Pulmonary interstitial emphysema (PIE) is an iatrogenic pulmonary condition of the premature infant with immature lungs. PIE occurs almost exclusively in association with mechanical ventilation. The ventilatory pressure used to keep the alveolar ducts open also may cause the alveolar duct to rupture (usually at the junction of the bronchiole and alveolar duct); this in turn leads to the escape of air into the pulmonary interstitium, lymphatics, and venous circulation. [1, 2, 3] The development of PIE is demonstrated in the images below.
PIE may be identified with a frontal chest radiograph. Disease progression is assessed with sequential studies.  In an ill infant, it may be difficult to differentiate PIE from lucent overdistension of the bronchioles, although overdistended distal airways tend to be round and of uniform diameter, whereas PIE tends to be ovoid in the direction of the bronchovascular bundles; in addition, the lesions of PIE tend to be of different sizes. It is also difficult to differentiate PIE from early bronchopulmonary dysplasia (BPD) with uneven patterns of aeration.
On radiography, PIE appears as linear, oval, and occasional spherical cystic air-containing spaces throughout the lung parenchyma. The interstitial changes are initially linear but may become more cystic as the air in the interstitium congregates locally. Subpleural cysts also develop and may rupture, producing a pneumothorax. The heart tends to get smaller as intrathoracic pressure increases; this results in diminished venous return into the chest. Overall lung volume is increased; however, the lungs are less compliant because they are splinted at a large volume by the air within the interstitium. Gas exchange is reduced by the increase in distance between the pulmonary vascular bed and the airspaces.
The images below demonstrate the radiographic characteristics of PIE.
Linear gas collections in the periphery of the lung, in association with an increased demand for respiratory support, are diagnostic of PIE. Increasing lung volumes also strongly support a diagnosis of PIE. Early BPD may present as focal areas of hyperaeration secondary to partial bronchial obstruction. The pattern closely resembles PIE, although the lucency is less linear in BPD. At histopathology, many lungs of infants with BPD demonstrate PIE that was not seen radiographically.