Mechanical ventilation principles in children — formative SAQs

SAQ 1 — Severe paediatric acute respiratory distress syndrome and lung-protective settings (20 marks, ~15 minutes)

An 8-year-old is intubated for severe pneumonia and is found to have paediatric acute respiratory distress syndrome. The team asks you to set the ventilator and justify your lung-protective strategy. ^[2]

Questions

1. State the lung-protective tidal volume target and explain why it is set against predicted body weight. (5 marks) ^[2]
2. What is the inspiratory plateau pressure ceiling, and why is the plateau pressure preferred over the peak pressure for monitoring it? (5 marks) ^{[2] [3]}
3. Explain the role of positive end-expiratory pressure and permissive hypercapnia in the strategy. (5 marks) ^{[1] [2]}
4. What trial evidence supports a structured lung-protective protocol in paediatric acute respiratory distress syndrome? (5 marks) ^[5]

Model answer (must-hit)

1. The target tidal volume is about 6 to 8 millilitres per kilogram of predicted body weight, reduced toward 4 to 6 in severe disease. It is set against predicted rather than actual body weight because the injured lung is not uniformly diseased: a small aerated baby lung is all that remains available to receive the breath, so a volume scaled to actual weight would overdistend it. Predicted body weight better reflects lung size and so limits volutrauma. ^{[2] [1]}
2. The inspiratory plateau pressure is held at or below 28 centimetres of water. The plateau pressure is measured during an inspiratory pause with no flow, so it reflects compliance of the lung and chest wall and caps the transpulmonary stress that drives injury, whereas the peak pressure also reflects airway resistance and rises with a kinked tube or secretions even when the lung is unchanged. The plateau pressure is therefore the number that governs lung protection. ^{[2] [3]}
3. Positive end-expiratory pressure holds alveoli open at end-expiration, raising functional residual capacity, improving oxygenation and preventing the cyclic collapse and reopening that causes atelectrauma. Permissive hypercapnia accepts a mildly elevated carbon dioxide, so long as the pH is preserved above about 7.20, in order to allow the smaller tidal volumes and lower pressures that protect the lung, rather than chasing a normal carbon dioxide with injurious settings. ^{[1] [2]}
4. Wong and colleagues conducted a nonrandomized controlled trial of a structured lung-protective protocol built on these targets and found it was associated with improved survival in paediatric acute respiratory distress syndrome, which is why the strategy is now the default rather than a rescue measure. ^[5]