The heart partially or completely surrounded by air, with the pericardium sharply outlined by air density on either side.
Pneumopericardium can usually be distinguished from pneumomediastinum, since air in the pericardial sac should not rise above the anatomic limits of the pericardial reflexion on the proximal great vascular pedicle. Also on radiographs obtained with the patient in the decubitus position, air in the pericardial sac will shift immediately, while air in the mediastinum will not shift in a short interval between films.
Occasionally, it may not be possible to distinguish pneumopenicardium from pneumomediastinum on plain film. (source wikidoc)
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