Pulmonary Interstitial Emphysema

Pulmonary interstitial emphysema is air that has escaped from over distended terminal airspaces and dissects into the perivascular connective tissue and lymphatics of the lung. It is the earliest form of air leak in ventilated preterm infants and often precedes pneumothorax.

It occurs mainly in infants under 32 weeks of gestation with respiratory distress syndrome, usually within the first 7 days of positive pressure ventilation. Its frequency has fallen with antenatal corticosteroids, surfactant and gentle ventilation.

Risk factors are extreme prematurity, high peak inspiratory pressure or tidal volume, high mean airway pressure, long inspiratory time, surfactant deficiency, meconium aspiration, pulmonary infection and unsynchronised ventilation.

Localised disease affects one lobe or segment. Diffuse disease is bilateral. Early disease appears in the first 48 hours. A persistent localised form can form cystic lesions that last for weeks.

Pathophysiology.

Poorly compliant lungs aerate unevenly, so open alveoli beside collapsed units are over distended and rupture at the bronchovascular sheath.

Air tracks along vessels towards the hilum and pleura and can produce pneumothorax or pneumomediastinum.

Interstitial air compresses small airways and capillaries. This raises dead space, worsens ventilation perfusion mismatch, lowers compliance and impairs venous return, so higher pressures are demanded and further leak follows.

• Sudden rise in oxygen requirement or CO2 retention in a ventilated preterm infant, often after initial improvement.
• Falling compliance, rising ventilator pressures, bradycardia, hypotension and reduced chest movement. Focal signs are often absent.

Red flags.

• Abrupt collapse with hypotension suggests tension pneumothorax or pneumopericardium.
• Persistence beyond 7 to 10 days suggests cystic change and progression to chronic lung disease.

• Chest X-ray is diagnostic: non-confluent cystic or linear radiolucencies radiating from the hilum to the periphery, hyperexpansion, a flattened diaphragm and, when unilateral, mediastinal shift.
• Serial films over days show persistent cysts that do not change with respiration, which distinguishes them from air bronchograms.
• Arterial blood gas typically shows respiratory acidosis with hypoxaemia.
• Echocardiography when hypotension is unexplained.

Differential diagnosis. Pneumothorax, congenital pulmonary airway malformation, lobar emphysema, cystic change of bronchopulmonary dysplasia, pneumatocele after staphylococcal infection and air bronchograms of respiratory distress syndrome.

Ventilation.

• Use the lowest peak inspiratory pressure and positive end expiratory pressure that maintain gas exchange. Accept PaCO2 45 to 60 mmHg with pH over 7.25.
• Short inspiratory time of 0.2 to 0.3 seconds, synchronised or volume targeted ventilation, and early move to non-invasive support.
• Change to high frequency oscillation or jet ventilation if conventional ventilation needs high pressures, using the lowest mean airway pressure that maintains saturation.

Positioning and selective intubation.

• For localised unilateral disease, nurse the infant with the affected lung down to reduce its ventilation and promote air resorption.
• Selective intubation of the contralateral main bronchus for 24 to 48 hours is a specialist option for severe unilateral disease that fails other measures.

Other measures.

• Surfactant for respiratory distress syndrome. Morphine 10 to 20 micrograms/kg/h IV for sedation when ventilation is difficult.
• Antibiotics only when infection is suspected.
• Needle aspiration and chest drain for tension pneumothorax.
• Surgical lobectomy is a rare option for a persistent localised lesion that shifts the mediastinum and compromises ventilation.

Monitoring. Chest X-ray daily or on alternate days, blood gases every 4 to 6 hours at first, blood pressure and perfusion.

Referral and escalation. Refer to a centre with high frequency ventilation when pressures keep rising. Involve paediatric surgery for persistent localised lesions.