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PH-RS05

Pneumothorax

How a slight negative pressure in the pleural space holds the lung against the chest wall, why air entering that space lets the lung collapse, and how a one-way leak can build pressure that stops blood returning to the heart.

By the end, you can

  • Describe the pleura and the negative intrapleural pressure that keeps the lung expanded.
  • Distinguish primary spontaneous, secondary, traumatic and iatrogenic pneumothorax.
  • Explain why air in the pleural space collapses the lung and how this affects gas exchange.
  • Explain how a tension pneumothorax causes obstructive shock.
  • Relate the symptoms, signs and X-ray appearance to the mechanism.
  • Recognise the classic appearances, types and tests of pneumothorax, and explain how decompression, drainage and pleurodesis act on its mechanism.

What it is

A pneumothorax is air in the pleural space, the thin space between the lung and the chest wall, causing part or all of the lung to collapse. A tension pneumothorax is one in which air keeps entering but cannot leave, so pressure rises until it compresses the heart and great veins: an emergency.

3 types to tell apart
Primary spontaneous
In people without known lung disease, usually from rupture of small air blisters (blebs) at the lung apex; commonest in tall, young men and in smokers.
Secondary spontaneous
Complicating lung disease such as COPD, cystic fibrosis or pneumonia; less lung reserve makes it more dangerous.
Traumatic and iatrogenic
From rib fractures, penetrating injury, or procedures such as central line insertion and lung biopsy.

US spelling: millilitres → milliliters.

Normal function

The pleura hold the lung open

Each lung is wrapped in visceral pleura, which folds back at the hilum to line the chest wall and diaphragm as parietal . Between the two layers is a thin film of fluid, a few millilitres, that lets them slide while holding them together.

The lung’s elastic tissue constantly pulls it inwards, while the chest wall tends to spring outwards. These opposing forces make the pressure in the pleural space slightly below atmospheric (about −5 cmH₂O at rest), and that suction keeps the lung inflated against the chest wall, including at its .

When the chest expands on breathing in, the pleural pressure becomes more negative and the lungs expand with it, drawing air in through the .

Point to it on the model
Opposing pulls keep the lung expandedSchematic
NormalLungElastic tissue pulls inwards
  • Recoil inwards (from Pleural space to Lung)
Pleural spaceSlightly below atmospheric: about −5 cmH₂O
  • Spring outwards (from Pleural space to Chest wall)
NormalChest wallSprings outwards

Schematic. Air in the pleural space removes the suction that couples the two.