A hydropneumothorax is a pneumothorax with an accompanying pleural effusion appearing as a horizontal edge (See figures A and B). Hydropneumothorax is a common chest radiograph finding. The diagnosis is usually made with upright PA and lateral chest films. This diagnosis can also be made with supine radiographs, ultrasound, or chest CT. A simple pneumothorax can be modeled by placing a balloon inside of glass and inflating the balloon to partially fill the glass (See figure C). The glass acts like the chest wall and the balloon represents the lung. The space between the two, the plural cavity, is filled with air simulating a pneumothorax. A hydropneumothorax can be modeled with the same glass and balloon by adding a volume of air and fluid to the space between the balloon and glass. The air fluid interface creates a horizontal edge which is the characteristic finding of a hydropneumothorax (See figure D). Lastly, a pleural effusion can be simulated by filling the space between the glass and the balloon entirely with fluid (See figure E). The fluid, which is denser than air, creates a meniscus surrounding the balloon.
Treatment of a hydropnemothorax, when indicated, is with tube thoracostomy. The underlying etiology of the hydropneumothorax should also be sought and treated as well.