Type 1 · lung failure
PaO₂ ↓ · PaCO₂ normal or low
V/Q mismatch, shunt, diffusion defect.
Bronchiolitis, pneumonia, PARDS, oedema.
True shunt needs PEEP, not more FiO₂.
Type 2 · pump failure
PaCO₂ > 50 with pH < 7.30
Alveolar hypoventilation.
CNS depression, neuromuscular, exhaustion
at the end of severe obstruction.
Type 3 · perioperative
FRC falls
Anaesthesia, positioning, opiates, splinting.
Basilar collapse, then V/Q mismatch.
Type 4 · shock
The diaphragm competes for cardiac output
Respiratory muscles take up to 40% of it.
Ventilation offloads them, for the organs.
Distress is compensated; failure is not. A "normal" PaCO₂ in an exhausted tachypnoeic child is a rising one.