A 45-year-old man incurs blunt chest trauma in a boating accident. On examination he has marked right chest wall pain. A chest radiograph shows a fractured 7th rib on the right side. Over the next 2 days he has subcutaneous soft tissue swelling with nonpainful crepitance on palpation of the right chest wall. Leakage of which of the following is most likely producing this swelling?

The trauma produced an air leak with interstitial emphysema. There could have been a pneumothorax as well, but in this case air escaped and dissected into soft tissues. A process that increases intraparenchymal lung pressure and ruptures the lung, such as positive pressure ventilation, can also lead to this complication. Interstitial emphysema looks worse than it feels, and within days the air is resorbed. When the leak occurs centrally, the term mediastinal emphysema can be used. The term pulmonary interstitial emphysema (PIE) describes this process in newborns, often with mechanical ventilation under positive pressure, and in the setting of respiratory distress syndrome. Rupture of the stomach is most likely to leak gastric acidic contents into the peritoneum to produce peritonitis, and be recognized by free air under the diaphragm on radiographs. Blood in the pleural space is called hemothorax; within soft tissues blood can form a hematoma. Leakage of lymph is rare, because the lymphatics have little or no pressure within them; blockage of the thoracic duct may produce a chylothorax. Pus in the pleural space is called empyema, and typically complicates an existing pneumonia.