Sternum and xiphoid process

Anatomical projection of heart contours onto the sternum

In clinical practice, the percussion-determined borders of relative cardiac dullness reflect the true anatomical contours of the heart projected onto the anterior chest wall. The skeletotopy of the heart is conventionally described using three main borders, each clearly spatially referenced to the sternum and the xiphoid process.

• The right border is formed by the superior vena cava and the margin of the right atrium (atrium dextrum). It originates at the upper margin of the 2nd rib cartilage—precisely at its attachment site to the sternum on the right. Descending toward the upper margin of the 3rd rib cartilage, the line deviates 1.0–1.5 cm laterally from the right border of the sternum. Further down, the contour runs as a convex arch from the 3rd to the 5th rib, maintaining a constant distance of 1–2 cm from the right border of the sternum.
• The left border has a more complex anatomical substrate. It begins at the lower margin of the 1st rib on the left, immediately adjacent to the sternum. In this segment, the contour is formed by the aortic arch.
• The inferior border is formed by the margin of the right and partially the left ventricles. At the level of the 5th rib on the right, it smoothly transitions from the right border, runs obliquely downward and to the left, and then crosses the sternum immediately above the base of the xiphoid process. Next, the line extends toward the sixth intercostal space on the left. Interestingly, the inferior border of relative cardiac dullness is usually not distinguished as a separate landmark on percussion.

Projection of great vessels and heart chambers

In addition to the general borders, individual chambers and great vessels can be projected relative to the sternum:

• The superior vena cava (v. cava superior) forms the upper part of the right contour (starting from the 2nd intercostal space). Its trunk is projected at the right sternal border at the level of the 1st rib cartilage attachment, with the vessel protruding beyond the right margin of the bone by exactly half its diameter.
• The pulmonary trunk is projected at the level of the second intercostal space, located 2.0–2.5 cm lateral to the left sternal border.
• The left auricular appendage corresponds to the level of the 3rd rib to the left of the sternum. From the lower margin of the 3rd rib (2.0–2.5 cm to the left of the linea sternalis sinistra), the contour runs in a convex lateral arch toward the fifth intercostal space.
• The area of absolute dullness is the region of the anterior cardiac surface that is not covered by the lungs and lies directly against the chest wall. Anatomically, this zone is formed by the right ventricle, which comprises the anterior and right parts of the heart along with the interventricular septum.

Sternal angle and xiphoid process in clinical practice

The topography of the sternum is of critical importance for physical examination and medical procedures.

The sternal angle is a universal landmark located approximately 5 cm above the center of the right atrium. This anatomical constant remains unchanged regardless of the patient's body position. In clinical practice, it is used to determine central venous pressure (CVP) via the jugular veins: the physician measures the height of the blood column in the vein above the level of the sternal angle and adds 5 cm to obtain the absolute CVP value.

The sternal angle also serves as a starting point for locating auscultation sites. Palpation from this angle identifies the cartilage of the 2nd rib, beneath which lies the second intercostal space. This is precisely where great vessel valves are auscultated, which directly corresponds to their projection on the left contour of the cardiac silhouette:

1. In the second intercostal space to the right of the sternum is the auscultation point for the aortic valve.
2. In the second intercostal space to the left of the sternum, the pulmonary valve is auscultated.
3. At the base of the sternal xiphoid process, the auscultation point of the tricuspid valve is located. This location precisely corresponds to the inferior border of the heart formed by the ventricles.

Additional maneuvers are used to improve auscultation quality. For example, leaning the patient's torso forward facilitates auscultation of the aortic valve, whose projection is located near the right sternal border. The left lateral decubitus position shifts the left ventricle closer to the anterior chest wall, improving the detection of mitral valve pathology.

Adjacent topographic landmarks

Knowledge of skeletotopy relative to the sternum is essential not only for internal medicine but also for resuscitation procedures:

• During defibrillation using an automated external defibrillator (AED), the left electrode is fixed on the anterolateral surface of the left hemithorax. This site strictly corresponds to the projection of the cardiac apex (5th intercostal space, 1.5–2.0 cm medial to the linea medioclavicularis sinistra).
• If open-chest cardiac massage is required, surgical access is achieved by opening the chest in the 4th intercostal space on the left, which also relies on the normal projection of the heart in this zone.
• It is important to consider that the anterior pleural borders (lines of transition from costal to mediastinal pleura) on the right and left are asymmetric. This asymmetry behind the sternum arises due to the position of the heart itself, which displaces the left pleural reflection.

Class and exam questions

What else to study on this topic

• Projection of the cardiac apex and apex beat
• Topography of the cardiac sulci (coronary and interventricular)
• Asymmetry of the anterior pleural borders
• Radiographic anatomy of the cardiac silhouette contours
• Topographic landmarks for defibrillation and open-chest cardiac massage

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.