Underwater Seal Chest Drain System
This is another favourite of the college.
Specifically, 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 came up previously in Question 26.1 from the first paper of 2009. In short, the ICU trainee should be prepared to enter into a detailed discussion of chest drain systems.
Briefly, the features and cardinal distinctions of three-bottle vs single-bottle systems are as follows:
- The single-bottle system is just the underwater seal bottle.
- The underwater seal provides counterpressure to pleural pressure (about 3cm)
- Thus, a positive pleural pressure of more than 3cm H2O will force air and fluid out into the single bottle.
- 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.
- Thus, the single bottle becomes a less efficient pleaural drainage device, the longer it remains in use (and the more it fills with pleural waste).
- As long as this bottle remains well below the patient, no fluid will get sucked up into the chest.
The three-chamber drain, in contrast:
- In contrast, in a three-bottle system the depth of the vent tube determines the negative pressure, and the amount of fluid collecting in the collection bottle does not determine the 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.
- The volume of fluid collecting in the first chamber thus has no influence on the suction, and there is no loss of efficiency with ongoing drainage.
- There is also less likelihood of fluid refluxing back up the tubing into the chest cavity with a sudden decrease in intrathoracic pressure (and so, less risk of infection).
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.