What strategy can be used to prevent mechanical ventilator-induced lung injury in a pediatric patient in the Pediatric Intensive Care Unit (PICU) on mechanical ventilation (MV)?

Permissive Hypercapnia to Prevent Ventilator-Induced Lung Injury

The correct answer is D: Increase PCO2 to maintain permissive hypercapnea. This lung-protective strategy accepts higher CO2 levels (targeting pH >7.20) while avoiding the injurious effects of excessive tidal volumes and pressures that cause ventilator-induced lung injury in mechanically ventilated pediatric patients 1.

Why Permissive Hypercapnia is the Correct Strategy

The Paediatric Mechanical Ventilation Consensus Conference explicitly recommends accepting higher PCO2 levels in acute pulmonary patients, targeting pH >7.20 rather than normal CO2 levels (35-45 mmHg), unless specific diseases like pulmonary hypertension dictate otherwise 1. This approach allows clinicians to use lower tidal volumes (≤10 mL/kg ideal body weight) and lower peak pressures (≤28-30 cmH2O depending on disease type), which are the cornerstone of preventing ventilator-induced lung injury 1, 2.

• The fundamental principle is that avoiding volutrauma and barotrauma takes priority over normalizing CO2 3, 4
• Permissive hypercapnia enables lung-protective ventilation by allowing reduced minute ventilation without forcing clinicians to use injurious ventilator settings 5
• The target pH >7.20 provides a clear safety threshold while accepting elevated PCO2 1

Why the Other Options Are Incorrect and Dangerous

Option A: Zero PEEP - Actively Harmful

Zero PEEP is contraindicated and will cause atelectrauma through repetitive alveolar collapse and reopening 3, 4. The guidelines explicitly recommend:

• PEEP of 5-8 cmH2O as baseline, with higher PEEP dictated by disease severity 1
• PEEP prevents alveolar collapse and is essential for lung protection 1
• In severe disease (PARDS with PEEP ≥10 cmH2O), even higher PEEP is necessary 1, 2
• Zero PEEP would eliminate one of the primary protective mechanisms against ventilator-induced lung injury 5

Option B: Increase Tidal Volume - Causes Volutrauma

Increasing tidal volume directly causes volutrauma, the primary mechanism of ventilator-induced lung injury 3, 4. Guidelines mandate the opposite:

• Keep tidal volume ≤10 mL/kg ideal body weight, potentially lower (6-8 mL/kg) in severe disease 1, 2
• Large tidal volumes overdistend alveoli and trigger inflammatory cascades (biotrauma) 3
• Clinical studies show that clinicians often fail to reduce tidal volumes even when pH is elevated, representing missed opportunities for lung protection 6

Option C: Hyperventilation - Compounds Injury

Hyperventilation to "wash out" CO2 requires either increased tidal volumes or increased respiratory rates, both of which worsen lung injury 5, 4:

• Increased tidal volumes cause volutrauma 3
• Increased respiratory rates reduce expiratory time, potentially causing air-trapping and auto-PEEP, especially in obstructive disease 1
• This strategy contradicts the fundamental principle of accepting higher CO2 to protect the lungs 1

Practical Implementation of Permissive Hypercapnia

Monitor arterial or capillary blood gas PCO2 and maintain pH >7.20 as your primary ventilation target 1:

• Measure PCO2 in arterial or capillary blood samples regularly 1, 2
• Consider transcutaneous CO2 monitoring for continuous assessment 1
• Measure end-tidal CO2 in all ventilated children as a trending tool 1, 2

Set ventilator parameters to minimize lung injury while accepting elevated CO2 1, 2:

• Tidal volume ≤10 mL/kg ideal body weight 1, 7, 2
• Peak inspiratory pressure ≤28-30 cmH2O (depending on restrictive vs. obstructive disease) 1, 2
• PEEP 5-8 cmH2O baseline, higher in severe disease 1, 7
• Plateau pressure monitoring to ensure safe pressures 1

Critical Exception: Pulmonary Hypertension

The one absolute contraindication to permissive hypercapnia is pulmonary hypertension, where normal pH must be maintained 1:

• Hypercapnia and acidosis cause pulmonary vasoconstriction, worsening pulmonary hypertension 1
• In acute pulmonary hypertension crisis, increasing FiO2 up to 1.0 may be required 1
• These patients require careful balancing of ventilation strategies 2

Common Pitfall to Avoid

The most common error is reflexively "correcting" elevated CO2 by increasing minute ventilation, which forces use of injurious ventilator settings 6. Studies demonstrate that clinicians frequently fail to decrease peak pressures or rates even when pH is >7.45, missing opportunities for lung protection 6. Accept the elevated CO2 as long as pH remains >7.20 1.