavicle and first rib will not enter the chest and cause a pneumothorax before finding the subclavian vein. A needle directed too steeply in its approach will quickly enter and exit the triangle where the subclavian vein is found, penetrate the intercostal space, and puncture the lung ( Fig. 4 ).

The second rib inserts into the sternomanubrial junction, also called the angle of Louis. This can be easily palpated in most people as a horizontal ridge in the sternum where the two planes that make up the sternum intersect ( Fig. 5 ). The interspace immediately below the angle of Louis is the second interspace. The angle of Louis serves as a landmark to rapidly locate the second rib and second interspace for placement of a catheter to decompress a tension pneumothorax.

The first rib is short, broad, and flat, and arches sharply from posterior to anterior ( Fig. 6 ). The second rib is longer than but very similar to the first rib ( Fig. 7 ). The first slip of the serratus anterior muscle attaches to the second rib approximately one third of the arc from posterior to anterior—this slip also attaches to the inferior aspect of the first rib. Posterior to this attachment, the scalenus posterior muscle attaches to the second rib.

When performing a thoracotomy, counting ribs can identify the correct interspace. Once the latissimus dorsi muscle has been divided and the serratus anterior muscle divided or swept anterior, the scapula is elevated. Thin fibrous attachments hold the undersurface of the scapula to the chest wall. A hand placed deep to the scapula, posterior near the spine, and apically can palpate ribs. The first rib is identified by its conspicuously broad and flat contour. Inferior to this, the second rib can be identified by the attachment of the scalenus posterior muscle. This muscle body is palpable by sweeping the finger from posterior to anterior along the second rib ( Fig. 8 ). Less distinct will be the third rib, which seems to “turn the corner” from the apex of the chest to the lateral chest wall ( Fig. 9 ). In a lateral decubitus position, the tip of the scapula overlies the sixth interspace. In a male, the nipple overlies the fourth interspace.

Muscles of the chest wall

Integral to safe thoracentesis, placement of a tube thoracostomy, or a thoracotomy is understanding the layers of the chest wall and the anatomy of the interspace.

The paired pectoralis major muscles cover the majority of the anterior chest wall. The pectoralis major muscle originates from the clavicle and anterior aspects of ribs 1 through 6 inserting on the proximal humerus. Its origin from the chest wall is broad and an anterior thoracotomy will divide or separate its fibers. Inferiorly, the rectus abdominus muscle inserts onto the costal cartilages of ribs 5 through 7 and the xiphoid process. Lateral to this, the muscle fibers of the external oblique insert onto ribs 5 through 12. The external oblique muscle interdigitates with the serratus anterior muscle as it inserts on ribs 1 through 8 ( Fig. 10 ). Most thoracotomies do not traverse the interspaces guarded by the rectus abdominis and external oblique. These muscles will be encountered with thoracoabdominal incisions crossing the costal margin.

Laterally and posteriorly, two musculofascial layers guard the ribs. The more superficial layer contains the latissimus dorsi muscle laterally. Posteriorly, at the auscultatory triangle, or posterior border of the latissimus dorsi, this layer becomes a thin but tough layer of fascia, which more posteriorly envelopes the trapezius muscle. The second musculofascial layer contains the serratus anterior muscle laterally, becoming a broader sheet of thin but tough fibrous tissue posteriorly and then becoming the rhomboid major muscle and then the rhomboid minor muscle posteriorly and superiorly ( Fig. 11 ). A tube thoracostomy will traverse these muscle layers to reach the ribs and interspaces. Knowing where you are in these layers allows precious time to be saved in traversing them and getting to where you need to be to complete the procedure.

A typical tube thoracostomy is placed in the fifth interspace at the anterior axillary line. The muscle bodies traversed are thinner here. From superficial to deep, the surgeon will separate skin, subcutaneous adipose tissue, the latissimus dorsi/trapezius musculofascial layer, and then the serratus anterior musculofascial layer. At this depth, the shiny surface of the periosteum of the ribs and the oblique fibers of the external intercostal muscle can be seen. As discussed later, tube thoracostomies are performed over the superior aspect of the rib. It is much easier to locate the superior aspect of the rib when you do not have intervening layers of muscle and fascia.

