The thorax lies between the neck and abdomen and includes the mediastinum, pleural cavity and the lungs. The thoracic cage is formed by the ribs originating from the vertebral column joining anteriorly at the sternum.
The diaphragm forms the base of the thorax whilst the thoracic inlet forms the superior aspect (located between the first two ribs). On the anterior surface of the thoracic cage lies the pectoralis major muscle group and overlying skin. Understanding the surface anatomy of the thorax will help to identify important clinical landmarks and is therefore crucial to performing a competent clinical examination.
There are 12 pairs of ribs that form the thoracic cage, with the first 7 pairs referred to as true ribs attaching to the sternum via the costal cartilage. The following 5 pairs of ribs are known as false ribs and attach to the sternum also via the costal cartilage but instead share the connection. Furthermore, the final two ribs of the five do not attach to the sternum and are referred to as floating ribs.
Importantly, the first rib is unique being atypical in shape and articulating with the manubrium (head of the sternum). The following 11 ribs articulate posteriorly with the inferior aspect of the vertebral blody and are curved both antero-posteriorly and cranio-caudally to articulate with the sternum (see image below).
Between each rib lie the intercostal muscles. The external intercostal muscle passes from the lower aspect of one rib to the superior border of the next caudal rib with the external intercostal membrane lying posterior to this. Deep to this lies the internal intercostal muscle and the internal intercostal membrane. Beneath this, lies the intercostal neurovascular bundle (NVB) containing the intercostal vein, artery and nerve. Importantly, the vein lies superior to the subcostal groove whilst the artery and nerve are located inferior to the groove. The intercostal nerve innervates the intercostal muscles and the overlying skin. Deeper still, lies another muscle layer called the innermost intercostal muscle, anteriorly becoming the tansversus thoracis muscle (see image below).
Awareness of the location of the intercostals vessels is important in the clinical setting. For example, when inserting a chest drain care is taken to avoid the neurovascular bundle by inserting the needle just above the rib at either the 4th, 5th or 6th intercostals space.
On the posterior side of the diaphragm, two muscle fibres attach on either side of the lumbar vertebrae, known as the right and left crus. The diaphragm has a number of openings located at the central tendon for the inferior vena cava (IVC), the muscle itself for the oesophagus and the aorta (together with the thoracic duct and azygos vein) between the left and right crus.
The thoracic outlet is defined as the superior opening of the thoracic cavity. Anatomically it is bounded by the first thoracic vertebra, first pair of ribs,costal cartilage and superior border of the manubrium. Key structures pass through this outlet including the trachea, oesophagus, blood vessels as well as lymph nodes and lymphatic vessels.
During free breathing the ribs which lie superiorly move in the antero-posterior direction whilst the lower more inferior ribs move laterally. Hence, on clinical examination, good chest expansion should be indicated by increased antero-posterior movement in the upper chest and lateral expansion of the lower chest.
Essentially, during breathing in the diaphragm contracts increasing thoracic volume and lowering intra-thoracic pressure, drawing air into the lungs. When breathing out the diaphragm relaxes, lowering thoracic volume and increasing intra-thoracic pressure forcing air out of the lungs. This process is often assisted by the intercostal muscles in periods of increased demand.
The Anterior Chest Wall
The second costal cartilage articulates with the manubriosternal joint which helps orientate the clinician when counting the number of intercostal spaces. This is particularly important in identifying the apex beat, which may be displaced as occurs in aortic stenosis, severe hypertension or dilated cardiomypoathy. In order to identify any displacement, one must be able to work out where the apex beat normally lies, which is in the fifth intercostal space in the mid-clavicular line. Other sites include the 5th rib which corresponds with the lower border of the pectoralis major and the 6th rib with the serratus anterior however both of these landmarks have limited clinical value. (See image below).
The Posterior Chest Wall
The midvertebral line is a theoretical line that runs down the middle of the back and in the thin patient the spinous processes may be visible. The 7th cervical vertebrae spinal process will most often be palpable. Another notable landmark is the scapula, consisting of the superior and inferior angle which is at the level of the 2nd and 7th thoracic vertebrae respectively. (See image below).
All surface lines are theoretical but used in clinical practice to help locate specific sites and commonly used as reference points including:
Angle of Louis/Sternal Angle - Is formed between the junction of the manubrium and body of the sternum. This has important clinical value since it corresponds with the level of the second rib.
Midsternal line - A line running down the middle of sternum.
Midclavicular line - A vertical line located midway between the jugular notch and the acromioclavicular joint.
Lateral sternal line - A line running along the sternal margin.
Parasternal line - Located midway between the laternal sternal line and the mammary line.
Anterior axillary line - A vertical line from the axillary folds, running between the middle of the clavicle and the lateral end of the clavicle.
Posterior axillary line - Runs parallel with the anterior axillary line but instead along the posterior axillary fold.
Midaxillary line - A vertical line from the apex of the axilla between the anterior and posterior axillary line. Scapular line - A vertical line running from the inferior angle of scapula.
