Position in the Thoracic Cavity

Here we are looking at Learning objective 1.

Describe the normal position of the heart within the chest cavity, relating it particularly to the rib cage and sternum.

We are also looking at Learning Objective 4.

Recognise the main features of the heart on a plain posterior/anterior x-ray and appreciate those parts of the heart that are not evident from this perspective.

Normal Position Of The Heart

The heart is protected by the thoracic cavity which consists of the sternum and the ribcage. It lies majorly behind the sternum but the apex of the heart protrudes towards the left hand side of the chest.

The surface markings of the heart as a follows:

Left atruim- 2nd left costal cartilage, lateral to the sternum.
Left ventricle (apex)- 5th left intercostal space, in the midclavicular line.
Right atrium- 3rd right costal cartilage
Right ventricle- 6th right costal cartilage.

It is important to note that the 5th intercostal space is lower than the 6th costal cartilage. This often gets students confused but like most of anatomy the best way to get your head around it is to count the ribs and the corresponding intercostal spaces. After doing so you will realise this for yourself.

Figure showing the surface markings of the heart

Mediastinum

The mediastinum is the central compartment of the thorax. It contains most of the thoracic organs and is a passageway for organs which terminate in the abdomen from the thorax.

An imaginary line runs from the junction between the manubrium and the body of the sternum, known as the sternal angle and thoracic vertebra T4. This imaginary line divides the mediastinum into two parts; the superior mediastinum and the inferior mediastinum. The inferior mediastinum is subdivided into the anterior mediastinum, middle mediastinum and posterior mediastinum. See the figure on the right for further illustration.

The contents of each division of the mediastinum are listed below:

Superior mediastinum: the arch of the aorta along with its branches and the great vessels.
Anterior mediastinum: contains no major organs.
Middle mediastinum: the heart, pericardium, tracheal bifurcation, left and right main bronchi. The great vessels that are found in the superior mediastinum originate from the middle mediastinum.
Posterior mediastinum: descending aorta and oesophagus.

See the figures below for further illustration.

Figure showing the mediastinum and all its subdivisions.

Figure showing the contents of the superior mediastinum

Figure showing the great vessels originating from the middle mediastinum

Pericardium

The pericardium is a fluid-filled sac that surrounds the heart and the root of the great vessels. It is made up of a tough outer layer and a thin inner layer called fibrous pericardium and serous pericardium respectively.

The fibrous pericardium is continuous with the diaphragm. It prevents overfilling of the heart and protects the heart from infection in the event of surrounding organs being compromised, such as the lungs. The attachment of the fibrous pericardium to the diaphragm also keeps it fixed in position.

The serous pericardium is divided into the outer parietal layer which lines the inner surface of the fibrous pericardium and the internal visceral layer (also known as the epicardium) which lines the external surface of the heart. Between these two layers is the pericardial cavity which contains serous fluid, a lubricator that reduces the friction generated during the pumping action of the heart.

The figures toward the right hand side of this page shows the arrangement of the pericardium from the external layer to the internal layer.

Figure showing the detailed layers of the pericardium

Figure showing the order of layers of the heart wall

Posterior/Anterior X-ray

Most of the structures of the heart cannot be seen from an X-ray. The 2 most prominent features are the aortic arch and the apex of the heart. It is important to be able to point these features out on both an x-ray and an actual model as you can be examined on it.

Figure showing the structures of the heart on an x-ray

Figure showing the mediastinal structures on the chest x-ray

Clinic Corner

Knowing the surface markings of the heart is clinically important because physicians need this information to know where to place a stethoscope to listen to the sounds generated by the closing of the valves of the heart.

Mitral valve- Its closing is heard over the apex of the heart in the 5th intercostal space located in the left sternal margin.

Tricuspid valve- is heard in the right sternal margin of 5th intercostal space.

Aortic valve- is heard in the 2nd intercostal space at right sternal margin.

Pulmonary valve- is heard in the 2nd intercostal space at the left sternal margin.

There is a condition called cardiac tamponade which occurs when blood or fluids fill the pericardium, preventing the heart from pumping blood effectively. This is a medical emergency as it can lead to organ failure, shock and ultimately death.

Common causes include;

Gun shot wounds
Blunt trauma to the chest from a road traffic accident
Breast or lung cancer which has metastasised to the pericardial sac
Pericarditis, among others.

Symptoms include;

Low blood pressure
Weakness
Rapid breathing
Fainting
Dizziness
Loss of consciousness.

Diagnosis involves the observation of three signs known as Beck's triad and an echocardiogram.

Treatment is by draining the fluid from the pericardial sac, using a needle. This procedure is known as pericardiocentesis.

The video below shows an emergency pericardiocentesis being carried out. Do watch if interested.

Figure showing the filling of the pericardium with blood in the event of cardiac tamponade

Neonatal autopsy showing cardiac tamponade

Image credit: National Institutes of Health (USA) 2017, Mikael Haggstrom 2009, Katherine Sanders 2017, Matt Baguley 2017, Bruce Blaus 2013, Matt Quinn 2018, Mikael Haggstrom 2017, Bruce Blaus 2014, Qiao's pathology 2016 - All Image Usage Complies with CC BY-SA 3.0.

Youtube video: The New England Journal of Medicine 2012.

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