Outside the lungs and inside the chest wall is a double layer of tissue, called the pleural lining. Between the two layers is a very small amount of fluid, which allows the layers to glide smootly over each other as the lungs and ribcage move during breathing.
Occasionally, a large amount of fluid can collect in this space. There are many possible causes of the fluid build-up, including infection, malignancy, trauma, surgery and cardiac failure. A collection of excessive fluid in the pleural space is problematic because it prevents the lungs from fully expanding and may cause substantial pain.
Sometimes the fluid can be drained by inserting a large needle between two ribs. However, sometimes a drainage tube in inserted between two ribs and the end of the tube is attached to a system that allows drainage but prevents backflow, thereby draining the excessive fluid over time.
Various complications can develop during the period that the intercostal tube is in situ (including blockage, infection, internal injury), so it is important to encourage rapid drainage of the pleural fluid collection.