Single versus repetitive doses of natural surfactant as treatment of respiratory distress syndrome in premature lambs.

After delivery by caesarean section at 117-120 and 125-130 days gestational age, 20 lambs were supported with infant ventilators at an FiO2 of 1.0. Throughout the experimental period the respiratory settings were held constant. All lambs were treated by tracheal instillation of 50 mg natural sheep surfactant lipid/kg body weight at 1 h of age, after respiratory failure had been documented. All lambs responded with a decrease in PaCO2 and increases in PaO2 and pH, but these responses were of greater magnitude with advancing gestational age. Without waiting for respiratory failure to reappear, repetitive doses of natural surfactant were administered and the effects studied. Retreatment by instillation of 12.5 mg surfactant/kg at 2, 3, 4, 5, and 6 h of age was ineffective at 117-120 days gestational age, but sustained the initial response to the bolus of 50 mg surfactant/kg at 125-130 days gestational age. This indicates that retreatment with natural surfactant should be started before the reappearance of respiratory failure in order to be effective, and that the response to retreatment with small, repetitive doses of surfactant is dependent on gestational age.