Midline sternotomy scar - Indicates previous coronary artery bypass grafting (CABG), aortic valve replacement or transplantation.
Left submammary scar - Indicates previous mitral valvotomy.
Infraclavicular scar - Indicates previous pacemaker insertion.
Information on auscultation sites of the heart can be found in the Cardiology Section - Valvular Disease and Murmurs. Importantly, auscultatory areas do not correspond with the surface markings of the heart valves. Instead the auscultatory areas are often where the sounds are heard best and transmitted. (See image below).
Access to the thoracic artery is via the left side of the chest known as a left thoracotomy through the 5th intercostal interspace.
Achieved by inserting needle left of the xiphoid process.
Achieved through left thoracotomy and excising the 5th costal cartilage.
There are a number of anatomical abnormalaties that can occur within the thorax. This has important implications to the cardiorespiratory system and clinical examination techniques. This includes the following:
Dextrocardia is a congenital condition where the heart is located on the right side of the thorax instead of the left. This anatomical variation can be associated with severe defects of the heart but ultimately depends upon the type of dextrocardia. On clinical examination, an apex beat would be felt in the fifth intercostal space in the mid clavicular line on the right hand side. Nevertheless, the diagnosis of this congenital condition is ideally made by an ECG or imaging. Dextrocardia has important implications with regards to the placing of ECG leads in the reversed position and particularly cases of defibrillation.
Pectus carinatum is protrusion of the ribs and sternum anteriorly and is an anatomical deformity. This can occur independantly or in association with other genetic disorders, including trisomy 18 or 21. There is little impact in development of vital organs however it may prevent such organs from functioning optimally.
Pectus excavatum is another congenital condition that causes the sternum to form inward, creating a depression in the centre of the chest. Again, this condition may occur independantly or in association with other syndromes. If severe, this can affect normal organ development and function, with patients often having mitral valve prolapse (MVP).
Thoracic outlet syndrome
Thoracic outlet syndrome is a condition which presents with pain in the neck/shoulder as well as numbness and tingling of the fingers. This is caused by compression of the nerves and blood vessels that pass through the superior thoracic outlet. This is often brought about by an extra cervical rib or an abnormally tight fibrous band connecting the spinal vertebrae to the rib. The diagnosis can typically be made with a history and examination but may involve further tests. Ideally, initial management will involve physiotherapy. If this proves ineffective surgery can be opted for involving removing the extra cervial rib or bypass surgery to reroute the blood supply.
Pericardial effusion occurs when fluid gathers within the percardial cavity. This increases intrapericardial pressure restricting the hearts ability to function normally, referred to as cardiac tamponade. Common presenting symptoms include chest pain, breathlessness and decreased blood pressure. Treatment varies according to the cause but if this progresses to cardiac tamponade fluid which has accumulated within the pericardial space will need to be drained, a procedure known as pericardiocentesis. Beck's triad refers to the clinical signs that occur during cardiac tamponade which includes low arterial blood pressure, distended neck veins and muffled heart sounds.
Superior vena cava syndrome
SVC syndrome occurs as a result of obstruction of the SVC and is a medical emergency. Most cases of SVC commonly caused by a cancerous tumour that is compressing the SVC including a bronchogenic carcinoma. Other causes include Burkitt's lymphoma, lymphoblastic lymphoma and acute lymphoblastic leukaemia. Common symptoms include dysponea, facial swelling, neck distension and a cough. Management includes medical therapy (glucocorticoids or diuretics) or surgery but this condition often has a poor prognosis.
The mediastinum refers to the central mass of tissue that houses the heart, great vessels and the oesophagus and is divided into the superior and inferior mediastinum. (See image below).
The superior mediastinum lies above the transverse plane at the level of the 4th thoracic vertebrae posteriorly and manubriosternal joint anteriorly. This compartment contains the great vessels passing to and from the thoracic inlet. The inferior mediastinum lies below the level of the 4th thoracic vertebrae but above the diaphragm and is further subdivided into the anterior, middle and posterior compartments. In the anterior compartment lies the thymus, the middle compartment the heart, whilst the oesophagus and aorta lie in the posterior compartment.
The heart is enclosed by the pericardial sac which is a double layered serous membrane. Inferiorly, the apex is fixed to the central tendon of the diaphragm and superiorly to the root of the great vessels, including the aorta and pulmonary trunk. A detailed description of the anatomy of the heart and its blood supply can be found in the Cardiology Section - Blood Supply of the Heart.
The aorta ascends from the heart, arches and then begins to descend at the level of the manubiosternal joint. The arch of the aorta is commonly seen on the chest x-ray as the aortic knuckle giving rise to three branches known as the left common carotid, left subclavian and brachiocephalic artery. The descencing thoracic aorta passes between the left and right crus of the diaphragm at the level of the 12th thoracic vertebrae. During its descent paired branches of vessels arise passing posteriorly to supply the spine and posterior trunk wall. From the right ventricle the pulmonary trunk arises which divides into the left and right pulmonary arteries that pass into the hilum of the left and right lung respectively.
