THE HEART SHADOW IN THE MEDIASTINUM

 

On a chest radiograph, the central mediastinum area is occupied by a large radio-opaque shadow termed the central mediastinal shadow which is the anatomical location of the heart and great vessels. If any heart disease is suspected, it is very important to carefully examine its size, position and shape. Enlargement of the heart shadow might indicate the presence of a cardiac problem such as heart failure following a myocardial infarction. However, the heart shadow is variable in shape even in normal subjects, mainly due to the wide variation in body build of different people. Additionally, respiration will also affect the heart shadow.

Using the image above as a key, examine the right border of the central mediastinal shadow.

1

In its superior parts, the right margin of the shadow represents the superior vena cava (1). This is a large vein which receives blood draining from the upper part of the body ie head, neck, upper limbs and part of the upper chest. Inferior to the superior vena cava, the border is formed by the right atrium (2) of the heart. Inferior to the right atrium, a small part of the inferior vena cava can sometimes be identified as it enters the chest through the diaphragm, prior to returning blood to the heart (3).

Now look at the left border.

The left border of the central mediastinal shadow is formed superiorly by the prominent left projecting shadow of the arch of the aorta (4), also known clinically as the aortic knuckle. Immediately inferior to the aortic arch is a shadow formed by the left pulmonary trunk (5). Lateral to this shadow, a larger white shadow within the lung field is the left pulmonary artery (6) shadow. In a more inferior direction, the left border of the central mediastinal shadow is composed of the left margin of the heart shadow. In its superior region, the heart shadow is formed by the tip of the auricleof the left atrium (7). Below the auricular shadow, the border is represented by the left ventricle (8) of the heart. The point where the cardiac shadow meets the diaphragmatic shadow is called the left cardiophrenic angle (9).

2

The right heart structures

By marking a radiograph in the appropriate areas, it is possible to trace out the anatomical course of the vessels connected to the right side of the heart. You will be required to know the anatomy of these vessels.

Note the position of the superior vena cava (A). The traced outline shows this vessel to lie to the right of the aortic knuckle. The right heart shadow is formed by the right atrium (B). Most of this atrium is projected to the posterior on the PA film and thus a relatively small area may be seen on this outline. Note the approximate position of the right auricle (C).

3

The inferior vena cava enters the lower part of the atrium. Sometimes, it can be identified as a white shadow filling the right cardiophrenic angle (D). The approximate position of the right ventricle is show (E). This never can be identified in outline as a separate structure on the PA radiograph due its orientation. It thus is important that you appreciate the normal orientation of the heart in the living subject.

Blood leaves the right ventricle via the pulmonary trunk (F). This divides into right and left pulmonary arteries just to the left of the ascending aorta and below the arch of the aorta.

The left heart structures.

4

The outline of the left heart structures may also be studied by outlining the vessels and chambers of the heart. The right pulmonary (A) and left pulmonary (B) veins drain the blood from the lungs back to the heart. The left atrium lies behind the ascending aorta and thus cannot be seen on these views (C).

Blood then passes through the mitral valve into the left ventricle (D). This forms the left border of the heart shadow. Blood then leaves the left ventricle and passes upwards and to the right to enter the ascending aorta (E). This then curves over towards the left and passes backwards as the arch of the aorta (the so-called aortic knuckle ) (F).

The approximate positions of the brachiocephalic (G), left common carotid (H) and left subclavian (J) arteries are indicated.

These vessels can be outlined clearly using aortic arch angiography.

THE THORACIC AORTA 1

5

The course of the thoracic aorta may appear as a shadow behind the central mediastinal structures on a PA chest radiograph.

In the radiograph displayed, identify the arch of the aorta ( aortic knuckle ) (A). This lies at the level of the sternal angle (angle of Louis).

The arch of the aorta passes backwards and to the left behind the lower part of the manubrium of the sternum to reach the level of the 4th thoracic vertebra (B). It is clinically important to appreciate that the aortic arch lies superior to the left bronchus and also is to the left of the trachea and oesophagus since a lesion of one of the structures will frequently involve all the others.

6

The thoracic aorta is the part of the descending aorta lying in the posterior mediastinum. It is said to commence level with the lowest part of the body of the 4th thoracic vertebra. As the aorta passes downwards through the chest, it approaches the median plane and enters the abdomen through the aortic hiatus anterior to the vertebral column.

The descending aorta gives off the 9 lower posterior intercostal arteries which pass laterally into the intercostal spaces. The outline of the descending aorta is shown on the marked radiograph (C).

http://www.liv.ac.uk/HumanAnatomy/phd/mbchb/hrtatk/ha1.html

7