A chest drain is a tube inserted into the thorax in order to drain fluid or air out of the pleural cavity. In a case of pneumothorax, the chest drain is connected to a fluid-filled bottle, which forms an underwater seal. This seal acts as a one-way valve allowing air to leave the pleural cavity, but not to enter it (it’s a bit like the opposite effect you see in a tension pneumothorax). The column of fluid “swings” with the changes in intrapleural pressure. This prevents air entering the pleural cavity via the drain, but allows the expulsion of any intrapleural air during expiration when the intrapleural pressure becomes less negative and the column of fluid “swings” down.

pneumothorax chest drain

Figure 1: A simple model of a chest drain and bottle used in pneumothorax. The lungs – with a minor leak - are represented as a punctured balloon in a sealed jar. A chest drain is connected to a bottle with an underwater seal. During inspiration, intrapulmonary pressure (Pip) becomes more negative, drawing air into the lungs. At the same time, fluid from the underwater seal rises slightly in the column of water in the bottle (the distance it rises is the change in Pip, measured in cmH2O). Until the hole in the lung heals, a little air may leak into the pleural cavity during inspiration. On expiration, Pip becomes less negative and the fluid in the underwater seal falls. Any air that leaked into the pleural cavity during inspiration will be forced out through the chest drain and bubble into the bottle. Thus, a functioning chest drain bottle is seen to “swing” (the fluid in the seal rising and falling) and bubble. When the pneumothorax heals, the drain will swing (mirroring normal changes in Pip) without bubbling. If it fails to swing, the drain may be blocked, or have been inadvertently withdrawn.

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