Artificial surfactant in preterm rabbits with and without respiratory distress syndrome: difference of in vitro and in vivo activities.

Sixty-eight rabbit fetuses of 27 days gestation were tracheotomised, artificially ventilated, and their lung mechanics studied in a body-enclosing plethysmograph. The animals were treated by tracheal instillation of natural surfactant concentrate or large unilamellar vesicles containing dipalmitoylphosphatidylcholine:egg phosphatidylglycerol, 9:1. Both preparations were highly surface active in terms of film adsorption and surface tension-lowering potential. Before treatment, the lung mechanics were analysed to indicate the presence of respiratory distress syndrome (RDS). Controls received 0.15 M saline. Differences were found between the in vitro and in vivo activities of both preparations in some animals. In 30 preterm animals with partial lung maturity and without respiratory distress syndrome, no significant effect could be achieved with either the natural or the artificial surfactant. In 38 animals with severe RDS, the tidal volume and compliance increased markedly within 15 min of substitution of both preparations. Compliance increased to 178% of the initial value in ventilated, control animals, to 391% in animals treated with natural, and to 344% in animals treated with artificial surfactant.