A thoracotomy can be fashioned to divide or spare these muscles as needed in order to gain access to the rib cage. A full thoracotomy will divide the latissimus dorsi laterally and the trapezius posteriorly. The incision sweeps from horizontal across the lateral chest to vertical and parallel to the spine posteriorly ( Fig. 12 ). Deep to this layer, the serratus anterior can be swept anteriorly or divided. Posteriorly, the fascial layer coming off the serratus anterior is divided and then the rhomboid major and rhomboid minor muscles are divided. The innervation of the trapezius muscle and rhomboid muscles runs from medial to lateral. The more muscle body that is left medially, the more muscle function will be retained. Enough muscle needs to be left attached to the scapula to allow suture repair of the muscle, and the muscle should not be stripped from the scapula. The posterior and vertical aspect of this incision where the trapezius and rhomboids are divided is done to elevate the scapula off the chest wall, to access the interspaces underneath.

A thoracotomy can be extended anteriorly, dividing the pectoralis major muscle overlying the interspace of interest. The sternum can be split transversely, and a thoracotomy continued on the contralateral side. This is termed a “clam shell” thoracotomy. The left and right mammary artery will be found 1 cm lateral to and on either side of the sternum, deep to the ribs and intercostal muscles, but superficial to the pleura. These vessels can be cauterized if speed is needed but are prone to spasm and late bleeding and should be sought and ligated when possible. Frequently, as perfusion is restored, they will begin to bleed and should then be ligated.

Intercostal space

Each intercostal space, from superficial to deep, has two layers of muscle; an artery, a vein, and a nerve; and a diminutive inner layer of muscle. The external intercostal muscles run obliquely with fibers in the same orientation as the external oblique muscle of the abdomen (fingers in pockets). Deep are the internal intercostal muscles running in the opposite direction. The intercostal artery, vein, and nerve run along the inferior aspect of each rib, occasionally running underneath a ledge in the costal groove. To avoid injury to these three structures, tube thoracostomies and thoracotomies are directed over the superior aspect of each rib or through the middle of the interspace but not the inferior aspect of the rib ( Fig. 13 ). The innermost intercostal muscles are located deep to the neurovascular bundle and run in the same direction as the internal intercostal muscles. Although mentioned in anatomy texts, surgically, the innermost intercostal muscles do not need to be considered separately from the internal intercostal muscle ( Fig. 14 ). The intercostal arteries originate as segmental branches off the descending aorta. The intercostal space, including the underlying pleura, can be harvested as a posteriorly based pedicled muscle flap ( Fig. 15 ). This flap is useful for reinforcing bronchial or esophageal repairs.

The internal mammary artery originates from the subclavian arteries bilaterally and descends on the inside of the chest wall, approximately 1 cm lateral to the sternum bilaterally ( Fig. 16 ).

Pleural space

Normally the lung is coupled to the chest wall by the vacuum that exists between the visceral and parietal pleura. With penetration of the chest wall air is allowed into the pleural space from the outside, or more commonly, penetration of the lung allows air to escape from air spaces within the lung (alveoli, bronchioles, bronchi) into the pleural space. The coupling of the visceral and parietal pleura is broken and the potential space, which is the pleural space, becomes a real space. The elasticity of the lung causes it to collapse, and a pneumothorax is formed. The pleural space extends superiorly to where it rises above the circumference of the first rib to inferiorly where the diaphragm inserts on the costal margin and the 12th rib. Lung may or may not be present between the diaphragm and ribs in the lowermost recesses of the pleural space. Anterior to the pericardium and posterior to the sternum, the two pleural cavities can abut but rarely communicate.

