Respiratory Physiology, Respiratory Failure, and Mechanical Ventilation
Core physiology
Oxygen delivery is the product of cardiac output and arterial oxygen content: CaO2 = 1.34 x Hb x SaO2 + 0.003 x PaO2. This is why a severely anemic child can have a reassuring saturation yet inadequate delivery. Respiratory system compliance is volume divided by pressure; airway resistance determines the peak-to-plateau pressure gradient. A rising peak pressure with a stable plateau indicates increased resistance (secretions, bronchospasm, kinked tube). A rising plateau indicates reduced compliance (pneumothorax, atelectasis, pulmonary edema, abdominal distension).
Ventilator strategy
For pediatric ARDS use lung-protective settings: tidal volume 5-8 mL/kg predicted body weight (PBW), PEEP titrated to oxygenation and hemodynamics, and driving pressure (plateau minus PEEP) ideally below 15 cm H2O. Permissive hypercapnia is acceptable unless intracranial pressure is elevated. Prone positioning improves mortality in moderate-to-severe ARDS. Time constants explain why a child with severe bronchospasm needs a longer expiratory time: auto-PEEP and dynamic hyperinflation cause hypotension, which is treated by briefly disconnecting the circuit, not by giving a fluid bolus.
Traps
Normal saturation does not mean adequate oxygen delivery; a pale, tachycardic child with SpO2 of 100% can be in shock. Peak pressure is not a measure of compliance. Do not chase a normal PaCO2 during lung-protective ventilation. Extubation criteria include an air leak, adequate cough, minimal secretions, and hemodynamic stability, not a specific oxygen level.
Worked rule
PBW for a 150 cm boy: 45.5 + 0.91 x (150 - 152.4) = 43.3 kg. At 6 mL/kg the tidal volume is 260 mL, noticeably less than the 300 mL obtained by guessing 50 kg from actual weight, which may overdistend.