Risk factors. The primary risk factors for air leak are mechanical ventilation and lung disorders. Risk factors common in premature infants include respiratory distress syndrome (RDS), sepsis, and pneumonia. Surfactant therapy for RDS has markedly decreased the incidence of pneumothorax. Risk factors common in term infants are aspiration of meconium, blood, or amniotic fluid; pneumonia; and congenital malformations.

Pathogenesis. Air leak syndromes arise via a common mechanism. Transpulmonary pressures that exceed the tensile strength of the noncartilaginous terminal airways and alveolar saccules can damage the respiratory epithelium. Loss of epithelial integrity permits air to enter the interstitium, causing pulmonary interstitial emphysema. Persistent elevation in transpulmonary pressure facilitates the dissection of air toward the visceral pleura and/or the hilum via the peribronchial and perivascular spaces. In rare circumstances, air can enter the pulmonary veins and result in air embolism. Rupture of the pleural surface allows the adventitial air to decompress into the pleural space, causing pneumothorax. Following a path of least resistance, air can dissect from the hilum and into the mediastinum, resulting in pneumomediastinum, or into the pericardium, resulting in pneumopericardium. Air in the mediastinum can decompress into the pleural space, the fascial planes of the neck and skin (subcutaneous emphysema), or the retroperitoneum. In turn, retroperitoneal air can rupture into the peritoneum (pneumoperitoneum) or dissect into the scrotum or labial folds.

Pneumothorax. Spontaneous pneumothorax occurs in 0.07% of otherwise healthy appearing neonates. One in 10 of these infants is symptomatic. The high inspiratory pressures and uneven ventilation that occur in the initial stages of lung inflation may contribute to this phenomenon. Pneumothorax is more common in newborns treated with mechanical ventilation for underlying pulmonary disease.

Clinical signs of pneumothorax range from insidious changes in vital signs to the complete cardiovascular collapse that frequently accompanies a tension pneumothorax. As intrathoracic pressure rises, there is decreased lung volume, mediastinal shift, compression of the large intrathoracic veins, and increased pulmonary vascular resistance. The net effect is an increase in central venous pressure, a decrease in preload, and, ultimately, diminished cardiac output. A pneumothorax must be considered in mechanically ventilated infants who develop unexplained alterations in hemodynamics, pulmonary compliance, or oxygenation and ventilation.