Tension pneumothorax is the progressive build-up of air within the pleural space, usually due to a lung laceration which allows air to escape into the pleural space but not to return. Positive pressure ventilation may exacerbate this 'one-way-valve' effect.
Progressive build-up of pressure in the pleural space pushes the mediastinum to the opposite hemithorax, and obstructs venous return to the heart. This leads to circulatory instability and may result in traumatic arrest. The classic signs of a tension pneumothorax are deviation of the trachea away from the side with the tension, a hyper-expanded chest, an increased percussion note and a hyper-expanded chest that moves little with respiration. The central venous pressure is usually raised, but will be normal or low in hypovolaemic states.
However these classic signs are usually absent and more commonly the patient is tachycardic and tachypnoeic, and may be hypoxic. These signs are followed by circulatory collapse with hypotension and subsequent traumatic arrest with pulseless electrical activity (PEA). Breath sounds and percussion note may be very difficult to appreciate and misleading in the trauma room.
Tension pneumothorax may develop insidiously, especially in patients with positive pressure ventilation. This may happen immediately or some hours down the line. An unexplained tachycardia, hypotension and rise in airway pressure are strongly suggestive of a developing tension.
The X-ray on the right is a post-mortem film taken in a patient with severe blunt trauma to the chest and a left tension pneumothorax. It illustrates the classic features of a tension:
Deviation of the trachea away from the side of the tension.
Shift of the mediastinum
Depression of the hemi-diaphragm