Pulmonary Interstitial Emphysema

High-resolution computed tomography (HRCT) images [Figure 1] of a 10-year-old boy presented with multilobar pneumonia and respiratory failure. He was treated with broad-spectrum antibiotics and oxygen through a high-flow nasal cannula at a flow rate of 40 L/min. The next day he complained of central chest pain and increased breathlessness. HRCT image shows pneumomediastinum (black arrow) and air in the pulmonary parenchyma (interstitium) (black arrows). Air tracking into the interstitium is very rare, and this is termed as pulmonary interstitial emphysema (PIE).Figure 1:: High-resolution computed tomogram of a boy suffering from multilobar pneumonia requiring oxygen supplementation through a humidified high-flow nasal cannula at a flow rate of 40 L/min. The child developed mediastinal emphysema (black arrows) and interstitial emphysema (red arrows)PIE is an unusual condition wherein air dissects through alveolar walls into the adjacent interstitial tissues.[1] This can be isolated or part of thoracic air leak syndrome (ALS). The term ALS is used to describe symptoms of respiratory distress occurring in conjunction with an air leak. Air leak is a clinical phenomenon associated with the leakage or escape of air from an air-containing space like the trachea, bronchi, or esophagus into spaces that usually do not have air. PIE occurs more commonly in neonates but can be seen in adults as well.[2] Neonatal etiologies include respiratory distress syndrome, prematurity, meconium aspiration syndrome, positive pressure ventilation or mechanical ventilation with high peak pressures, pulmonary infection (pneumonia and sepsis), amniotic fluid aspiration, and incorrect endotracheal tube placement.[2] Causes in adults include barotrauma, blunt thoracic trauma, obstructive lung diseases, infectious diseases, and idiopathic pulmonary fibrosis.[2-4] Recently, the use of a heated humidified high-flow nasal cannula (HHFNC) has been reported to cause air leaks due to the high flow of oxygen which distends small airways and alveoli.[5,6] The pathogenesis of PIE is usually attributed to the rupture of airspaces at the lobular periphery or the rupture of overdistended small bronchioles. This occurs mainly during positive pressure ventilation.[1,3] Imaging shows the lung parenchyma filled with a mix of spherical cystic, linear, and oval air-containing lucencies [Figure 1]. Early changes appear to be linear but slowly progress to more cystic formation in the interstitium. Linear radiolucency is about 3–8 mm long and <2 mm in width. Cystic-like radiolucency measures 1–4 mm in diameter. Imaging also shows the presence of air leak into the mediastinum (pneumomediastinum). Air can dissect into the pleural space (pneumothorax) or subcutaneous space (subcutaneous emphysema). During inspiration, the lung volumes may increase, but premature lungs have decreased lung compliance resulting in hyperdistended lungs on imaging.[2] Heated HHFNC may induce air leaks due to the high flow of oxygen which distends small airways and alveoli. In a case study, an otherwise healthy 16-year-old boy with cerebral palsy developed pneumomediastinum and died of its complications. He was receiving 20 l/m HHFNC therapy when he developed pneumomediastinum.[5] Subcutaneous emphysema, pneumothorax, and pneumomediastinum should be considered a serious complication during HHFNC therapy.[6] In severe infections such as multilobar pneumonia or sepsis, lung tissues are easily friable and even a small increment in airway pressure may cause air leakage. This is due to intense inflammation and vasculitis. PIE is usually treated conservatively with close monitoring and optimal mechanical ventilator setting.[1,3] Optimal use of antibiotics and other supportive measures including oxygen through HFNC at a lower flow rate resulted in complete recovery in the index case. CONCLUSION This rare complication due to air tracking through the interstitium as a part of ALS can occur following oxygen delivery at a high flow rate through HFNC. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.