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  • presence of air in the pleural space
  • look for:
    • abnormal lucency (particularly sudden decreases in density - the air in the pleural space has a lower density than the lung) or
    • very sharp borders that outline lung edges in non-anatomical positions
    • note that pneumothoraces are best seen in the most superior part of the lung but that this is not the apices in a supine film
    • a tension pneumothorax develops when air progressively enters the pleural space but cannot escape, creating a high pressure in the pleural space. The resulting shift of the superior and the inferior vena cava  towards the opposite side causes decreased venous return and hypotension. Radiological signs:
      • shift of the heart and mediastinum to the opposite side
      • depression or inversion the diaphragm

Tension pneumothorax

Tension pneumothorax

Pleural effusions

The most common causes are congestive heart failure, parapneumonic effusions and haemothorax. It is not possible to diagnose the nature of the pleural flood based only on the plain film and ultrasound examination and paracentesis is often required.

In an erect or sitting x-ray the diaphragmatic contour and lateral costophrenic sulcus may be obscured. In a supine patient fluid is usually manifested by a generalized opacity of the lung on the affected side, or may be seen accumulating at the apex (pleural fluid cap).

Left pleural effusion


On the supine chest x-ray up to 400-500ml of pleural fluid can go undetected. On the erect chest x-ray usually only 50-100ml of pleural fluid is likely to go undetected. Remember that the fissures are in continuity with the pleural space. Therefore, fluid can fill and distend the fissures. This phenomenon is often refered to as pseudotumor because of it’s resemblance to a tumor.

©Charles Gomersall, January, 2018 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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