The venous system includes the right braciocephalic vein and left brachiocephalic vein formed from the union of the internal jugular and subclavian veins. These veins descends vertically to form the superior vena cava (SVC). The SVC and the right atrium form the right border of the mediastinum with the left border being formed by the LV and aortic knuckle. The inferior vena cava (IVC) passes through the central tendon to enter the inferior aspect of the right atrium.
The intercostal arteries arise from the paired branches of vessels of the aorta from T4-T12. The superior intercostal artery, a branch of the subclavian artery, supplies the first two intercostal spaces. The internal thoracic artery is also a branch of the subclavian artery and provides pairs of vessels level with each intercostal space. As mentioned the intercostal veins lie superiorly in each intercostal space located within the subcostal groove draining into the larger azygos and smaller hemiazygos vein.
The left and right pleural cavities are lined by the pleura comprising the parietal layer attached to the chest wall and a visceral layer covering each lung. This creates a closed space known as the pleural cavity which if perforated results in an inability to develop the negative pressure required to draw air into the alveolar spaces. A chest drain may then be directed superiorly to the upper border of the rib, to avoid the neurovascular bundle. This is in the 4th interspace in the mid-axillary line.
Each lung is connected to the mediastinum via the hilum which contains the pulmonary veins and arteries. The point of bifurcation of the left and right primary bronchus is at the level of the carina. The right lung has three lobes and the left lung has two lobes with each lobe being supplied by a second-order bronchus that further subdivides into bronchopulmonary segments and finally alveoli. In the right lung the oblique fissure seperates the lower lobe from the middle and superior lobes. The horizontal fissure seperates the superior from the middle lobe. In the left lung the two lobes are seperate by the oblique fissure.
The thorax has a complex lymphatic system. The thoracic duct arises inferiorly from the cisterna chyli posterior to the right crus of the diaphragm. During its course, it connects with other mediastinal lymph nodes that are mainly clustered around the trachea and main primary bronchi. The thorax also contains the thymus which develops alongside the parathyroid glands in the neck before descending into the superior/anterior mediastinum.
The oesophagus runs vertically along the midline of the thoracic inlet, posterior to the trachea. It enters the inferior mediastinum deviating to the right passing through the diaphragm at the level of the 10th thoracic vertebrae, where the right crus wraps around it. Access to the oesophagus is achieved via a right thoracotomy.
The thorax has a complex nerve innervation. As mentioned, the intercostal nerve lies inferior to the subcostal groove providing branches to the intercostal muscle, overlying pleura and skin. Posteriorly, on either side of the thoracic vertabrae, lie the sympathetic ganglia. This is connected to the intercostal nerve by grey/white ramus communicans representing the outflow and inflow for the sympathetic nervous system (SNS) respectively. The ganglia at each level are connected to the ganglion above and below forming the sympathetic chain. The first two intercostal nerves carry the sympathetic outflow to supply the head and neck whereas the lower six intercostal nerves carry the outflow for structures inferior to the diaphragm. Importantly, three sympathetic nerves arise from the sympathetic chain. They are the greater splanchic nerve from T7-9, lesser splanchic from T10-11 and least splanchic from T12.
The phrenic nerve arises from the 3rd, 4th and 5th cervical nerve roots passing into the thoracic inlet to supply the diaphragm. This is a mixed nerve with a motor component supplying the diaphragm and a sensory component supplying the diaphragmatic pleura and peritoneum. Should the peritoneum become irritated patients may sometimes complain of pain in the corresponding shoulder, this is known as referred pain. Another commonly known nerve is the left and right vagus nerve which supplies parasympathetic fibres to the thorax. The recurrent laryngeal nerve is a branch of the vagus nerve that hooks around the subclavian artery. This nerve is sensory to the mucosa of the trachea and motor to the muscles of the vocal chord. Branches of this nerve are given off inferior to the arch of the aorta to form the superficial and deep cardiac plexi.
Figure 1 (Top left) - This is an example of a normal chest x-ray. There is good exposure with both costophrenic angles clearly visible as well as the primary bronchi.
Figure 2 (Top right) - This is an example of heart failure (HF). Classic features of HF include alveolar oedema, interstitial oedema, cardiomegaly, dilated prominent upper lobe vessels and pleural effusion.
Figure 3 (Bottom left) - This is an example of dextrocardia. Note the fact that the heart is on the right hand side instead of the left. This is a congenital condition.
Figure 4 (Bottom right) - This is an example of cardiomegaly. This can be demonstrated if the heart measures more than 50% of the cardiothoracic ratio.
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Figure 1) Rib Cage
Reproduced with kind permission from Dr Darling. The encyclopedia of science. Anatomy and Physiology. http://www.daviddarling.info/encyclopedia/R/rib-cage.html
Figure 2) Intercostal Muscle
Reproduced with kind permission from Dr Darling. The encyclopedia of science. Anatomy and Physiology. http://www.daviddarling.info/encyclopedia/I/intercostal_muscles.html
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