2nd part of aorta; entirely in superior mediastinum; arches over root of left lung; gives 3 branches to head/neck/upper limbs.

Extent

Begins anteriorly, ends posteriorly; both ends at same horizontal level (sternal angle/T4-T5)

Course

• Upwards → backwards → to the left
• Posterior to lower half of manubrium; summit reaches middle of manubrium
• In front of tracheal bifurcation
• Arches over left principal bronchus + left pulmonary artery

Relations

Anterior/Left (order)

1. Left phrenic nerve
2. Inferior cervical cardiac branch of left vagus
3. Superior cervical cardiac branch of left sympathetic trunk
4. Left vagus nerve
5. Left superior intercostal vein

Left superior intercostal vein: deep to phrenic nerve, superficial to vagus

• Also: left lung/pleura, thymic remnants

Superior

• 3 branches right→left: brachiocephalic trunk, left common carotid, left subclavian
• Left brachiocephalic vein crosses anterior to all three

Posterior/Right

• Trachea, esophagus, left recurrent laryngeal nerve, thoracic duct, vertebral column, deep cardiac plexus

Inferior (concavity)

• Left principal bronchus, pulmonary trunk bifurcation, left pulmonary artery, ligamentum arteriosum, left recurrent laryngeal nerve, superficial cardiac plexus
• Left recurrent laryngeal hooks under arch near ligamentum arteriosum → tracheo-esophageal groove

Branches (B-C-S)

Brachiocephalic trunk: origin behind manubrium centre; up-back-right; ends behind right SC joint; → right common carotid + right subclavian; anterior to trachea; thyroidea ima may arise here (tracheostomy risk)

Left common carotid: ascends left of trachea; behind left SC joint (has thoracic course, unlike right)

Left subclavian: most posterior branch; ascends vertically; behind left SC joint; longest thoracic course; close to left mediastinal pleura

Variations

• Common brachiocephalic + left common carotid origin
• Aberrant right subclavian → dysphagia lusoria
• Thyroidea ima → tracheostomy bleeding risk
• Right-sided arch → congenital heart disease association

Ligamentum Arteriosum

• Connects: inferior arch (distal to left subclavian) ↔ root of left pulmonary artery
• Postnatal remnant of ductus arteriosus
• Relations: posterior = left recurrent laryngeal nerve; anterior = superficial cardiac plexus; superior = arch; inferior = left pulmonary artery
• Surgical landmark for left recurrent laryngeal nerve

Ductus Arteriosus

• Fetal: pulmonary trunk/left PA → aorta; bypasses non-functioning lungs
• Derived from distal left 6th aortic arch
• Functional closure: hours–days after birth; anatomical closure: weeks (→ ligamentum arteriosum)
• Triggers: ↑O2 tension after first breath; ↓prostaglandins after placental separation

Applied Anatomy

PDA

• Failure of closure → left-to-right shunt (aortic > pulmonary pressure)
• Continuous machinery murmur, best at left 2nd ICS near sternum
• Wide pulse pressure, bounding pulses; long-standing → Eisenmenger reversal/cyanosis
• Rx: indomethacin/ibuprofen (premature infants); PGE1 keeps duct open if needed; catheter/surgical closure

Coarctation of Aorta

• Narrowing near aortic isthmus, close to ligamentum arteriosum

• Features: upper limb HTN, weak lower limb pulses, radiofemoral delay, claudication, rib notching, reverse-3 sign
• Collateral: subclavian → internal thoracic/costocervical → anterior/posterior intercostal arteries → descending aorta below coarctation
• Rib notching: ribs 3–8, inferior borders

Aneurysm of Arch

• Causes: atherosclerosis, syphilitic aortitis, Marfan, cystic medial degeneration, trauma

• Tracheal tug — downward tugging felt in suprasternal notch; classic for arch aneurysm

Aortic Knuckle — arch shadow, upper left mediastinal border on PA CXR; prominent in age, HTN, atherosclerosis, aneurysm

Dysphagia Lusoria — aberrant right subclavian artery passes behind esophagus → compression, especially with solids

Left Recurrent Laryngeal Nerve — at risk in arch surgery/aneurysm, mediastinal lymphadenopathy, esophageal/lung surgery → hoarseness, vocal cord paralysis, bovine cough, aspiration risk

The arch of aorta is the second part of the aorta. It lies entirely in the superior mediastinum, arches over the root of the left lung, gives the three great branches to the head, neck and upper limbs, and is related to the ligamentum arteriosum and left recurrent laryngeal nerve.

Study sequence:

Extent → course → relations → branches → ligamentum arteriosum → applied anatomy

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Extent

Key point:

• The arch begins anteriorly and ends posteriorly.
• Both ends lie at the same horizontal level: the sternal angle / T4-T5 level.

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Course

From its origin, the arch passes:

1. Upwards
2. Backwards
3. To the left

It arches over the root of the left lung and continues as the descending thoracic aorta.

Important course facts:

• It lies posterior to the lower half of manubrium sterni.
• Its summit reaches the level of the middle of manubrium.
• It passes in front of the tracheal bifurcation.
• It arches over the left principal bronchus and left pulmonary artery.

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Position

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Relations of the Arch of Aorta

The relations are best learned by surfaces.