Diaphragm

The diaphragm is the movable dome-shaped muscle that separates the thoracic and abdominal cavities. With full exhalation, the dome of the diaphragm can rise to the level of the fourth interspace anteriorly (nipple level). With full inhalation, the diaphragm flattens, bringing the thoracic cavity down to the level of the costal margin anteriorly and the 12th rib posteriorly. The muscle fibers of the diaphragm originate from the sternum, the ribs, and the vertebral column. All three groups insert on a tough, fibrous central tendon. Fibers of the sternal portion are short, arising as small slips from the back of the xiphoid process. Laterally on either side of the xiphoid, fibers originate from the inner surface of the lower six costal cartilages (costal margin). Posteriorly, fibers originate from a thick band arching over the quadratus lumborum (lateral arcuate ligament) and the psoas major (medial arcuate ligament). The paired lateral arcuate ligaments extend from the tip and lower margin of the 12th ribs and arch over the quadratus lumborum muscle to the transverse processes of L1. The paired medial arcuate ligaments complete the journey, arching over the psoas major from the tip of the transverse process of the first lumbar vertebrae to the tendinous portion of each diaphragmatic crus ( Fig. 17 ).

The posterior medial portion of the diaphragm is composed of two crura—an anatomic right crus originating from the upper three lumbar vertebral bodies and an anatomic left crus originating from the upper two lumbar vertebral bodies. Anterior to the aorta, the medial margins of the two crura form a poorly defined arch called the median arcuate ligament. Anterior to this arch, either the anatomic right crus (64%) or the anatomic left crus (2%) or both (34%) form the esophageal hiatus. Although anatomists name the crura left or right by their origin from the left or right side of the vertebral bodies, surgeons name the crura left or right by their relationship to the esophagus. In the abdomen, visualization of the esophagus and division of the crus running to the left of the esophagus will expose the distal thoracic aorta above the level of the celiac artery and renal arteries. A clamp can be applied here to obtain vascular control. Alternatively, the Conn aortic root compressor or a small Richardson retractor wrapped with a laparotomy pad can be used in this position to occlude the aorta by compressing it against the posteriorly located vertebral body ( Fig. 18 ).

The phrenic nerve and twigs from the lower intercostal nerves innervate the diaphragm. The phrenic nerve originates primarily from the C4 nerve root, but receives innervation from C3 and C5 (C3, C4, and C5 keep the body alive). In the neck, the phrenic nerve originates lateral to the scalenus anterior muscle and descends from lateral to medial on the superficial surface of this muscle, deep to the sternocleidomastoid muscle. It enters the thoracic inlet and is found on the medial aspect of the mediastinum just deep to the pleura bilaterally. Superiorly, it is very anterior in the chest and vulnerable to injury, especially with a median sternotomy and dissection of the great vessels, where it is often not readily visible in the wound, but very close to the dissection. On the left, it descends outside the pericardium, deep to the pleura, passing over the arch of the aorta, anterior to the hilum of the lung, and anterior to the inferior pulmonary ligament. As it nears the diaphragm, it is often invested in a veil of pericardial fat, hanging like a curtain between the pericardium and the diaphragm. The nerve reaches the diaphragm just lateral to the left border of the heart and in a plane slightly more anterior than the right phrenic nerve (see Fig. 33 ). The right phrenic nerve descends along the right lateral border of the superior vena cava and passes anterior to the hilum of the lung, anterior to the inferior pulmonary ligament. It is also invested in a veil of pericardial fat as it approaches the diaphragm. The right phrenic nerve enters the diaphragm just lateral to the inferior vena cava (see Fig. 32 ).

Both left and right phrenic nerves immediately trifurcate into three muscular branches after entering the hemidiaphragm. One is directed anteromedially toward the sternum, one anterolaterally, and a third posteriorly. The posterior branch bifurcates into a branch directed toward the 12th rib and one toward the crus. Safe incisions in the diaphragm are fashioned to avoid cutting major branches of the phrenic nerve ( Fig. 19 ). A peripheral and circumferential incision will avoid all but distal twigs of the phrenic nerve. Radial incisions can be placed but must be done with care to avoid major branches of the phrenic nerve.

Because the primary innervation of the diaphragm, the phrenic nerve, enters centrally and spreads centrifugally, the diaphragm can be transposed to higher or lower origins from the thoracic cage while maintaining its innervation. This is occasionally required in repair of a diaphragmatic rupture when surface area of the diaphragm is lost, or the chest wall has lost its rigidity and can no longer subserve its cylinder function. Care should be taken to maintain a dome shape to the diaphragm. A diaphragm that is flattened at rest will pull the walls of the thorax closer together when contracting. With contraction, instead of increasing intrathoracic volume, the diaphragm will now decrease intrathoracic volume and become a muscle of expiration ( Fig. 20 ).

