• CXR—most easily seen on erect views where absent lung markings are seen lateral to a well-defined lung border. However, ventilated patients are often imaged in a supine position; pneumothorax may be easily missed as it may be lying anterior to normal lung giving the misleading appearance of lung markings on the radiograph. Supine pneumothorax should be considered if the following are seen:

• Hyperlucent lung field compared to the contralateral side

• Loss of clarity of the diaphragm outline

• 'Deep sulcus' sign, giving the appearance of an inverted diaphragm

• A particular clear part of the cardiac contour

• A lateral film may help. Tension pneumothorax results in marked mediastinal shift away from the affected side.

• Ultrasound — may be helpful but is highly operator dependent

• CT scan — very sensitive and may be useful in difficult situations, e.g. ARDS, and to direct drainage of localised pneumothorax

Pneumothorax must be distinguished from bullae, especially with long-standing emphysema; inadvertent drainage of a bulla may cause a bronchopleural fistula. Assistance should be sought from a radiologist.

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