Pathophysiology

Pulmonary surfactant is a mixture of phospholipids and proteins produced by lamellar bodies of type II alveolar cells. These phospholipids reduce alveolar surface tension, preventing the alveoli from collapsing.

Surfactant deficiency is most likely to occur in preterm infants, because:

• Surfactant production begins at approximately 20 weeks gestation.

• Distribution throughout the lungs begins at 28-32 weeks' gestation and does not reach sufficient concentration until 35 weeks gestation.

Surfactant deficiency → little or no reduction of alveolar surface tension → increased alveolar collapse → atelectasis → decreased lung compliance and functional residual capacity → hypoxemia and hypercapnia

• Hypoxemia and hypercapnia → vasoconstriction of the pulmonary vessels (hypoxic vasoconstriction) and respiratory acidosis → intrapulmonary right-to-left shunt → increased permeability due to alveolar epithelial damage → fibrinous exudation within the alveoli → development of hyaline membranes in the lungs (hyaline membrane disease)