6.2 Pediatric Acute Respiratory Distress Syndrome (PARDS II Criteria, Oxygenation Index & Lung Protection)

Pediatric Acute Respiratory Distress Syndrome (PARDS)

Clinical Core: PARDS can follow a pulmonary or systemic insult. Use the child’s respiratory support and oxygenation measurements to recognize the syndrome and select lung-protective settings.

Pathophysiology of PARDS

Direct insults include pneumonia, aspiration and pulmonary contusion. Systemic insults include sepsis, severe trauma and transfusion-related acute lung injury. Injury to the alveolar epithelium and capillary endothelium can increase permeability and impair alveolar fluid clearance. Edema and surfactant dysfunction favor collapse, reducing aerated lung volume and compliance.

Blood passing through poorly ventilated or collapsed units contributes to hypoxemia. Impaired perfusion of ventilated units can increase dead space. The distribution and severity of these changes vary. Large breaths in a lung with reduced aerated volume can overdistend the remaining units. PALICC-2 pathobiology · Pediatric endothelial study · Pediatric surfactant study

PALICC-2 Diagnostic Framework and Severity Stratification

In 2023, the Pediatric Acute Lung Injury Consensus Conference updated the diagnostic criteria (PALICC-2) to improve early recognition and accommodate non-invasive diagnostic modalities.

PALICC-2 diagnosis

Use the pediatric framework in children under 18 without active perinatal lung disease. Onset is within 7 days of a known clinical insult. The failure is not fully explained by cardiac failure or fluid overload. New unilateral or bilateral pulmonary parenchymal opacities are required and should not be primarily explained by atelectasis or pleural effusion. Use the support-specific oxygenation thresholds and consider cardiac or chronic lung baseline where relevant.

Oxygenation Metrics: OI versus OSI

For invasive ventilation, OI and OSI account for the mean airway pressure needed to achieve the measured oxygenation. PALICC-2 uses P/F or S/F with qualifying full-face noninvasive ventilation.

$\text{Oxygenation Index (OI)} = \frac{\text{Mean Airway Pressure (MAP in }\text{cmH}_2\text{O)} \times FiO_2 \times 100}{PaO_2 \text{ (in mmHg)}}$

$\text{Oxygenation Saturation Index (OSI)} = \frac{\text{Mean Airway Pressure (MAP in }\text{cmH}_2\text{O)} \times FiO_2 \times 100}{SpO_2 \text{ (percentage)}}$

Applying the oxygenation criteria

Prefer arterial oxygen tension when available. Measure at steady state. If using a saturation-based index, ensure $SpO_2 \le 97\%$. PALICC-2 separates criteria for diagnosis from severity classification at least four hours after diagnosis.

Children on nasal support can meet separate criteria for possible PARDS. Apply the age, timing, imaging and clinical criteria before classifying. PALICC-2 severity categories do not apply to the specified special populations with cyanotic heart disease or baseline invasive ventilation for chronic lung disease.

Worked example: At a steady mean airway pressure of 16 cm H₂O, inspired oxygen 0.60 and arterial oxygen tension 60 mmHg, $OI=16 \times 0.60 \times 100/60=16$. If the child meets PARDS criteria and these values are measured at least 4 hours after diagnosis, this meets the invasive severe threshold. PALICC-2 definition and Tables 4–5

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Lung-Protective Mechanical Ventilation Principles

Mechanical ventilation supports gas exchange. Excessive stress or repetitive collapse can contribute to ventilator-induced lung injury:

• Volutrauma: Alveolar overdistension secondary to excessive tidal volume.
• Barotrauma: High transpulmonary pressures leading to gross air leaks (pneumothorax, pneumomediastinum).
• Atelectrauma: Shear injury generated by the cyclic, repetitive collapse and reopening of unstable alveoli during tidal ventilation.
• Biotrauma: Mechanical stress can promote local and systemic inflammatory signaling.

Tidal volume, pressure and PEEP

PALICC-2 suggests tidal volumes of 6–8 mL/kg, using the lesser of predicted or actual body weight. Use 4–6 mL/kg when needed to stay within pressure limits. Use caution below 4 mL/kg. In the absence of transpulmonary pressure measurements, aim for plateau pressure at or below 28 cm H₂O. A higher plateau pressure of 29–32 cm H₂O may be appropriate with reduced chest-wall compliance. The suggested driving pressure limit is 15 cm H₂O under static conditions. Calculate it as plateau pressure minus total PEEP measured during an expiratory hold when appropriate. Titrate PEEP to oxygenation, oxygen delivery, hemodynamics and static compliance, generally at or above the ARDS Network lower PEEP/higher oxygen table. Reassess the response to each change.

Ventilation and oxygenation goals

To maintain lung-protective volumes and pressures, permissive hypercapnia to a pH of 7.20 can be reasonable. Consider exceptions such as intracranial hypertension, severe pulmonary hypertension, selected congenital cardiac lesions, hemodynamic instability or important ventricular dysfunction. Avoid routine bicarbonate for respiratory acidosis. In mild/moderate PARDS, the suggested saturation range is 92–97%. In severe PARDS, a value below 92% may be accepted after optimizing PEEP to limit oxygen exposure. Avoid prolonged values below 88% or above 97% while on supplemental oxygen. When saturation is below 92%, monitor oxygen delivery and utilization, including central venous saturation when appropriate. PALICC-2 ventilation recommendations

Adjuncts for Persistent Gas-Exchange Failure

• Prone positioning: The pediatric evidence does not establish a standard duration or outcome benefit. Consider it for hypoxemia persisting despite other interventions. Secure the tube and lines, use a trained team and assess oxygenation and tolerance in the prone position. The pediatric randomized trial did not show a ventilator-free-day benefit. PALICC-2 recommendation 4.3 · Pediatric prone trial
• Neuromuscular blockade: Consider the minimum effective blockade with adequate sedation when protective ventilation cannot be achieved with sedation alone. Reassess benefit and monitor depth with the clinical response and the local protocol.
• Inhaled nitric oxide: Avoid routine use. Consider a monitored trial in selected children with documented pulmonary hypertension or severe right ventricular dysfunction, or as a bridge to extracorporeal support. Assess response early and serially. Monitor for methemoglobin and nitrogen dioxide with the delivery system. FDA nitric oxide prescribing information
• ECMO evaluation: Discuss promptly with an experienced team if a potentially reversible cause of severe PARDS prevents adequate gas exchange despite lung-protective strategies. Use the trajectory, hemodynamics, cause and risks in serial review. PALICC-2 does not specify a single OI or pH threshold for cannulation. PALICC-2 recommendations on adjuncts and ECMO