Surfactant metabolism.

Alveolar surfactant pool sizes are small in the preterm animal with RDS, and these pool sizes probably increase to approximate those found in preterm animal without RDS by 3 to 4 days after birth. De novo synthesis and secretion of surfactant phospholipids are slow processes. Therefore, surfactant treatments are the only way to acutely increase the pool size. Surfactant phospholipids are very slowly catabolized in the preterm lung. The surfactant phospholipids used for treatment rapidly associate with lung tissue and are processed by recycling through lamellar bodies within type II cells. Surfactant proteins already present within the preterm lung can associate with the exogenous surfactant. These proteins have metabolic pathways quite different from those for the lipids. Surfactant treatments do not seem to interfere with endogenous metabolic pathways for surfactant synthesis and secretion. Surfactant treatments of infants with RDS are beneficial in part because of the favorable metabolism characterized by a preterm lung that can recycle surfactant and does not catabolize the exogenous surfactant.