Paediatric Acute Respiratory Distress Syndrome

Paediatric acute respiratory distress syndrome is acute hypoxaemic respiratory failure with diffuse lung inflammation, onset within 7 days of a known insult, and new infiltrates on chest imaging that cannot be fully explained by heart failure or fluid overload.

Severity is graded by the oxygenation index, calculated as the mean airway pressure times FiO2 times 100, divided by PaO2 in mmHg: mild 4 to under 8, moderate 8 to under 16, and severe 16 or over.

When oxygenation is only assessed by pulse oximetry, the oxygen saturation index is used. In children with non-invasive support, a PaO2 to FiO2 ratio of 300 or under with full face mask CPAP or bilevel ventilation at 5 cmH2O or more defines at risk or mild disease.

It accounts for about 1 to 3% of paediatric intensive care admissions. Mortality is about 10 to 20% overall and higher in severe disease and in immunocompromised children.

Causes are pneumonia (the commonest, often viral), sepsis, aspiration, near drowning, trauma, pancreatitis, transfusion related lung injury and inhalation injury.

Pathophysiology.

Injury to alveolar epithelium and endothelium causes increased permeability, protein rich oedema, surfactant dysfunction and neutrophil driven inflammation. Alveoli collapse and fill with fluid, producing shunt and reduced compliance.

Ventilation can worsen injury through overdistension and cyclic collapse (ventilator induced lung injury).

• Rapid onset of tachypnoea, increased work of breathing, hypoxaemia that responds poorly to oxygen, and bilateral crackles.
• Features of the underlying cause: fever, sepsis, aspiration or trauma.
• Later: cyanosis, fatigue, hypercapnia and haemodynamic compromise.

Red flags.

• Oxygen requirement increasing despite high flow oxygen or non-invasive support.
• Oxygenation index of 16 or over, rising CO2, or shock.
• Immunocompromise, malignancy or marrow transplant.

• Clinical criteria with chest X-ray or CT showing bilateral infiltrates, and arterial blood gas to calculate the oxygenation index.
• Exclude cardiac failure and fluid overload with echocardiography.
• Identify the cause: cultures, respiratory viral panel, tracheal aspirate or bronchoalveolar lavage, blood tests, and imaging.

Differential diagnosis. Cardiogenic pulmonary oedema, congenital heart disease, severe bronchiolitis, diffuse alveolar haemorrhage, interstitial lung disease, pulmonary embolism and fluid overload.

Treat the cause. Antimicrobials for infection, source control for sepsis, and drainage of effusions or pneumothorax.

Lung protective ventilation.

• Tidal volume 5 to 8 mL/kg of predicted body weight (nearer 3 to 6 mL/kg with poor compliance), with plateau pressure of 28 cmH2O or less (up to 29 to 32 cmH2O with reduced chest wall compliance).
• Titrate PEEP to oxygenation and haemodynamics, typically 8 to 15 cmH2O in moderate to severe disease.
• Permissive hypercapnia with pH 7.15 to 7.30 if there is no contraindication (raised intracranial pressure, pulmonary hypertension).
• Oxygen saturation target 92 to 97% with mild disease and 88 to 92% with severe disease and high PEEP.

Adjuncts.

• Sedation and analgesia titrated to comfort, with neuromuscular blockade for severe disease and patient ventilator dyssynchrony.
• Prone positioning for 12 to 16 hours daily in severe disease.
• Conservative fluid management once shock is resolved, aiming for euvolaemia.
• Early enteral nutrition, stress ulcer prophylaxis and thromboprophylaxis in adolescents.
• Inhaled nitric oxide 5 to 20 ppm as rescue for refractory hypoxaemia or pulmonary hypertension, stopping if no response. Surfactant and routine corticosteroids are not advised.
• Extracorporeal membrane oxygenation for severe refractory hypoxaemia or hypercapnia when the cause is potentially reversible.

Monitoring. Oxygenation index, compliance, plateau pressure, haemodynamics, fluid balance and daily screening for extubation readiness.

Referral and escalation. Early discussion with a paediatric intensive care and extracorporeal support centre when oxygenation index is rising above 15.