Virtually all pulmonary air leak syndromes begin with some degree of PIE. When high airway pressures are applied to an infant’s lungs (e.g., during mechanical ventilation), the distal airways and alveoli often become overdistended—that is, they develop bleb or emphysema-like areas—and rupture (see Figure 35-1). In addition, gas trapping from an insufficient expiratory time can also cause alveolar overdistention and rupture. Once the gas escapes, it is forced into (1) the loose connective tissue sheaths that surround the airways and pulmonary capillaries, and (2) the interlobular septa containing pulmonary veins. In severe cases, the gas continues to spread peripherally by dissecting along the peribronchial and perivascular spaces to the hilum of the lung, producing the classic radiographic appearance of PIE that shows bubbles of air in the interstitial cuffs (Figure 35-2 and Figure 35-3).

The overdistention associated with PIE forces the lungs into a full inflation position and thereby decreases lung compliance (remember that static lung compliance is reduced at both very low and very high lung volumes). Moreover, air trapped within the interstitial cuffs compresses the airways and increases airway resistance. In addition, the trapped air in the interstitial spaces impairs lymphatic function, resulting in fluid accumulation in the interstitial cuffs and alveoli. Once the interstitial gas reaches the hilum of the lung, it either (1) coalesces to form large hilar blebs, or (2) tracks beneath the visceral pleura to form large subpleural pockets of air. In either case, the accumulation of gas can be large enough to significantly compress the lung or mediastinal structures.

A pneumomediastinum may occur when the excessive air associated with a PIE continues to track—and accumulate—through the perivascular and peribronchial cuffs and causes the gas in the hilum area to rupture into the mediastinum. In addition, the high gas pressures associated with a pneumomediastinum may also dissect into the pleural space and the fascial planes of the neck and skin, resulting in the condition known as subcutaneous emphysema.

A pneumothorax may occur because of the alveolar overdistention and subsequent rupture commonly associated with a PIE (Figure 35-4). A pneumopericardium can develop from direct tracking of interstitial air along the great vessels into the pericardial sac (Figure 35-5). Gas pressure in the pericardium restricts atrial and ventricular filling, resulting in a decreased stroke volume and, ultimately, a reduced cardiac output and systemic hypotension. During the late stages, inflammatory changes of the airways lead to increased capillary leakage and excessive bronchial secretions.

A pneumoperitoneum may develop from the tracking of gas along the sheaths of the aorta and vena cava and eventually may burst into the peritoneal cavity. Clinically, the infant with a pneumoperitoneum manifests a sudden onset of abdominal distention. The pneumoperitoneum may be large enough to block the descent of the diaphragm and may require drainage. Finally, the excessive gas accumulation associated with a pneumoperitoneum may end up in the scrotum in male babies or the labia in females.

In very rare cases, an intravascular air embolism may be seen. It is hypothesized that the air is actually pumped under high pressure through the pulmonary lymphatics into the systemic venous circulation. Intravascular air causes an abrupt cardiovascular collapse and is frequently diagnosed when air is observed in vessels on chest radiographs taken to establish the cause of cardiovascular collapse.

The major pathologic changes associated with pulmonary air leak syndromes are as follows:

Etiology and Epidemiology

Preterm infants who weigh less than 1000 g at birth have an increased risk for the early occurrence of pulmonary air leak syndromes (often within the first 24 to 48 hours of life), especially because of the weak noncartilaginous structures of their distal airways. The most frequent causative factor resulting in pulmonary air leak syndromes in preterm infants is the use of mechanical ventilation. Pulmonary air leak syndromes commonly result from the use of high levels of positive end-expiratory pressure (PEEP), high peak inspiratory pressures (PIPs), and prolonged inspiratory times (ITs). Occasionally, full-term babies will develop a spontaneous tension pneumothorax.