Surfactant deficiency causes high surface tension, alveolar collapse and stiff lungs
Expansion
Immature type II pneumocytes produce insufficient surfactant, so:
- Surface tension is high, and by Laplace’s law small alveoli collapse into larger ones
- Compliance falls, so the work of breathing rises steeply
- Widespread atelectasis creates a large shunt with refractory hypoxaemia
- Proteinaceous exudate forms hyaline membranes, which further impair diffusion
Clinically: tachypnoea, grunting (an attempt to generate auto-PEEP by expiring against a partly closed glottis), intercostal recession, nasal flaring and cyanosis, with a ground-glass appearance and air bronchograms on radiography.
Risk falls with gestation: surfactant appears from about 24 weeks and is adequate by about 35. Maturity can be assessed by the lecithin-sphingomyelin ratio.
Management follows the physiology: antenatal corticosteroids to accelerate production, CPAP to splint alveoli open, and exogenous surfactant. Diabetes in the mother delays maturation, while chronic intrauterine stress accelerates it.