Factors increasing the susceptibility of infants and young children to hypoxaemia
| *For example, arising from falls in inspired pO2, respiratory infections, or chronic lung disease. |
| Predisposition for paradoxical inhibition of respiratory drive (up to 1–2 months of age) |
| In early infancy, infections/hypoxia may present with apnoea/hypoventilation |
| More compliant rib cage |
| Negative intrathoracic (pleural) pressures less effective (i.e. less suck is generated) |
| Provides less support of lung volume, particularly during active sleep |
| Reduced surfactant (preterm newborns) |
| Increases tendency to atelectasis and hypoxia |
| Preterm infants would not usually travel, except as part of a medical transfer |
| Increased proportion of the pulmonary vascular bed with muscular arterioles (early infancy) |
| Airway or alveolar hypoxia* causes pulmonary vasoconstriction5 |
| Pulmonary vascular responses to inspired hypoxia are greater in older infants than in newborns,6 and in those with perinatal vascular insults |
| Rise in pulmonary vascular resistance contributes to right to left shunting, ductal opening (in the early neonatal period), further ventilation-perfusion mismatch, and hypoxia |
| Increased airway reactivity in response to hypoxia (infancy) |
| Airway or alveolar hypoxia* in infants can cause bronchoconstriction7,8 |
| Infants at 26 weeks of age show greater desaturation on histamine challenge than infants 4 weeks old9 |
| Lung volume at end expiration similar to closing volume10 (early infancy) |
| Small airway closure, and hence non-ventilated units, occur more readily, e.g. during active sleep, feeding, and crying |
| Reduced upper and lower internal diameters of the airways |
| Airway conductance falls from birth to 2 months of age11 |
| Reductions in diameter from e.g. respiratory infection, reduces airway patency sooner,12 increasing tendency to airway closure and ventilation-perfusion mismatch |
| Fewer alveoli (early childhood) |
| Growth in the alveolar region greater than that in the airways in early infancy13 |
| Increases the susceptibility to mismatch between ventilation and perfusion |
| Fetal haemoglobin present up until 4–6 months of age |
| Oxygen dissociation curve is shifted to the left, so oxygen is given up less readily to the tissues |
| At any given pO2, the SaO2 is higher, consistent with the higher values reported at 3100 m in neonates compared to values at 4 months of age14 |