Thoracic Venous Drainage
Overview
The venous drainage of the thorax is dominated by the azygos venous system, serving as a vital collateral channel between the SVC and IVC. When either cava is obstructed, this system becomes a recognisable bypass pathway on contrast CT and CT venography.
Azygos System Architecture
- Azygos Vein: Ascends the right paravertebral gutter, arching over the right lung root at T4 to drain into the SVC. Drains the right posterior intercostal veins.
- Hemiazygos Vein: Drains the lower left thoracic cage (T9-T12), crossing horizontally at T9 to join the azygos vein.
- Accessory Hemiazygos Vein: Drains the upper left cage (T5-T8), crossing at T7-T8 to feed the azygos vein.
Clinical Significance of SVC Obstruction
WarningSVC Syndrome
When the SVC is obstructed (most commonly by bronchogenic carcinoma or lymphoma compressing it), blood re-routes inferiorly via the azygos system to reach the IVC. On CT, the azygos vein appears dramatically dilated, and collateral veins are visible in the chest wall.
Azygos Arch Variant
An azygos lobe is a well-known anatomical variant where the azygos vein invaginates into the right upper lobe, creating a visible fissure on CXR. It is not pathological but must not be mistaken for pleural disease or a mass.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
The azygos vein appears as a rounded opacity in the right tracheobronchial angle on PA CXR. Normal diameter is less than 10 mm erect. A measurement >10 mm indicates azygos engorgement from raised CVP, SVC obstruction, or portal hypertension.