SN4
SHORT NOTE
5M
★ High-yield
Describe the azygos system of veins and its clinical importance.
Definition
The azygos system is a set of longitudinal veins on the posterior thoracic wall that drains the thoracic wall and forms an important anastomosis between the superior and inferior venae cavae.
Azygos Vein
- Right posterior intercostal veins (4th to 11th)
- Right superior intercostal vein (2nd and 3rd spaces)
- Hemiazygos and accessory hemiazygos veins
- Oesophageal, mediastinal, pericardial and bronchial veins
- Right subcostal and ascending lumbar veins
Hemiazygos and Accessory Hemiazygos
| Vein | Formation | Drains | Ends |
|---|---|---|---|
| Hemiazygos | Left ascending lumbar and left subcostal | Lower left posterior intercostal veins (9th–11th) | Crosses to the right at T8 to join the azygos |
| Accessory hemiazygos | Descends on the left | Left posterior intercostal veins (4th–8th) | Crosses at T7 to join the azygos |
| Left superior intercostal | — | 2nd and 3rd left spaces | Left brachiocephalic vein |
- The pattern is highly variable, more so than almost any other venous system
Clinical Importance — Caval Anastomosis
- In SVC obstruction the flow reverses — blood passes downward through the same channels to the inferior vena cava; the direction of flow in the dilated veins is the clinical discriminator
- The azygos also communicates with the vertebral venous plexus (Batson), which is valveless — a route for the spread of prostatic, breast and thyroid carcinoma to the vertebrae, skull and brain, bypassing the lungs
Relations of Surgical Importance
- Lies on the right side of the thoracic aorta and thoracic duct
- The arch of the azygos is a landmark in right thoracotomy, and must be divided in an oesophagectomy
- It grooves the medial surface of the right lung
- Azygos lobe — an anatomical variant in which the vein cuts into the right upper lobe, producing an accessory fissure visible on the chest radiograph
Applied Aspects
- The azygos vein dilates in portal hypertension, carrying blood from the oesophageal varices — part of the portosystemic anastomosis
- Widening of the azygos shadow on a chest radiograph suggests raised right atrial pressure, fluid overload, or obstruction
- Injury during thoracic surgery causes brisk haemorrhage, since the vein is large and thin-walled
- The system explains why an occluded inferior vena cava is compatible with life — a striking demonstration of the value of collateral circulation
All questions in Thorax
- Q1Define the mediastinum and describe its divisions with their contents. Add a note on the structures at the level of the sternal angle. ★
- Q2Describe the chambers of the heart, its conducting system and its blood supply. Add a note on the surface marking of the valves. ★
- Q3Describe the lungs with their lobes, fissures and root. Explain bronchopulmonary segments and add a note on the pleura. ★
- Q4Describe the diaphragm under origin, openings, nerve supply and actions. Add a note on its applied anatomy. ★
- Q5Describe the intercostal space with its muscles, vessels and nerves. Add a note on the thoracic cage and applied aspects. ★
- SN1Describe the coronary circulation with its branches and peculiarities. ★
- SN2Describe the pericardium with its layers and sinuses. Add a note on cardiac tamponade. ★
- SN3Describe the course and distribution of the phrenic nerve. Explain the referred pain to the shoulder. ★
- SN4Describe the azygos system of veins and its clinical importance. ★
- SN5Describe the course and drainage of the thoracic duct. Add a note on chylothorax. ★
- SN6Describe the formation, course and relations of the superior vena cava. Add a note on SVC obstruction. ★
- SN7Describe the breast with its lymphatic drainage. Add a note on the surface anatomy of the thorax.