12.1. Landmarks of the Thoracic Wall
There are several defined vertical lines that demark regions of the anterior and posterior thoracic wall (Fig. 23). These lines are used to describe the location of surface landmarks and the locations of injuries or lesions on or within the thorax. The anterior median line runs vertically in the midline. It is also referred to as the midsternal line. The midclavicular line bisects the clavicle at its midpoint and typically runs through or close to the nipple. Three lines demarcate the axilla. The anterior axillary line runs vertically along the anterior axillary fold, and the posterior axillary line runs parallel to it along the posterior axillary fold. The midaxillary line runs in the mid-line of the axilla, at its deepest part. The scapular line runs vertically on the posterior thorax, through the inferior angle of the scapula. The posterior median line, also called the midvertebral or midspinal line, runs vertically in the midline on the posterior thorax.
The sternum lies subcutaneously in the anterior median line and can be palpated throughout its length. The jugular notch is found at the upper margin of the sternum, between the medial ends of the clavicle. The jugular notch is easily palpated and can usually be seen as a depression on the surface. The jugular notch represents the anterior junction of the superior mediastinum and the root of the neck. It lies at the level of the T2 vertebra posteriorly. The manubrium intersects with the body of the sternum about 4 cm inferior to the jugular notch, at the manubrio-sternal joint; this joint creates the sternal angle, which is normally visible on the surface of the thorax.
The sternal angle demarcates the inferior border of the superior mediastinum and lies at the level of the intervertebral disc between T4 and T5. The second rib articulates with the sternum at the sternal angle, making this site an excellent landmark for determining rib number. Immediately adjacent to the sternal angle is rib 2; the other ribs can be found by counting up or down from rib 2. Intercostal spaces are numbered for the rib above. On the posterior thorax, the fourth rib can be found at the level of the medial end of the spine of the scapula and the eighth rib at the inferior angle.
The manubrium overlies the junction of the brachiocephalic veins to form the superior vena cava (Fig. 23). The superior vena cava passes at the level of the sternal angle and at, or slightly to the right of, the border of the manubrium. The superior vena cava typically enters the right atrium behind the costal cartilage of the third rib on the right; it is sometimes accessed for various procedures, and knowledge of this surface anatomy is critical for such a procedure.
The xiphoid process is the inferior part of the sternum and lies in a depression, called the epigastricfossa, at the apex of the infrasternal angle formed by the convergence of the costal margins at the inferior border of the thorax (Fig. 23). The loca tion of the xiphisternal joint is used as a landmark to determine hand position for cardiopulmonary resuscitation.
The breasts are also surface features of the thoracic wall. In women, the breasts vary greatly in size and conformation, but the base of the breast usually occupies the space between ribs 2 and 6, from the lateral edge of the sternum to the midaxillary line. The nipples, surrounded by an area of darker pigmented skin called the areola, are the prominent features of the breast. In men, the nipple is located anterior to the fourth intercostal space in the midclavicular line. Because of the variation in breast anatomy in the female, the location of the nipple is impossible to predict.
12.2. The Lungs and Pleura
The pleural sac is outlined by the parietal pleura as it projects onto the surface of the lungs (Fig. 23). From the root of the neck, these projections follow the lateral edge of the sternum inferi-orly. On the left, the border of the parietal pleura moves laterally at the level of fourth costal cartilage to accommodate the cardiac notch within the mediastinum. The pleura follows a line just superior to the costal margin, reaching the level of the tenth rib at the midaxillary line. Posteriorly, the inferior margin of the plural cavity lies at the level of T12, and the medial margin follows the lateral border of the vertebral column to the root of the neck.
