blunt or penetrating trauma, esophageal perforation, iatrogenic, pulmonary infections, gas forming organisms, cocaine inhalation, air extension from pneumothorax
air per se is of no clinical significant in mediastinum, maybe mild substernal pain
imaging
ring around artery sign: air around usually right pulmonary artery
sail sign: elevation of thymus
air anterior to pericardium on lateral xray
continuous diaphragm sign: air trapped posterior to pericardium
tubular artery sign: air adjacent to major branches of aorta
double bronchial wall sign: air around bronchus depicts walls clearly
differentials
medial pneumothorax: air bounded on lateral side by thin margin of pleura
Mach effect: seen on convex surfaces. abrupt change in density between heart and lung. This lucency around convex borders. No opaque line seen as in pneumomediastinum