Pericardium

The pericardial space is considerably smaller than the pleural space and a small increase in the volume of fluid in this space can have a dramatic impact on cardiac function. The parietal pericardium is a thick, fibrous sac with an inner serosal surface containing the heart, the proximal ascending aorta, the distal superior vena cava, the distal inferior vena cava, the pulmonary trunk and bifurcation, proximal left and right main pulmonary arteries, and a short segment of all four distal pulmonary veins. From this description, it can be visualized that all vessels flowing into and out of the heart have short segments contained in the pericardial sac ( Fig. 21 ). Also, these vascular structures fix the heart in the pericardial sac. If the heart is allowed to rotate, these structures will be twisted or kinked, impeding venous return. Because they are the lowest pressure conduits, the superior vena cava and inferior vena cava are the most vulnerable to kinking and impedance of flow. With decreased blood flow into the heart, there is decreased blood flow out of the heart, and systemic blood pressure falls. This is the physiology of hypotension associated with tension pneumothorax and with cardiac herniation.

There are two sinuses behind the heart. The oblique pericardial sinus is a cul-de-sac behind the heart bounded by pericardial attachments to the inferior vena cava and the four pulmonary veins. Because of the oblique sinus, with a median sternotomy, a hand can be placed around the apex of the heart and the apex gently elevated into the wound. This allows visualization of the lateral and posterior walls of the left ventricle, including the vascular distribution of the diagonal, circumflex, and obtuse marginal coronary arteries. This maneuver is generally poorly tolerated without opening the right pericardium vertically, parallel to the phrenic nerve. This allows the right side of the heart to fall into the right pleural space and maintain filling as the heart is lifted. In addition, severe Trendelenburg position and an apically placed suction retraction device will aid exposure and improve hemodynamics. Internal defibrillating paddles should be open and ready prior to performing this maneuver, as ventricular fibrillation is not uncommon.

The transverse pericardial sinus allows a finger or clamp to be placed along the right side of the ascending aorta, behind the aorta and pulmonary trunk, and be visualized to the left of the pulmonary trunk and superior to the left superior pulmonary vein in the vicinity of the left atrial appendage (see Fig. 21 ).

The pericardium can be drained through a median sternotomy, left or right thoracotomy via a subxiphoid approach, or laparotomy. From a left or right thoracotomy, an incision is made anterior or posterior to and parallel to the phrenic nerve. From the left side of the chest the left ventricle and from the right side of the chest the right atrium will be encountered in the pericardial space behind these incisions ( Fig. 22 ).

From a laparotomy, a modification of the subxiphoid approach can be used to enter the pericardium. Alternatively, the central portion of the diaphragm makes up the inferior fibrous parietal pericardial sac. An incision in the diaphragm in this location will enter the pericardial sac, visualizing the inferior wall of the heart.

Subxiphoid space

The subxiphoid space is a favored access to the pericardium for diagnosis and treatment of pericardial effusions. Both the linea alba and the diaphragm attach to the xiphoid. The peritoneum on the diaphragm is continuous with the peritoneum on the deep surface of the posterior fasciae of the anterior abdominal wall. An incision from above the xiphoid process to 4 cm below will pass through skin, fat, and linea alba. Incising the linea alba will reveal the xiphoid superiorly and the peritoneum inferiorly. The diaphragmatic attachments to the xiphoid can be divided flush with the xiphoid and the xiphoid resected to the level of the sternal body/costal margin. A large vein is routinely encountered at the angle between the xiphoid, costal margin, and sternal body. Posterior retraction of the diaphragm and superior retraction of the sternum will reveal the pericardial reflection on the diaphragm, which is often covered with fat, which must be bluntly dissected with a Kittner dissector sponge stick to reveal the underlying pericardium. Incising the pericardium will enter the pericardial space. The acute margin of the right ventricle will be visible through this incision ( Fig. 23 ). Because this incision is at the corner where two perpendicular planes meet, fluid can be aspirated in two directions. First, straight posterior, parallel to the diaphragm, along the inferior border of the heart, and second, superior, parallel to the sternal body, anterior to the anterior surface of the heart ( Fig. 24 ).