Respiratory distress syndrome in immature lambs. Prevention through antenatal accelerated conditioning of the lung.

We tested the effectiveness of constant distending pressure applied to immature lungs in preventing respiratory distress syndrome. Fetal lambs of 131 to 134 days gestation were delivered by cesarean section, but the umbilical circulation was kept intact for CO2 removal through the natural in situ placenta. The lungs were inflated to a pressure of 35 cm H2O (Group I, 11 animals) or 25 cm H2O (Group II, 14 animals), after which the airway pressure was maintained at 15 cm H2O through apneic oxygenation until total static compliance exceeded 0.5 ml (cm H2O)- 1kg -1. After a mean of 1.1 and 5.7 h, respectively, the animals were delivered and were given mechanical ventilation for 24 h. Twenty-four animals reached this aimed-for compliance and survived the period of mechanical ventilation in excellent health. A control group of fetal lambs was delivered immediately and treated with mechanical ventilation. Three of 10 control animals developed severe respiratory distress syndrome and died; 1 additional animal survived but with central nervous system involvement from severe hypoxia. We conclude that pulmonary inflation to 35 cm H2O pressure, followed by a constant distending pressure of 15 cm H2O, held until compliance reaches 0.5 ml (cm H2O)- 1kg -1, is an important element in the prevention of respiratory distress syndrome.