ABSTRACT
Pulmonary air leaks are the result of alveolar overdistention with resultant tissue rupture at the alveolar bases. The alveolar overdistention occurs in conditions associated with high transpulmonary pressure swings, air trapping and uneven alveolar ventilation. The alveolar connecting channels, the pores of Kohn, are reduced in immature infants,102 which exacerbates uneven ventilation. Once tissue rupture has occurred, gas tracks to the mediastinum along the sheaths of the pulmonary blood vessels, which are in apposition to the alveolar bases. The interstitial air may track to the mediastinum to form a pneumomediastinum or penetrate directly into the pleural cavity to cause a pneumothorax. A pneumothorax may also occur if a subpleural bleb ruptures.135 Air from the pneumomediastinum can move to extrathoracic areas, resulting in subcutaneous emphysema or rupture into the pleural cavity to form a pneumothorax.103 The type of pulmonary air leak produced is determined by the direction and ease with which the gas can then move. The movement of gas is impeded by the presence of connective tissue, which is more extensive in the immature infant than in the adult. High levels of interstitial water, also found in premature infants, particularly those with respiratory distress syndrome (RDS), impede gas flow along the perivascular spaces. In such patients, widespread interstitial emphysema is more likely to occur than a pneumothorax.146
