Question

Pneumothorax is normally treated by insertion of a chest tube to draw air out of the pleural cavity. Explain why this would allow the collapsed lung to reinflate. Support your answer by drawing a diagram of the lungs, pleurae, and pleural cavity before and after inserting a chest tube. Label your diagram with the appropriate alveolar pressure and intrapleural pressure, assuming an atmospheric pressure of 760 mm Hg.

Answer 1

Before we understand how intercostal drainage tube/ chest tube (ICD) works in pneumothorax let us acquaint ourselves with some pressures relevant to this question.

1. Atmospheric pressure = 760 mmHg
2. Intraalveolar pressure - Pressure inside the alveoli
   • During inspiration, the intraalveolar pressure is less than the atmospheric pressure (<760mmHg). this promotes movement of air inside the lung
   • At the end of inspiration, the intra-alveolar pressure is the same as the atmospheric pressure (760 mmhg)
3. Intrapleural pressure - it is the pressure inside the pleural cavity
   • This pressure always negative and is -4 mmHg less than atmospheric pressure
   • This constant negative pressure is maintained by the inwards recoil of the alveoli and the outward movement of the chest wall.
   • As the alveoli have a tendency to collapse ( due to surface tension and elastic recoil), it is important to keep them from collapsing.
   • Because if they collapse, it will be difficult to open them ( its easier to blow air into a partially inflated balloon compared to a complete deflated balloon)
   • This negative intrapleural pressure is important to keep the alveoli open all the time.
   • The intra-pleural pressure becomes more negative during inspiration and less negative during expiration

Pneumothorax occurs when the alveoli rupture or the breach in the continuity of the chest wall, the intrapleural space is exposed to positive pressures ( either the exterior or the alveoli)

• This leads to equalization of pressures
• In other words, the relatively low pressure of the intrapleural space (compared to the atmosphere) promotes the movement of air inside the pleural space.
• As a result, the intrapleural pressure becomes positive, and the lung collapses.
• Now the pressure inside the intrapleural space is equal to the atmosphere.

When an ICD inserted into the chest, one end of the tube is inside the chest cavity (pleural cavity to be precise) and the other end is inside a pneumobag

• This external tip of the tube is always placed underwater.
• The reason behind this is, that it creates an underwater seal. This underwater seal is like a one-way valve
• It promotes air from moving out of the pleural cavity but prevents it from entering it.

So when an ICD inserted, the following changes take place

1. the defect in the chest wall is selectively block but the underwater seal
2. Selective because it allows only unidirectional flow.
3. When the patient inspires, the intrapleural pressure becomes negative relative to the atmospheric pressure.
4. If the tube was not inplace, it would have allows air to enter through the defect
5. But, with the tube ( and of course the underwater seal), the air is prevented from entering into the pleural cavity from the atmosphere
6. You will observe, a column of fluid moves upwards into the tube during inspiration.
7. During expiration, it intrapleural pressure becomes positive.
8. This pushes the air from the pleural cavity outside via the tube.
9. You will see bubbles coming out of the pneumobag.
10. This process continues till all the air inside the pleural cavity evacuates.
11. When the pneumothorax resolves, there will be no air bubbling but the column movement persists.'