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Chest Imaging

Tension pneumothorax

closed pneumothorax caused by a check-valve mechanism in which the tissues surrounding the opening into the pleural cavity act as a valve allowing air to enter pleural space during inspiration, but preventing its escape during expiration. For a pneumothorax to increase in volume, the pressure within the pleural space must be relatively negative. The pressure of the pneumothorax exceeds atmospheric pressure during the later phase of respiration; thus the correct term is expiratory tension pneumothorax. Haemodynamic and morphological changes may occur and reflect impairment of venous return to the right heart. Because the respiratory and haemodynamic consequences of tension pneumothorax may not be accompanied by a radiographic equivalent in some circumstances, the diagnosis is clearly a clinical judgement rather than being radiographic.

Chest radiograph usually demonstrates compression of mediastinal structures (e.g. flattening of the heart border and superior and inferior vena cava), inversion of diaphragm, displacement of mediastinum or anterior junctional line and azygoesophageal recess. Although total or subtotal lung collapse occurs in tension pneumothorax, it may not be seen in patients with pleural adhesions or stiff lung parenchyma, as in adult respiratory distress syndrome (ARDS). Tension pneumothorax must be treated by a chest tube regardless of the size of pneumothorax seen on chest radiograph. The presence of the chest tube on the affected side does not exclude the possibility of tension pneumothorax, because pleural adhesions can cause loculations, which may be under tension, or alternatively the chest tube may not function.

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