Anatomy, Thorax, Heart Aorta

Structure and Function

The thoracic aorta originates from the left ventricle and is guarded by the aortic valve. Just above the aortic valve cusps, the aorta gives rise to the right and left main coronary arteries, which travel within the coronary grooves of the heart and perfuse the myocardium. The initial portion of the aorta, which ascends posterior to the sternum, is referred to as the ascending aorta and extends approximately to the level of the T4 vertebral body. At this level, the vessel becomes the aortic arch and begins to curve posteriorly and to the left of the vertebral bodies within the posterior mediastinum. The aortic arch begins and ends at the level of the second rib and lies within the superior mediastinum. On chest radiography, the aortic arch is visible as the aortic knob. The aortic arch gives rise to 3 branches, including:

• Brachiocephalic artery: The brachiocephalic artery, also known as the innominate artery, is the first vessel to branch from the aortic arch. The artery travels superiorly and toward the right before bifurcating into the right subclavian artery and right common carotid artery.

• Left common carotid artery: The left common carotid artery is the second vessel to branch from the aortic arch.

• Left subclavian artery: The left subclavian artery is the third vessel to branch from the aortic arch.

The final section of the aortic arch, located just distal to the origin of the left subclavian artery, is known as the aortic isthmus. Mild aortic narrowing occurs near the ligamentum arteriosum, a remnant of the ductus arteriosus. At the level of the T4 vertebral body, the vessel continues as the descending thoracic aorta and travels inferiorly to the aortic hiatus of the diaphragm at T12, where it exits the thorax. The descending thoracic aorta gives rise to multiple vessels before exiting the thoracic cavity, including arteries that supply the pericardium, bronchi, mediastinum, and esophagus. Additional branches include the superior phrenic, posterior intercostal, and subcostal arteries.

Embryology

The aorta develops during the third gestational week.[1] The ascending aorta and aortic arch develop independently from 2 different embryologic structures. The ascending aorta develops from the truncus arteriosus, a component of the developing heart. The truncus arteriosus is a single outflow tract that forms during cardiac development and originates from both the left and right ventricles. The aorticopulmonary septum then divides the truncus arteriosus into 2 separate outflow tracts, which later become the pulmonary artery and ascending aorta.

The aortic arch forms through the development of the branchial arch arteries. Six branchial arch arteries, also known as the pharyngeal arch vessels, develop into the following structures. These develop into the following:

• First branchial arch artery: Forms the maxillary and external carotid arteries.
• Second branchial arch artery: Forms the stapedial arteries.
• Third branchial arch artery: Contributes to the formation of the common carotid arteries and the proximal internal carotid arteries.
• Fourth branchial arch artery: Forms the main portion of the aortic arch and the proximal right subclavian artery. The left subclavian artery develops from the left seventh intersegmental artery, one of a series of arteries arising from the dorsal aorta.
• Fifth branchial arch artery: Does not form a persistent vessel and subsequently regresses.
• Sixth branchial arch artery: Contributes to the formation of the pulmonary arteries and the ductus arteriosus.[2][3]

Surgical Considerations

Coarctation of the Aorta

Coarctation of the aorta is a congenital condition characterized by aortic narrowing that obstructs distal blood flow. The narrowing most commonly occurs near the ductus arteriosus and is classified as preductal, ductal, or postductal coarctation.

Preductal coarctation: Preductal coarctation occurs proximal to the ductus arteriosus. This form most often results from a congenital cardiac anomaly that decreases blood flow to the left side of the heart and aorta, leading to hypoplastic development.

Ductal coarctation: Ductal coarctation occurs at the insertion of the ductus arteriosus. This form most often becomes apparent at birth when the ductus arteriosus closes.

Postductal coarctation: Postductal coarctation occurs distal to the ductus arteriosus and is the most common form in adults. Patients often present with hypertension in the upper extremities and weak pulses in the lower extremities. The difference in perfusion results from increased blood flow through the aortic arch vessels and decreased blood flow to the descending aorta distal to the stenotic segment. Collateral vessels may enlarge to improve blood delivery to the descending aorta. Collateral circulation most commonly extends from the subclavian artery to the internal thoracic artery, anterior intercostal arteries, posterior intercostal arteries, and finally the descending thoracic aorta. Dilation of the intercostal arteries may produce the characteristic finding of rib notching on chest radiography.[4][5]

Coarctation of the aorta can cause substantial morbidity in patients with severe narrowing and is associated with poor outcomes among patients who survive beyond 1 year of age without treatment.[6] Treatment options include surgical repair, balloon angioplasty, and endovascular stent placement. Although these options are available, the decision on an optimal treatment strategy can be complicated. No universally accepted treatment algorithm exists for these patients.[7] Management is at the physician's discretion, with age at presentation and complexity of coarctation influencing treatment choice. 

Clinical Significance

Aortic Aneurysm

A thoracic aortic aneurysm refers to dilation of the thoracic aorta, often involving the proximal ascending segment. In adults, chronic hypertension is a common underlying factor. In younger adults, the most common underlying factor is a connective tissue disorder such as Marfan syndrome or Ehlers-Danlos syndrome. An aortic aneurysm is clinically significant because progressive dilation weakens the aortic wall, increasing the risk of aortic rupture or dissection. For this reason, elective repair is often recommended once an aneurysm has reached a diameter of 5.5 cm or greater.[8][9] Elective repair can reduce the risk of rupture and restore survival rates to nearly those of the general population.

Aortic Dissection

Aortic dissection refers to a disruption of the innermost layer of the aorta that allows blood to tunnel through the middle layer of the aortic wall. The initial tear most often occurs in the proximal portion of the ascending aorta, just distal to the aortic valve. Aortic dissections can then propagate distally or proximally through the aortic wall. Acute aortic dissection is a life-threatening emergency that may require urgent surgical repair to increase the likelihood of survival. Nearly 40% of patients with acute aortic dissection die immediately. Of those who survive the initial event, approximately 80% die within 2 weeks.[10]

Patent Ductus Arteriosus

The ductus arteriosus is a communication between the pulmonary artery and the aortic arch during fetal development. The vessel usually closes by the second day after birth in full-term infants. Persistence of this communication beyond 2 days is termed patent ductus arteriosus. Depending on the size of the persistent opening, patent ductus arteriosus may cause failure to thrive and heart failure.[1] Other aberrant conditions of development seen in the aorta are hypoplastic ascending aorta, interrupted aortic arch, right aortic arch, and double aortic arch.[1]