Question 24

a) Draw a simple line diagram of a single chamber chest drain using an underwater seal and label the main features including the connections. List its advantages and disadvantages.  
                                                           (30% marks) 
b)    Draw a simple line diagram of a double chamber chest drain with an underwater seal and label the main features including the connections. List its advantages and disadvantages.    
(30% marks) 
c)    Draw a simple line diagram of a three-chamber chest drain with an underwater seal and label the main features including the connections.  List its advantages and disadvantages.  (40% marks) 

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College answer


Drain simple pneumothoraces

Cannot drain fluid from pleural cavity safely 
Cannot apply suction safely 


Drain simple pneumothoraces and fluid

Cannot apply suction safely 


Drain simple pneumothoraces and complex fluid collections

Can apply suction

Complexity and cost 

Examiners Comments: 
Extremely poorly done with many candidates showing a complete lack of even a basic understanding of the 
set up or physics of pleural drains.   


The chest drain systems asked about are:

The single chamber system

Single bottle underwater seal pleural drain


  • Simple
  • Cheap
  • Easily improvised from unrelated equipment
  • For simple pneumothorax, there is usually no need for anything more sophisticated
  • The fluid level (i.e. valve pressure) is adjustable, though there are few scenarios where one might wish to adjust it. 


  • It is unsuitable for draining pleural fluid.  Air will vent out of the single bottle effortlessly, but any fluid drained will collect in the bottle, increasing the fluid level. As the fluid level rises, the pressure required to force air and fluid out of the chest cavity increases; i.e. the more fluid drains out of the patient, the deeper the tip of the tube, and the more pressure will be required to force further fluid/gas out of the pleural cavity.
  • If pleural fluid coes enter the bottle, froth will form. Protein from the pleural space tends to foam due to the bubbling of the drain, which fills the chamber with froth. This makes the level of the fluid difficult to read, and is aesthetically unappealing. 
  • Fluid may reflux into the patient's chest cavity. As long as this bottle remains well below the level of the patient's pleural space, no fluid will get sucked up into the chest. If the bottle is held above the level of the chest, everything inside it may regurgitate back into the pleural cavity, with non-hilarious consequences.

The double chamber system

two-chamber underwater seal pleural drain


  • Fixed underwater seal level, therefore consistent (low) resistance to air expulsion
  • Pleural fluid and water seal are separate: therefore, no froth will form.
  • The collection bottle permits the drainage of pleural fluid, so the case uses of this system are not limited to pneumothoraces.


  • It is less efficient at draining air cavities. The air of the first chamber becomes essentially an extension of the pleural air pocket, a large compressible volume of gas. Air expelled from the pleural cavity must compress this gas volume enough to overcome the underwater seal, which requires more effort than the single chamber drain. In this fashion, the two-chamber system impedes the drainage of pneumothorax and the re-expansion of the lung.
  • Without suction, the drainage is less efficient: the pressure difference between the drainage chamber and the underwater seal chamber is fairly low. One can apply a sucking subatmospheric pressure to the seal chamber, thereby increasing that gradient, but this system does not innately offer any mechanism by which one might regulate that pressure.

The three-chamber system

three-chamber underwater seal pleural drain


  • Adjustable pressure of the suction: in a three-bottle system the depth of the vent tube determines the negative pressure. The pressure can be adjusted to the desired level by manipulating the depth of the manometer vent tube in the third bottle. This also protects the pleural cavity from the unmoderated effects of wall suction.
  • Effective for both pneumothorax and pleural fluid: There is no loss of drainage efficiency with pleural fluid drainage, i.e. the volume of fluid collecting in the first chamber has no influence on either the suction or the underwater seal.
  • No likelihood of fluid refluxing back up the tubing: there is virtually no chance that pleural drain fluid will re-enter the chest cavity with a sudden decrease in intrathoracic pressure.


  • Continuous bubbling: while the drain is on suction, it constantly entrains room air, and bubbles gurgle around in the third chamber. Depending on how much you like this sound, this is either a feature or a bug.
  • Complexity is often quoted as a disadvantage, though one must consider that we are usually protected from this complexity by packaged pre-assembled drain systems (i.e. at no stage is one ever expected to actually assemble such a system from glass bottles and rubber stopcocks). 
  • No failsafe for suction failure: if the suction line is occluded, one is left with what is essentially a blocked two-chamber system. There will be no way for the pleural pressure to overcome the resistance of the water column in the third chamber, and the air pressure in the chambers will increase to the point of re-expanding the pneumothorax. 


NSW Health: Chest Drain - Set up of Atrium Oasis Dry Suction Under-Water Seal Drainage

Atrium have published their instructions online.

Additionally, they provide this training document which is surprisingly full of useful information.

Kam, A. C., M. O'brien, and P. C. A. Kam. "Pleural drainage systems." Anaesthesia 48.2 (1993): 154-161.

Walcott-Sapp, Sarah. "A history of thoracic drainage: from ancient Greeks to wound sucking drummers to digital monitoring." (2018).

Roe, Benson B. Perioperative management in cardiothoracic surgery. Little, Brown Medical Division, 1981