In the superior parts of the pleural cavity, the visceral pleura of the lungs is in close contact with the parietal pleura, with the lungs consequently filling the plural cavity. Both lungs and parietal pleura (cervical part) extend above the clavicles into the supraclavicular fossae, at the root of the neck. At the inferior reaches of the pleural cavities, the lungs stop short of filling the plural cavity, reaching only to the level of the 6th rib in the midclavicular line, the eighth rib in the midaxillary line, and the tenth rib posteriorly, creating the costodiaphragmatic recesses. The major (oblique) fissures of the lungs extend along a line from the spinous process of T2 to the costal cartilage of the sixth rib. The minor (horizontal) fissure of the right lung lies under the fourth rib.
Under pathological conditions, fluid can accumulate in the pleural cavity. This fluid normally drains inferiorly and accumulates in the costodiaphragmatic recess. Thoracocentesis refers to the procedure used to drain such fluid (Fig. 28). A needle is inserted into the costodiaphragmatic recess by passing it through the middle of the intercostal space, taking care to avoid the primary intercostal neurovascular bundle immediately below the rib above and collaterals above the rib below.
The heart and great vessels are covered by the sternum and central part of the thoracic cage (Fig. 23). The apex of the heart usually lies in the fifth intercostal space just medial of the midclavicular line. The upper border of the heart follows a line from the inferior border of the left second costal cartilage to the superior border of the right costal cartilage. The inferior border of the heart lies along a line from the right sixth costal cartilage to the fifth intercostal space, at the midclavicular line where the apex of the heart is located. The right and left borders follow lines connecting the right and left ends of the superior and inferior borders.
All four heart valves, the closing of which account for the heart sounds, lie well protected behind the sternum. The sounds of the individual valves closing are best heard at auscula-tory sites to which their sounds are transmitted. The bicuspid (mitral) valve is heard at the apex of the heart in the region of the fourth or fifth intercostal space on the left near the midclavicular line. The tricuspid valve can be heard along the left margin of the sternum at the level of the fourth or fifth intercostal space. The pulmonary valve is heard along the left border of the sternum in the second intercostal space. The aortic valve is heard at the second intercostal space on the right sternal border. (For more details on heart sounds, see Chapter 14.)
An understanding of the surface landmarks relative to the axilla and subclavian region is critical for successful access of the venous system via the subclavian vein. The subclavian vein passes over the first rib and under the clavicle at the junction of its middle and medial thirds; it courses through the base of the neck, where it passes anterior to the apex of the lung and the pleural cavity (Fig. 29). The subclavian vein is immediately
anterior to the subclavian artery and is separated from the artery medially by the anterior scalene muscle. To access the subcla-vian vein, a needle is inserted approx 1 cm inferior to the clavicle at the junction of its medial and middle thirds, and the needle is aimed toward the jugular notch, parallel with the vein to minimize risk of injury to adjacent structures.
The most common complication of subclavian venous access is puncture of the apical pleura, with resulting pneumothorax or hemopneumothorax. In addition, the subclavian artery, lying behind the vein, also has the potential to be injured by this procedure. If subclavian access is attempted on the left, one must also be aware of the junction of the thoracic duct with the subclavian vein. Injury to the thoracic duct can result in chylothorax, the accumulation of lymph in the plural cavity. This is difficult to treat and has an associated high morbidity. When access of the subclavian is attempted for cardiac lead placement, care must be taken to avoid piercing the subclavius muscle or costoclavicular ligament. Passing the lead through these structures tethers it to the highly mobile clavicle, which may cause premature breakage of the lead.
Options for accessing, in a minimally invasive fashion, the heart are limited by the vascular anatomy of the superior mediastinum and the axilla. Percutaneous access strategies are limited by the bony anatomy of the thoracic cage. How a device interacts with the thorax and accommodates basic thoracic movements, and movements of the upper extremity and neck, must be understood for design of devices that will endure in the body. Thus, a thorough understanding of the thoracic anatomy surrounding the heart is important to those seeking to design and deploy devices for placement and use in the heart. With an understanding of the important thoracic anatomical relationships presented in this chapter, the engineer should be able to design devices with an intuition for the anatomical challenges that will be faced for proper use and deployment of the device.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.