Hydropneumothorax. Coloured chest X-ray showing the left lung (blue, upper right) of a patient affected by hydropneumothorax. The lung has decreased in volume, resulting in a reduction in lung efficiency. The lung at left is unaffected. Hydropneumothorax is a condition which arises when fluid and air are trapped in the pleural cavity between the lung and the chest wall. There is a risk that the lung will collapse, which causes shortness of breath and pain on breathing. Hydropneumothorax is treated by draining the fluid and air from the cavity.