Air appears around heart but does not extend above great vessels
Air in the pericardial sac is limited by the normal pericardial reflections. Pneumo-pericardium is often secondary to an infectious process with associated pericardial fluid and thickening, which will produce an air-fluid level on horizontal beam radiographs. Pneumo-pericardium may produce a similar finding continuous diaphragmatic sign
Air within the pericardial sac will rise to a nondependent position on decubitus positioning, unlike mediastinal air, which is not mobile.