accessory hemiazygos vein

The accessory hemiazygos vein (accessory hemi-azygos vein) is a vein on the left side of the vertebral column that generally drains venous blood from the fifth through eighth intercostal spaces on the left side of the chest. It is part of the azygos venous system.

The accessory hemiazygos vein runs vertically on the left side of the vertebral column, above the hemiazygos vein. It typically extends from approximately the fifth to eighth intercostal spaces and crosses the vertebral column to drain into the azygos vein on the right side.

The accessory hemiazygos vein drains venous blood from the upper left intercostal spaces (ribs 5-8) and delivers it into the azygos system. This provides an alternate route for venous return from the thoracic wall when the superior vena cava or local drainage is compromised.

The accessory hemiazygos vein can become a collateral pathway in cases of superior vena cava syndrome (obstruction of the SVC), allowing venous blood to bypass the obstruction and return to the heart. Thoracic surgery and central venous catheter placement may involve knowledge of this vessel.

The hemiazygos vein drains the lower left intercostal spaces (approximately 9-11), while the accessory hemiazygos vein drains the middle intercostal spaces (approximately 5-8). Together with the left superior intercostal vein above, these three vessels cover the entire left side of the thoracic wall's venous drainage.