Intercostal Chest Drain Attached to Underwater Seal

The end of the tube is 2–3 cm below the level of the water in the bottle. If intrapleural pressure becomes negative, water rises up the tube, only to fall again when the intrapleural pressure falls towards atmospheric. The system operates as a simple one-way valve, allowing either air or fluid within the pleural space to drain out safely. Once a pneumothorax or effusion has resolved, the water level will generally be slightly negative throughout the respiratory cycle and reflect the normal fluctuation in intrapleural pressure. (Adapted from Brewis RAL (1985) Lecture Notes on Respiratory Diseases, 3rd edn. Oxford: Blackwell Scientific Publications, p. 290.)