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Anterior and Left Relations

Structures crossing the left/anterior surface of the arch include:

Additional superficial relations:

• Left lung and pleura
• Remains of thymus

Important memory point:

• Left superior intercostal vein lies deep to phrenic nerve and superficial to vagus nerve.

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Superior Relations

The superior / convex surface gives three branches from right to left:

1. Brachiocephalic trunk
2. Left common carotid artery
3. Left subclavian artery

The left brachiocephalic vein crosses anterior to these three branches.

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Posterior and Right Relations

Structures related posteriorly and to the right:

• Trachea
• Oesophagus
• Left recurrent laryngeal nerve
• Thoracic duct
• Vertebral column
• Deep cardiac plexus near tracheal bifurcation

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Inferior Relations

The inferior concavity of the arch is related to:

• Left principal bronchus
• Bifurcation of pulmonary trunk
• Left pulmonary artery
• Ligamentum arteriosum
• Left recurrent laryngeal nerve
• Superficial cardiac plexus

The left recurrent laryngeal nerve hooks under the arch near the ligamentum arteriosum and ascends in the tracheo-oesophageal groove.

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Branches of the Arch of Aorta

The arch gives three branches from its convex surface.

Memory: B-C-S

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Brachiocephalic Trunk

The brachiocephalic trunk is the first and largest branch.

Important relation:

• It lies anterior to the trachea.

Variation:

• Thyroidea ima artery may arise from it or directly from the arch; important during tracheostomy.

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Left Common Carotid Artery

Unlike the right common carotid, the left common carotid has a thoracic course.

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Left Subclavian Artery

It has the longest thoracic course among the three arch branches.

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Common Branch Variations

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Ligamentum Arteriosum

The ligamentum arteriosum is a short fibrous cord connecting:

• Inferior surface of arch of aorta just distal to left subclavian artery
• Root of left pulmonary artery

It is the postnatal remnant of the ductus arteriosus.

Relations

Surgical importance:

• It is a landmark for the left recurrent laryngeal nerve.

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Ductus Arteriosus

In fetal life, the ductus arteriosus connects the pulmonary trunk / left pulmonary artery to the aorta.

Function:

• Bypasses the non-functioning fetal lungs.
• Carries blood from pulmonary trunk to descending aorta.

Development:

• Derived from the distal part of the left 6th aortic arch.

Closure:

Triggers:

• Increased oxygen tension after first breath
• Reduced prostaglandins after placental separation

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Applied Anatomy

Patent Ductus Arteriosus

PDA is failure of ductus arteriosus to close after birth.

After birth:

• Aortic pressure exceeds pulmonary pressure.
• Blood flows from aorta to pulmonary artery.
• This is a left-to-right shunt.

Features:

• Continuous machinery murmur
• Best heard in left 2nd intercostal space near sternal margin
• Wide pulse pressure
• Bounding pulses
• Pulmonary hypertension in large cases

Complication:

• Long-standing pulmonary hypertension may lead to Eisenmenger reversal and cyanosis.

Treatment:

• Indomethacin or ibuprofen may help close PDA in premature infants.
• Prostaglandin E1 keeps the ductus open when needed in duct-dependent congenital heart disease.
• Catheter closure or surgical ligation may be required.

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Coarctation of Aorta

Coarctation is congenital narrowing of the aorta, commonly near the aortic isthmus, close to the ductus arteriosus / ligamentum arteriosum.

Types

Features

• Hypertension in upper limbs
• Weak pulses in lower limbs
• Radiofemoral delay
• Claudication of legs
• Rib notching
• Reverse 3 sign on chest X-ray

Collateral Circulation

Subclavian artery
→ Internal thoracic and costocervical branches
→ Anterior and posterior intercostal arteries
→ Descending aorta below coarctation

Rib notching occurs along inferior borders of ribs, classically ribs 3-8, due to enlarged intercostal arteries.

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Aneurysm of Arch of Aorta

An aneurysm is pathological dilatation of the arch.

Causes:

• Atherosclerosis
• Syphilitic aortitis, classically
• Marfan syndrome
• Cystic medial degeneration
• Trauma

Compression effects:

Tracheal Tug

Tracheal tug is downward tugging of the trachea felt in the suprasternal notch, classically associated with aneurysm of the arch of aorta.

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Aortic Knuckle

On PA chest X-ray, the arch of aorta forms the aortic knuckle on the upper left mediastinal border.

Prominent in:

• Old age
• Hypertension
• Atherosclerosis
• Aortic aneurysm

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Dysphagia Lusoria

Dysphagia lusoria is dysphagia due to an aberrant right subclavian artery.

Mechanism:

• Right subclavian artery arises as an abnormal distal branch of arch.
• It passes behind the oesophagus.
• It compresses the oesophagus, especially during swallowing solids.

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Left Recurrent Laryngeal Nerve

The left recurrent laryngeal nerve is related to the arch because it hooks under the arch near the ligamentum arteriosum.

At risk in:

• Aortic arch surgery
• Arch aneurysm
• Mediastinal lymph node enlargement
• Oesophageal surgery
• Left lung surgery

Injury causes:

• Hoarseness
• Vocal cord paralysis
• Bovine cough
• Aspiration risk

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