The underwater seal drain is a favourite of the college, likely because it is a widely misunderstood device which is in routine use. In Question 24.2 from the second paper of 2012 they asked the trainees to label the diagram of one, and then to explain how it differs from a single-chamber system. This topic also came up previously in Question 26.1 from the first paper of 2009. Again, in Question 24 from the second paper of 2018, the trainees were asked to draw all three bottle-based chest drain system setups, and to argue about their advantages and disadvantages. In short, the ICU trainee should be prepared to enter into a detailed discussion of chest drain systems.
From the perspective of published peer-reviewed resources, you can't do much better than Kam et al (1993), which has clear diagrams and all the details required to answer Question 24 from the second paper of 2018.
Single chamber underwater seal chest drain
The single bottle system is exactly that. It is a single bottle, open to air. The patient's chest tube is submerged under a level of water (usually about 2cm) which acts as a one-way valve. When the patient's pleural pressure exceeds the level of water (i.e. it is greater than 2cm H2O), the air in the tube will bubble out and escape into the atmosphere. When the patient takes a breath in, the negative intrapleural pressure will suck drain water up the tube, but no additional air can enter.
What are the advantages of the single bottle pleural drain?
- 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.
This system obviously has a few problems:
- 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.
In order to address some of the shortcomings of the single-bottle system, one may be tempted to add another bottle.
The two-chamber underwater seal pleural drain
This system separates the fluid collection chamber from the water seal chamber. That way, there is still an underwater seal to prevent the re-entrainment of air, and pleural fluid can collect in the first chamber without affecting the depth of the underwater seal.
So, the advantages of this thing are:
- 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.
There are of course disadvantages:
- 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 latter point brings us to the next system, the classical three-chamber underwater seal drain.
The three-chamber underwater seal drain
This system is much like the two-bottle system, but with an added chamber to help regulate the suction pressure, i.e. it is specifically designed to be used with suction. It was apparently developed at the Massachusetts General Hospital in 1945.
Advantages of the three-chamber 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.
Everything has disadvantages:
- 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.
To address the latter point, and ignoring the previous point, one may add a fourth bottle for safety.
The four-chamber pleural drain system
The major change in this system is the addition of another underwater seal, this time disconnected from the wall suction and vented to the atmosphere. The objective is to protect the patient from pneumothorax in the event of sudden suction failure.
Under normal circumstances, while the suction is working, the underwater seal prevents the air from the fourth chamber from entering the rest of the system, and so it only ever vents when the air pressure in the other two chambers exceeds the water seal pressure (i.e. about 2 cm H2O).
The local system
In case there is any interest, here is a diagram of our dearly beloved Atrium system, with labels. This thing might turn up in a viva station. The examiners would typically have a whole series of these things, all set up; and the candidate would be invited to pick one which they are familiar with.
Viva questions about this thing might include the following:
"Discuss the function of this device"
- One would need to mention that it is a dry suction underwater seal drain
"What are the safety features of this device?"
- Underwater seal (of course)
- Transparent collection chamber
- Positive pressure release valve
- Manual high negative pressure vent
- Suction control
- Air leak monitor
- Retractable stand and bedside hanging arms to prevent accidental spills.
"What is the purpose of the underwater seal?"
- The underwater seal prevents the inward movement of air. It excludes air from re-entering the chest cavity via the chest tube.