Also known as mediastinal emphysema
Definition: presence of free air in the mediastinum
Can be Spontaneous or secondary (to violation of aerodigestive tract)
Spontaneous<unk>  usually occurs due to sudden increase in intra-alveolar pressure causing alveolar rupture → air dissects into pulmonary interstitium and then into mediastinum, neck, or pericardium
Life threatening causes include esophageal rupture or tension pneumothorax
No identified cause
Smoking or tobacco use
Recreational drug inhalation (cocaine, methamphetamine, marijuana)
Intrinsic Lung and Airway
Asthma / COPD (most common co-morbidity)
Bronchiectasis
Interstitial lung disease
Lung cancer
Foreign body in the airway
Mycoplasma pneumoniae pneumonia
Influenza A
Iatrogenic
Endoscopy, bronchoscopy, or colonoscopy
Intubation
Central venous access procedures
Thoracostomy / VATS
Chest or abdominal surgeries
Traumatic
Thoracic trauma (blunt or penetrating)
Blast injury
Environmental pulmonary barotrauma (e.g. scuba diving, flight)
Other
Excessive vomiting (i.e., Boerhaave syndrome, anorexia nervosa)
Esophageal rupture (rare)
Hydrocarbon inhalation
Bowel rupture or other cause of air in abdominal cavity (tracts up into the chest)
Childbirth
Asthma / COPD (most common co-morbidity)
Bronchiectasis
Interstitial lung disease
Lung cancer
Foreign body in the airway
Mycoplasma pneumoniae pneumonia
Influenza A
Endoscopy, bronchoscopy, or colonoscopy
Intubation
Central venous access procedures
Thoracostomy / VATS
Chest or abdominal surgeries
Thoracic trauma (blunt or penetrating)
Blast injury
Environmental pulmonary barotrauma (e.g. scuba diving, flight)
Excessive vomiting (i.e., Boerhaave syndrome, anorexia nervosa)
Esophageal rupture (rare)
Hydrocarbon inhalation
Bowel rupture or other cause of air in abdominal cavity (tracts up into the chest)
Childbirth
Pneumomediastinum
Acute coronary syndromes 
Hemothorax
STEMI
Chylothorax
Non-STEMI
Pneumopericardium
Unstable angina