What is the pathophysiology of pulmonary interstitial emphysema (PIE)?

Updated: Jun 25, 2019
  • Author: Abhay J Bhatt, MD, MBBS; Chief Editor: Muhammad Aslam, MD  more...
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Answer

Pulmonary interstitial emphysema (PIE) is initiated when air ruptures from the alveolar air space and small airways into the perivascular tissue of the lung. Air leaks result from high intra-alveolar pressure more frequently due to mechanically applied pressure (insufflation), retention of large volumes of gas, and uneven ventilation, leading to rupture of the small airways or alveoli. 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. The process often occurs in conjunction with respiratory distress syndrome (RDS).

Other predisposing etiologic factors include meconium aspiration syndrome (MAS), amniotic fluid aspiration, and infection (which can include chorioamnionitis [11]  or congenital pneumonia). The highest incidence of PIE in tiny infants has been observed when intrauterine pneumonia complicates the respiratory distress syndrome (RDS). It is also been reported with respiratory syncytial virus (RSV) infection or other bronchiolitis in term infants which can be mimicked by congenital cystic adenomatoid malformation. [3, 12]

Positive pressure ventilation (PPV) and reduced lung compliance are significant predisposing factors. Studies have shown that hyperventilation and overinflation of the lungs increase the loss of surface active phospholipids. [13] However, in extremely premature infants, PIE can occur at low mean airway pressure and probably reflects the underdeveloped lung’s increased sensitivity to stretch. PIE has been rarely reported in the absence of mechanical ventilation or continuous positive airway pressure (CPAP). [7, 8] Incorrect positioning of endotracheal tube may be responsible for localized PIE. [14]

Infants with RDS have an initial increase in interstitial and perivascular fluid that rapidly declines over the first few days of life. This fluid may obstruct the movement of gas from ruptured alveoli or airways to the mediastinum, thereby causing an increase of PIE. This occurs more commonly in preterm infants due to the structural immaturity of the lung, mainly owing to a lack of elastic tissue and the presence of large interstitium as a result of poor alveolation. The entrapment of air in the interstitium may initiate a vicious cycle in which compression atelectasis of the adjacent lung then necessitates a further increase in ventilatory pressure with still more escape of air into the interstitial tissues.

Plenat et al described two topographic varieties of air leak: intrapulmonary pneumatosis and intrapleural pneumatosis. [15] In the intrapulmonary type, which is more common in premature infants, the air remains trapped inside the lung and frequently appears on the surface of the lung, bulging under the pleura in the area of interlobular septa. This phenomenon develops with high frequency on the costal surface and the anterior and inferior edges, but it can involve all of the pulmonary areas. In the intrapleural variety, which is more common in more mature infants with compliant lungs, the abnormal air pockets are confined to the visceral pleura, often affecting the mediastinal pleura. The air of PIE can be located inside the pulmonary lymphatic network. [16]

The extent of PIE can vary. It can present as an isolated interstitial bubble, several slits, lesions involving the entire portion of one lung, or diffuse involvement of both lungs. PIE does not preferentially localize in any one of the five pulmonary lobes.

PIE compresses adjacent functional lung tissue and vascular structures and hinders both ventilation and pulmonary blood flow, thus impeding oxygenation, ventilation, and blood pressure. This further compromises the already critically ill infant and significantly increases morbidity and mortality. PIE can completely regress or decompress into adjacent spaces, causing pneumomediastinumpneumothorax, pneumopericardium, pneumoperitoneum, or subcutaneous emphysema. [17]


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