Hydropneumothorax is defined as the presence of both air and fluid within the pleural space. An upright chest x-ray will show air fluid levels. The horizontal fluid level is usually well defined and extends across the whole length of hemithorax.
This can be remembered by 4 'S':
straight line dullness,
Iatrogenic: Introduction of air during pleural fluid aspiration in effusion
Presence of a gas-forming organism
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