Why tension pneumothorax causes shock
Tension pneumothorax is air trapped in the pleural space (the gap between lung and chest wall) until rising pressure compresses the veins and less blood returns to the heart.
Remember
Trapped air in the pleural space squeezes the veins that return blood to the heart, so the shock (a collapse in blood pressure and cardiac output) comes from falling venous return (the blood coming back to the heart), not from the collapsed lung alone.
A tension pneumothorax begins when the pleural membrane is breached and the tear works as a flap valve, a valve that opens in one direction only. Air is drawn into the pleural space as the chest expands on inspiration, but it cannot escape on expiration.
Because the air is trapped, the intrapleural pressure (the pressure in the pleural space) keeps rising. The lung on that side collapses, and the mediastinum (the central compartment of the chest, containing the heart and trachea) is pushed towards the opposite side.
The vena cavae (the large veins that return blood to the heart) are compressed, so venous return falls and the heart has less to pump. Cardiac output and blood pressure fall, and the pulse rises. Blood backs up in the neck, giving a raised jugular venous pressure (JVP, the height of the venous pulsation in the neck).
This is why tension pneumothorax causes shock rather than just breathlessness. Breathlessness comes from the collapsed lung, but the fall in blood pressure comes from the pressure effect on the veins and the heart.
On examination the affected side is hyperresonant (a hollow, drum-like percussion note over an air-filled space). Breath sounds are reduced or absent, because the lung has collapsed away from the chest wall, and the trachea can deviate away from the affected side. It is diagnosed on examination and treated immediately, without waiting for imaging, because it can cause cardiac arrest within minutes.
Key facts
- Valve
- air enters the pleural space on inspiration but cannot leave on expiration.
- Pressure
- intrapleural pressure rises progressively, collapsing the lung and pushing the mediastinum towards the opposite side.
- Veins
- compression of the vena cavae cuts venous return, so cardiac output and blood pressure fall.
- Neck
- blood backs up in the neck, which gives a raised jugular venous pressure.
- Examination
- the affected side is hyperresonant with reduced or absent breath sounds, and the trachea can deviate away from it.
- Action
- diagnose on examination and treat immediately, before any imaging.
Exam traps
- Pain from inflamed pleural membranes does not explain shock from air under pressure, because inflammation causes sharp pain and shallow breathing but does not raise intrapleural pressure or obstruct venous return.
- Blood in the pleural space can cause shock through loss of circulating volume, but a hyperresonant chest points to air rather than blood, because blood would dull the percussion note.
- A pleural effusion (fluid rather than air in the pleural space) is dull to percussion and develops far more slowly, so it does not give the sudden hyperresonant side of a tension pneumothorax.
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