Typical Intercostal Spaces,Arteries,Veins,Lymphatics,Internal Thoracic Artery

Topic Overview

Typical Intercostal Spaces

• A typical intercostal space is found between the 3rd and 6th ribs; it contains muscles, vessels, and nerves arranged in a definite order.

• Each space is filled by:

  1. External intercostal muscle

  2. Internal intercostal muscle

  3. Innermost intercostal muscle

  4. Intercostal vessels and nerve

Arrangement of structures (from above downward):
Vein – Artery – Nerve (V-A-N) located in the costal groove on the lower border of the upper rib.

Contents of a typical intercostal space:

• Posterior intercostal artery (from thoracic aorta).

• Anterior intercostal artery (from internal thoracic or musculophrenic artery).

• Intercostal vein (drains into azygos/hemiazygos or internal thoracic vein).

• Intercostal nerve (anterior ramus of thoracic spinal nerve).

• Lymphatics accompanying these vessels.

Muscular plane arrangement (from superficial to deep):

• External intercostal → Internal intercostal → Innermost intercostal (subcostal and transversus thoracis portions).

Function:

• Provides rigidity to thoracic wall, assists in respiration, and serves as passage for neurovascular bundles.

Intercostal Arteries

Each intercostal space receives two sets of arteries:
1. Posterior intercostal arteries and 2. Anterior intercostal arteries.

Posterior Intercostal Arteries

• Origin:

  • First two → from superior intercostal artery (branch of costocervical trunk of subclavian artery).

  • Remaining nine → from thoracic aorta.

• Course:

  • Each passes between internal and innermost intercostal muscles in the costal groove.

  • Accompanied by intercostal vein (above) and nerve (below).

• Branches:

  • Dorsal branch: supplies vertebrae, spinal cord, and back muscles.

  • Collateral branch: runs along upper border of rib below.

  • Lateral cutaneous branch: to overlying skin.

  • Muscular branches: to intercostal muscles.

Anterior Intercostal Arteries

• Origin:

  • From internal thoracic artery (upper six spaces).

  • From musculophrenic artery (lower spaces).

• Course:

  • Each space receives two anterior intercostal arteries—one near upper border and one near lower border.

  • They anastomose with posterior intercostal arteries in the mid-axillary line.

• Distribution:

  • Supply intercostal muscles, ribs, costal cartilages, and overlying skin.

Intercostal Veins

• Each intercostal space has posterior and anterior intercostal veins, corresponding to the arteries.

• Arrangement: Vein lies above the artery and nerve below in the costal groove.

Posterior Intercostal Veins

• Drain into:

  • Right side: into azygos vein.

  • Left side:

    • Upper two or three → left superior intercostal vein (to left brachiocephalic vein).

    • Remaining → hemiazygos or accessory hemiazygos veins.

• Communicate with vertebral venous plexuses, providing alternate venous return pathways.

Anterior Intercostal Veins

• Drain into internal thoracic or musculophrenic veins, which then open into brachiocephalic veins.

Clinical Relevance:

• The posterior intercostal veins form vital anastomoses between the systemic and portal circulation, significant in conditions like portal hypertension.

• Azygos system serves as a collateral pathway for venous blood to bypass the inferior vena cava if it is obstructed.

Lymphatics of an Intercostal Space

• The lymphatic drainage of each intercostal space is divided into anterior and posterior groups.

• Anterior group:

  • Lymphatics from the anterior part of each intercostal space drain into the anterior intercostal (internal mammary) lymph nodes.

  • These nodes lie along the internal thoracic artery.

  • The efferent vessels from these nodes join those of the tracheobronchial and brachiocephalic nodes to form the bronchomediastinal trunk.

  • This trunk opens into the right lymphatic duct on the right side and into the thoracic duct on the left.

• Posterior group:

  • Lymphatics from the posterior part of the intercostal space drain into posterior intercostal lymph nodes, located on the heads and necks of the ribs.

  • Efferents from the lower four posterior intercostal spaces unite to form a lymphatic trunk that descends to open into the cisterna chyli.

  • Efferents from the upper spaces drain into the bronchomediastinal trunks on both sides

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Internal Thoracic Artery

Origin:

• Arises from the inferior aspect of the first part of the subclavian artery, opposite the thyrocervical trunk.

• The origin lies about 2 cm above the sternal end of the clavicle.

Course:

• Descends medially and downward behind the sternal end of the clavicle and first costal cartilage.

• Runs vertically downwards, about 2 cm from the lateral border of the sternum, up to the 6th intercostal space.

• Terminates by dividing into the superior epigastric and musculophrenic arteries.

Relations:

• Anteriorly: Covered by pectoralis major, first six costal cartilages, external intercostal membranes, internal intercostal muscles, and first six intercostal nerves.

• Posteriorly: Related to endothoracic fascia and pleura up to the 2nd or 3rd costal cartilage, and below this level by the sternocostalis muscle.

• Accompaniment: Paired venae comitantes that unite near the 4th costal cartilage to form the internal thoracic vein, which drains into the brachiocephalic vein.

Branches:

1. Pericardiacophrenic artery – accompanies the phrenic nerve, supplies pericardium and pleura.

2. Mediastinal arteries – small branches to thymus, pericardium, and mediastinal fat.

3. Anterior intercostal arteries – two branches to each of the upper six intercostal spaces.

4. Perforating arteries – in 2nd to 4th spaces, supply mammary gland.

5. Terminal branches:

   • Superior epigastric artery – enters rectus sheath to anastomose with inferior epigastric artery.

   • Musculophrenic artery – supplies 7th–9th intercostal spaces and diaphragm

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Clinical Note:

• The internal thoracic artery is often used in coronary artery bypass grafting (CABG) because of its durability and resistance to atherosclerosis.

• It must be avoided during pericardiocentesis, as it lies 1 cm lateral to the sternum.