The pulmonary air leak syndromes (pneumomediastinum, pneumothorax, pulmonary interstitial emphysema [PIE], pneumatocele, pneumopericardium, pneumoperitoneum, and pneumoretroperitoneum) comprise a spectrum of diseases with the same underlying pathophysiology. Overdistention of alveolar sacs or terminal airways leads to disruption of airway integrity, resulting in dissection of air into extra-alveolar spaces. Very rarely air can enter pulmonary vasculature (pulmonary veins) and cause air embolus. Air can also leak into the subcutaneous layers of the skin, especially skin of the chest, neck, and face, causing subcutaneous emphysema.
The exact incidence of the air leak syndromes is difficult to determine. Pneumothorax is the most common of the air leak syndromes, reported to occur spontaneously in 1–2% of all neonates. The incidence increases in preterm infants to about 6%. The incidence also increases to 9–10% in infants with underlying lung disease (such as respiratory distress syndrome [RDS], meconium aspiration, pneumonia, and pulmonary hypoplasia) who are on ventilatory support, and in infants who had vigorous resuscitation at birth.
Overdistention of terminal air spaces or airways can result from uneven alveolar ventilation, air trapping, or injudicious use of alveolar-distending pressure in infants on ventilatory support. As lung volume exceeds physiologic limits, mechanical stresses occur in all planes of the alveolar or respiratory bronchial wall, with eventual tissue rupture. Air can track through the perivascular adventitia, causing PIE, or dissect along vascular sheaths toward the hilum, causing a pneumomediastinum. Rupture of the mediastinal pleura and into the thoracic cavity results in a pneumothorax. Pneumoretroperitoneum and pneumoperitoneum may occur when mediastinal air tracks downward to the extraperitoneal fascial planes of the abdominal wall, mesentery, and retroperitoneum and eventually ruptures into the peritoneal cavity.
Barotrauma. The common denominator of the air leak syndromes is barotrauma. Barotrauma results whenever positive pressure is applied to the lung. It cannot be avoided in the ill newborn infant needing ventilatory support, but its effects should be minimized. Peak inspiratory pressure (PIP), positive end-expiratory pressure (PEEP), inspiratory time (IT), respiratory rate, and the inspiratory waveform play important roles in the development of barotrauma. Contributing factors include high PIP, large tidal volume, and long IT. It is difficult to determine which of these parameters is the most damaging and which plays the largest role in the development of the air leaks.
Other causes of lung overdistention. Barotrauma is not the only cause of lung overdistention. Atelectatic alveoli in RDS may cause uneven ventilation and subject the more distensible areas of the lung to receive high pressures, placing them at risk for rupture. Small mucous plugs in the airway in meconium aspiration may cause gas trapping secondary to a ball-valve effect. Other events, such as inappropriate intubation of the right main stem bronchus, failure to wean after surfactant replacement therapy, and vigorous resuscitation or the development of high opening pressures with the onset of air breathing, ...