Drains head/neck, upper limbs, thoracic wall, upper trunk → right atrium.

Formation

Course

• Descends vertically behind right sternal border
• Extrapericardial part: superior mediastinum; Intrapericardial part: middle mediastinum
• Sequence: forms behind right 1st costal cartilage → descends → receives azygos vein before pericardium → pierces fibrous pericardium at right 2nd costal cartilage → opens into RA at right 3rd costal cartilage level

Tributaries

• Right + left brachiocephalic veins (forms SVC)
• Azygos vein (opens before pericardium)
• Mediastinal + pericardial veins (small)

Relations

Brachiocephalic Veins

• Formed behind SC joint: internal jugular + subclavian veins

Tributaries:

Left brachiocephalic vein crosses anterior to brachiocephalic trunk, left common carotid, left subclavian

CXR

• SVC forms upper part of right border of mediastinal/cardiac shadow on PA film

SVC Obstruction

• Causes: bronchogenic carcinoma, mediastinal lymphadenopathy, lymphoma, aortic aneurysm, thrombosis/catheter
• Thin-walled, low-pressure → early signs in mediastinal syndrome
• Features: facial/neck swelling, distended neck veins, dilated chest veins, cyanosis/congestion of upper body, dyspnoea, cough, headache

Collateral Pathways

Above azygos opening — upper body veins → superior intercostal/azygos system → azygos → SVC below obstruction → RA

• Dilated veins mainly chest wall, limited to costal margin

Below azygos opening — lateral thoracic ↔ thoracoepigastric ↔ superficial epigastric ↔ femoral ↔ external iliac ↔ IVC

• Dilated veins chest + abdomen, towards groin

Applied Anatomy

• Mediastinal syndrome — SVC often first compressed (thin-walled, low pressure)
• Central venous access — IJV + subclavian veins form brachiocephalic veins; relevant to CVC placement
• JVP — absent SVC valves → RA pressure changes transmitted to IJV (basis of JVP exam)
• Aortic nipple — distended left superior intercostal vein on PA CXR (e.g., raised venous pressure, SVC obstruction)

The superior vena cava (SVC) is the large vein that drains blood from the head and neck, upper limbs, thoracic wall, and upper part of the trunk into the right atrium. It is clinically important because obstruction produces characteristic collateral venous pathways and early signs in mediastinal compression.

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Formation

The SVC is formed by union of the right and left brachiocephalic veins.

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Course

The SVC descends vertically behind the right border of sternum.

It has two parts:

Course sequence:

1. Formed behind lower border of right 1st costal cartilage.
2. Descends vertically.
3. Receives the azygos vein just before entering pericardium.
4. Pierces fibrous pericardium at the level of right 2nd costal cartilage.
5. Opens into upper part of right atrium at lower border of right 3rd costal cartilage.

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Tributaries

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Relations

Anterior Relations

• Right lung and pleura
• Right internal thoracic vessels
• Anterior chest wall

Posterior Relations

• Trachea, posteromedially
• Right pulmonary artery
• Right principal bronchus

Right Relations

• Right phrenic nerve
• Pericardiacophrenic vessels
• Right mediastinal pleura and lung

Left Relations

• Ascending aorta
• Brachiocephalic trunk

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Brachiocephalic Veins

Each brachiocephalic vein is formed behind the sternoclavicular joint by union of:

• Internal jugular vein
• Subclavian vein

The right and left brachiocephalic veins unite to form the SVC.

Right vs Left Brachiocephalic Veins

Tributaries

The left brachiocephalic vein crosses anterior to the three branches of the arch of aorta:

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

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SVC in Chest X-Ray

In PA chest radiograph, the SVC forms the upper part of the right border of the mediastinal / cardiac shadow.

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Obstruction of Superior Vena Cava

SVC obstruction may occur due to:

• Bronchogenic carcinoma
• Mediastinal lymphadenopathy
• Lymphoma
• Aortic aneurysm
• Thrombosis or catheter-related obstruction

Because the SVC is thin-walled and low-pressure, signs of obstruction appear early in mediastinal syndrome.

Features of SVC Obstruction

• Facial swelling
• Neck swelling
• Distended neck veins
• Dilated superficial veins over chest
• Cyanosis and congestion of upper limb/head-neck region
• Dyspnoea, cough, headache

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Collateral Pathways

The pattern depends on whether the obstruction is above or below the entry of the azygos vein.

Obstruction Above Azygos Opening

Venous return can still pass through the azygos vein to the SVC below the obstruction.

Pathway:

Upper body veins
→ Superior intercostal and azygos system
→ Azygos vein
→ SVC below obstruction
→ Right atrium

Clinical sign:

• Dilated veins mainly over chest wall, limited up to costal margin.

Obstruction Below Azygos Opening

Blood must reach the inferior vena cava through caval-caval collateral channels.

Important pathway:

Lateral thoracic vein
↔ Thoracoepigastric vein
↔ Superficial epigastric vein
↔ Femoral vein
↔ External iliac vein
↔ Inferior vena cava

Clinical sign:

• Dilated veins over chest and abdomen, extending towards the groin.

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Key Distinction

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Applied Anatomy

Mediastinal Syndrome

The SVC is often the first major structure compressed in mediastinal masses because it is thin-walled and low pressure.

Central Venous Access

The internal jugular and subclavian veins form the brachiocephalic veins. Their anatomy is important during central venous catheter placement.

Jugular Venous Pulse

Absence of valves in the SVC allows venous pressure changes from the right atrium to be transmitted to the internal jugular vein, forming the anatomical basis of JVP examination.

Aortic Nipple

The left superior intercostal vein may appear as an aortic nipple on PA chest X-ray when distended, for example in raised venous pressure or SVC obstruction.

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