Extraluminal ectopic gas in subcutaneous soft tissues and muscles, lung interstitium, retroperitoneum, intraperitoneal spaces, and bowel wall
• Radiographic findings
Pneumothorax
– Radiolucent gas between visceral and parietal pleura
– Inferiorly displaced costophrenic angle on supine films (deep sulcus sign)
Pneumomediastinum
– Radiolucent streaks outlining heart and trachea
Pneumoperitoneum
– Best seen on upright and left decubitus films
– Supine films: Air outlining bowel or falciform ligament
Subcutaneous emphysema
– Radiolucent streaks outlining fat and muscles
TOP DIFFERENTIAL DIAGNOSES
• Perforated duodenal or gastric ulcer
• Iatrogenic introduction of ectopic gas
• Diverticulitis
• Other causes of pneumothorax, pneumomediastinum, pneumoperitoneum, or pneumatosis
• Ischemic enteritis
PATHOLOGY
• Positive pressure ventilation → alveolar rupture → air leakage into pulmonary interstitium
• Interstitial air can dissect along perivascular sheaths into mediastinum
• Mediastinal and pleural air can leak into peritoneal and retroperitoneal cavities
• Primary risk factors include interstitial lung disease, asthma, acute respiratory distress syndrome (ARDS), and mechanical ventilation with high tidal volumes
(Left) Axial CECT in a young man on a ventilator following a motor vehicle crash shows a tension pneumothorax on the right side and a smaller pneumothorax on the left. Gas dissects under pressure along the peridiaphragmatic fat .
(Right) Axial CECT in the same patient shows the extraluminal air from the thorax dissecting into the peritoneal cavity to outline bowel loops. There was no intraabdominal injury.
(Left) Axial CECT in an elderly man on positive pressure ventilation and with known large bilateral pneumothoraces and gas in the mediastinum shows the gas dissecting under pressure into the abdomen, including the retroperitoneum and mesentery .
(Right) Axial CECT in the same patient shows that in addition to the extensive retroperitoneal gas, intraperitoneal gas is also present . In some cases, gas can dissect into the bowel wall, simulating pneumatosis from bowel ischemia.
TERMINOLOGY
Synonyms
• Pulmonary barotrauma
Definitions
• Alveolar rupture caused by elevated transalveolar pressure during mechanical ventilation
IMAGING
General Features
• Best diagnostic clue
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Extraluminal ectopic gas in subcutaneous soft tissues and muscles, lung interstitium, retroperitoneum, intraperitoneal spaces, and bowel wall
on the right side and a smaller pneumothorax on the left. Gas dissects under pressure along the peridiaphragmatic fat 
.
to outline bowel loops. There was no intraabdominal injury.
and mesentery 
.
. In some cases, gas can dissect into the bowel wall, simulating pneumatosis from bowel ischemia.
