Corticosteroids and surfactant increase lung volumes and decrease rupture pressures of preterm rabbit lungs.

We measured the effects of corticosteroids and surfactant individually and in combination on lung pressure-volume relationships, rupture pressures, and rupture volumes. Pregnant does were injected with betamethasone (0.1 mg/kg per day im) or vehicle on days 24 and 25 of gestation, and fetal rabbits were delivered on days 26 and 27. Natural surfactant (50 mg/kg body wt) was instilled intratracheally into half of the lungs after tracheotomy. After nine cycles of inflation with air to 40 cmH2O and deflation, air pressure-volume curves were measured. Then the lungs were filled with air to rupture, and rupture volume and pressure were recorded. Both corticosteroids and surfactant caused an increase in maximal lung volumes (P less than 0.01) and a decrease of lung rupture pressures (P less than 0.01) compared with controls. The effects of corticosteroids plus surfactant on lung volumes were the sum of each effect individually, but rupture pressures were the same as those for corticosteroids or surfactant alone. Surfactant, in addition, caused an increase in lung stability at deflation, an effect that was not evident in the corticosteroid-treated groups. Measurements of saturated phosphatidylcholine in alveolar washes and lung tissue indicated comparable values in the corticosteroid and control groups. We conclude that changes in static properties and rupture pressures presumably reflect changes in lung structure caused by corticosteroids that are independent of a corticosteroid effect on surfactant pool sizes.