Surface projections of the heart
The surface projections of the heart represent points on the thoracic wall that map out the outline and valves of the heart. These include four borders (superior, right, inferior, left) and four valves (left atrioventricular, right atrioventricular, aortic, pulmonary). The main reference points used for the surface projections of the heart are the borders of the sternum and costal cartilages, the clavicle and intercostal spaces. The latter favour sound transmission, facilitating clinical maneuvers such as percussion, auscultation and palpation to pinpoint the cardiac location.
Therefore, surface projections of the heart are essential for basic clinical practice. During auscultation, clinicians use a stethoscope to listen to sounds above specific localizations of the heart, such as heart valves. Heart sounds and additional murmurs can reveal cardiovascular system diseases like valvulopathies or vascular malformations, such as a patent ductus arteriosus.
Superior - inferior border of the second left costal cartilage -> superior border of the third right costal cartilage
Right - third right costal cartilage -> sixth right costal cartilage
Inferior - sixth right costal cartilage -> fifth left intercostal space, midclavicular line
Left - fifth left intercostal space, midclavicular line -> inferior border of the second left costal cartilage
|Heart valve projections||
Left atrioventricular valve - posteriorly to the left side of the sternum at the level of the left fourth costal cartilage
Right atrioventricular valve - posteriorly to the right side of the sternum at the level of the right fourth costal cartilage
Aortic valve - posteriorly to the left side of the sternum at the level of the third intercostal space
Pulmonary valve - at the junction of sternum and left third costal cartilage
|Heart auscultation points||
Left atrioventricular valve - the left fifth intercostal space at the midclavicular line
Right atrioventricular valve - left fifth intercostal space, parasternally
Aortic valve - right second intercostal space, parasternally
Pulmonary valve - left second intercostal space, parasternally
|Clinical points||Aortic stenosis, aortic regurgitation, mitral stenosis, mitral regurgitation|
Knowing the exact heart topography is absolutely necessary also in radiology, for the correct evaluation of your patient’s thoracic X-ray. This is why knowing the surface projections of the heart is important for every future clinician, hence they will be discussed in this article.
Borders of the heart
The heart is placed within the middle mediastinum. Laterally and posteriorly it is surrounded by the lungs, while the sternum is located anteriorly. The superior border, or base of the heart, is the portion located opposite the apex where the big blood vessels enter and leave the heart. These aspects explain why the heart and great vessels project onto the middle of the thorax.
Even though the position of the heart can vary depending on the physical build and the position of the diaphragm, we can always identify its borders by following these heart landmarks:
- The superior border of the heart is a line that connects the inferior border of the second left costal cartilage and the superior border of the third right costal cartilage.
- The right border of the heart corresponds to a line that is convex to the right, which extends from the third right costal cartilage to the sixth right costal cartilage.
- The inferior border is marked by a line that joins the inferior end of the right border (six right costal cartilage) to the point where the fifth left intercostal space and midclavicular line intersect. Also, this point of intersection marks the apex of the heart.
- The left border corresponds to a line drawn from the inferior border of the second left costal cartilage to the intersection point between the fifth left intercostal space and midclavicular line. More simply, the line interconnects the left ends of the superior and inferior borders of the heart.
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Heart valves are placed between the heart chambers and at the roots of great vessels. Their function is to force blood to flow in only one direction, preventing backflow (regurgitation) due to intra-arterial pressure. In that way, the direction of blood flow is always constant and unidirectional throughout the human body. Therefore, valves are extremely important! Next time you hear an abnormal sound during valve auscultation, realize the impact of the pathology on the patient's entire circulatory system..
There are four existing heart valves:
- The left atrioventricular (mitral) valve between the left atrium and left ventricle. It projects posteriorly to the left side of the sternum, at the level of the left fourth costal cartilage.
- The right atrioventricular (tricuspid) valve between the right atrium and right ventricle. It is located posteriorly to the right side of the sternum at the level of the right fourth costal cartilage.
- The aortic valve between the left ventricle and aorta. One can find it at the level of the third intercostal space, posterior to the left side of the sternum.
- The pulmonary valve between the right ventricle and pulmonary trunk, projects at the junction of the sternum and left third costal cartilage.
