Azygos and Hemiazygos Venous Drainage

The azygos vein drains into the superior vena cava, while the hemiazygos vein typically drains into the azygos vein.

Normal Anatomical Drainage Pattern

The azygos vein terminates by arching over the right main bronchus and draining directly into the superior vena cava at approximately the level of the third to fourth thoracic vertebra 1, 2
During cardiac transplant procurement, the superior vena cava is dissected superiorly to the level of the azygos vein, which serves as an important anatomical landmark 1
The azygos vein serves as a collateral venous pathway between the superior and inferior vena cava systems, becoming a vital shunt when major pathways are obstructed 2

The hemiazygos vein crosses the midline (typically at the level of the eighth to tenth thoracic vertebra) and drains into the azygos vein 3, 4
The accessory hemiazygos vein, when present, also crosses to drain into the azygos vein at a higher level (typically around the seventh to eighth thoracic vertebra) 3, 4

In certain congenital heart conditions, particularly univentricular hearts and heterotaxia syndromes, absent infrahepatic inferior vena cava with azygous or hemiazygous connection can occur as an associated lesion 1
When the inferior vena cava is interrupted, the azygos system provides continuation of venous drainage, with the azygos vein becoming significantly enlarged (azygos continuation) 2
Systemic venous return abnormalities such as interrupted inferior vena cava with azygous continuation require specific imaging assessment during evaluation of complex congenital heart disease 1

During the Warden procedure for partial anomalous pulmonary venous connection, the azygous vein is ligated as part of the surgical technique 1
Knowledge of azygos system variations is critical for cardiothoracic surgeons during central venous access procedures and surgical interventions 5, 6

Normal anatomical variations of the azygos venous system occur in approximately 1.5% of the population 6
The most common variation is the connection between the hemiazygos and accessory hemiazygos veins draining into the left brachiocephalic vein (0.9% prevalence) rather than crossing to the azygos vein 6
Rare variations include transposition where the azygos vein forms on the left side and terminates into the left brachiocephalic vein 5

Failure to recognize azygos system variations can lead to complications during central venous catheterization, pacemaker/defibrillator implantation, and cardiothoracic surgery 5, 6, 4
In cases of absent left brachiocephalic vein, venous drainage may occur through the left superior intercostal vein, accessory hemiazygous, hemiazygous, and azygous veins into the right brachiocephalic vein 4
Cross-sectional imaging (CT or MRI) should be reviewed carefully before left-sided device implantation to identify potential venous anomalies 6, 4