Intercostal Arteries and Veins
The intercostal space is the space between the ribs, and it is comprised of three muscular layers and a neurovascular bundle. The blood supply and venous drainage of the intercostal spaces is essential to functional breathing. In this article we will discuss the gross and functional anatomy of the intercostal arteries and veins. We will also discuss the clinical relevance of the structures, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The intercostal arteries and veins run on the costal groove of each rib (below each rib), and supply the intercostal muscles with blood and drain away blood. There are three layers of intercostal muscles, the external that run in the oblique direction of your hands in your pockets, the internal intercostals which run in the perpendicular direction to the external intercostals, and the innermost intercostals, which run in the same direction as the internal intercostals. The intercostal neurovascular bundle runs between the internal and innermost intercostal muscles.
The anterior intercostal arteriesThese arteries supply the superior five to six intercostal spaces. They are branches of the internal thoracic artery (also known as the internal mammary artery), which is a branch of the first part of the subclavian artery. The internal thoracic artery divides at the level of sixth intercostal space into two terminal branches, the musculophrenic and superior epigastric artery. The anterior intercostal artery divides into a superior and an inferior branch. The inferior branch lies below the vein and above the nerve in the intercostal neurovascular bundle. The superior branch lies just superior to the rib above. They supply the intercostal muscles, pectoral muscles, breast and skin.
The musculophrenic artery continues to supply the remainder of the anterior intercostal arteries. It runs obliquely downward posterior to the cartilages of the false ribs (ribs 8-10). The artery then pierces the diaphragm, and gives off intercostal branches to the 7th to 9th intercostal spaces. It then terminates as a much smaller artery opposite the last intercostal space. The musculophrenic artery also supplies the pericardium as well as the abdominal muscles. Terminally, the anterior intercostal arteries anastomose with the posterior intercostal arteries.
The anterior intercostal veins
The anterior intercostal veins follow the same course as the arteries, and run in the costal grooves on the lower edge of the ribs. The veins run above the arteries, with the intercostal nerve the lowest of the three. They are the tributaries of internal thoracic and musculophrenic veins.
The posterior intercostal arteries
The superior intercostal artery is a branch of the costocervical artery, a branch of the second part of subclavian artery. It gives rise to the 1st and 2nd posterior intercostal arteries. The lower nine posterior intercostal arteries, are also known as the aortic intercostals, and rises from the posterior surface of the thoracic aorta. The thoracic aorta arches over the left pulmonary arteries and lies anterior to the tracheal bifurcation at T4. There are dorsal, collateral, muscular and cutaneous branches of the posterior intercostal arteries.
The right aortic intercostals are longer than the left aortic intercostals. The right aortic intercostals pass across the vertebrae, posterior to the oesophagus, azygous vein and thoracic duct. The left aortic intercostals cross along the sides of the thoracic vertebrae, and are covered by the left lung and pleura. They are crossed anteriorly by the superior intercostal vein superiorly, and the hemiazygous and accessory hemiazygous vein inferiorly.
The sympathetic trunk also runs down anterior to the arteries. Each artery divides into an anterior and posterior ramus. Each intercostal artery runs in the costal groove, on the lower edge of the rib. The vein runs above the artery, with the intercostal nerves the lowest of the three. The third posterior intercostal artery anastomoses with the superior intercostal artery on its respective side, and may be an important source of collateral blood flow to the second intercostal space. The lower two arteries anastomose with superior epigastric and lumbar arteries.
The posterior intercostal veins
These veins drain into the azygous (right side) and hemiazygous and accessory hemiazygous vein (left side). The first posterior intercostal vein drain into ipsilateral brachiocephalic or vertebral vein. Superior intercostal vein is formed on the right side by the joining of second third and fourth intercostal space which later drain into azygous vein. Left superior intercostal vein is responsible to drain second and third left posterior intercostal veins and open into left bronchial vein and has connection with accessory hemiazygous inferiorly. Accessory hemiazygous vein also drain all the veins from 5th (sometimes fourth) to eighth intercostal space while 9th to 11th intercostal space veins join the hemiazygous. The azygous vein drains the posterior thoracic and abdominal wall, and is formed by the confluence of the ascending lumbar veins and the right subcostal vein at vertebral level T12. The azygous vein arches over the right main bronchus, in order to join the superior vena cava. The azygous vein also drains the pericardial veins, bronchial veins and vertebral venous plexuses.
Enlarged azygous vein- This can result from overhydration, pregnancy, aortoazygous fistula, constrictive pericarditis, Superior vena cava obstruction, inferior vena cava obstruction, portal hypertension and any other cause of right heart output failure. It may also result from mediastinal or paratracheal malignancy that compressed the vein.
Inferior Rib Notching/Roesler sign- This is deformation of the inferior surface of the rib. It occurs due to dilated intercostal arteries, secondary to coarctation of the aorta, in order to bypass the narrowed aorta. The blood passes from the internal thoracic artery, to the anterior intercostals, the posterior intercostals and therefore to the aorta. It is only seen in long standing cases of the disease.
- The superior 6 intercostal arteries arise from the internal thoracic artery.
- The lower anterior intercostal arteries arise from the musculophrenic artery.
- The superior 2 posterior intercostal arteries arise from the costocervical trunk, and the lower 9 arise from the aorta.
- The anterior intercostal veins follow the arteries.
- The posterior intercostal veins drain into the azygous vein (right side), and hemiazygous and accessory hemiazygous veins (left side). The latter two veins ultimately drain into the azygous vein.
- The azygous vein then drains into the superior vena cava.