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To complicate matters, the auscultatory areas and the actual locations of the valves are quite different. Valves can be heard away from their anatomical position, further down the stream of the blood. This is because the loudest sound is produced only after the ejected blood passes through the valve. We can hear this sound as the bloodstream ejected during systole hits the vascular wall. As a result, the auscultatory areas of the heart valves are the following:
- Left atrioventricular valve - the left fifth intercostal space at the midclavicular line. Since this auscultation point lies over the apex of the heart, you can simultaneously hear the latter, too. The cardiac apex also represents the point of maximal impulse of the heart. This is where the cardiac impulse can be best palpated during the cardiovascular physical exam.
- Right atrioventricular valve - left fifth intercostal space, parasternally
- Aortic valve - right second intercostal space, parasternally
- Pulmonary valve - left second intercostal space, parasternally
Learn more about heart valve anatomy and valvular heart disease here:
The sound heard during auscultation is caused by the closing of the valves, when they audibly snap shut due to a sudden change of pressure. The normal heartbeat heard with a stethoscope sounds something like: “lub-dub, lub-dub”. The “lub” corresponds to the closure of the atrioventricular valves at the beginning of ventricular systole. It is called the first heart sound (S1). On the other hand, the “dub” is associated with the closure of the aortic and pulmonary valves at the end of the ventricular systole. This is called the second heart sound (S2) and it is best auscultated at the Erb’s point. This landmark is located on the left third intercostal space, parasternally, between the pulmonic and tricuspid areas.
In addition to the physiological S1 and S2 sounds, additional ones can occasionally be heard. These include splits, third (S3) and fourth (S4) heart sounds, as well as murmurs. S3 and S4 are associated with diastole and the cycle of the atria. They are usually physiological in children and skinny adults, but pathological in the general adult population. On the other hand, murmurs are always a sign of cardiovascular dysfunction.
So far, we’ve learned about the surface projections of the heart and how this organ is heard when a stethoscope is placed over the thorax. It’s now time to understand how these anatomical cardiac borders and features appear on standard chest X-rays. Such radiological images depict a 2D image of the thoracic wall and contents, including the heart. The X-ray can be performed in the anteroposterior direction (AP), posteroanterior (PA) or from either lateral perspective (left, right).
In a PA image of the thorax, the cardiac silhouette, or shadow, is placed in the middle of the thoracic cavity. The silhouette has two margins, left and right.
The right margin of the heart consists of two lateral convex arches separated by an obtuse angle. In adults, the lower arch belongs to the right atrium, while the upper arch represents the ascending aorta. However, in children, the upper arch is actually flat and is formed by the superior vena cava. This difference originates from the fact that as we get older, the ascending aorta becomes elongated and slightly dilated. These changes are due to the high pressure bloodstream ejected by the left ventricle hitting the walls of the aorta. The right atrium makes an acute angle with the diaphragm, which is called the right cardiophrenic angle.
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The left margin of the cardiac silhouette has two convex arches separated by a concavity. The upper convex arch, called the aortic knob, is the place where the aortic arch continues as the descending aorta. The concave arch corresponds with the pulmonary trunk and left pulmonary artery. The lower convex arch marks the left ventricle and it ends by merging with the diaphragmatic silhouette. This forms the left cardiophrenic angle. Note that the most lateral point of the lower arch never passes over the left midclavicular line under physiological conditions.
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During auscultation, every clinician should be able to recognize pathological sounds upon the valves, called murmurs. The processes that affect the valves can be classified as stenoses or insufficiencies. Any heart valve can be affected by such pathological changes. These pathologies are collectively known as valvular heart disease. Some examples include aortic stenosis, aortic regurgitation, mitral stenosis and mitral regurgitation.
Stenotic processes will cause the valve to harden and not open properly. Thus, it will obstruct the blood flow from one heart chamber to another, or to the great vessels. On the other hand, insufficiencies usually present with loose valves which cannot close entirely. In this case, the blood will flow retrogradely every time during systolic increases of pressure, which is called regurgitation. Based on the affected valve and type of valvular pathology, murmurs are present either during systole or diastole. The timing and location of the murmur then lead us to the diagnosis.
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