Accumulation of alveolar surfactant following delivery and ventilation of premature lambs.

We studied the accumulation of surfactant in the alveoli and airways from birth to 10 h of age in 60 lambs delivered prematurely at a mean gestational age of 136 days. The lambs were delivered by cesarean section and ventilated by varying peak inspiratory pressures only to normalize arterial blood gas and pH values. Alveolar surfactant accumulation was measured either by alveolar wash after timed sacrifice or by repetitive sampling of the airways using an isotope dilution technique. The fall in specific activity of phosphatidylcholine with time was measured after the intratracheal administration of either a trace dose of 3H-labeled natural surfactant and synthetic 14C-dipalmitoylphosphatidylcholine (DPPC) at birth or a treatment dose of the same mixture at 5 h of age (about 55 mg of total surfactant lipid/kg body weight). From the change in phosphatidylcholine-specific activities and the quantities and specific activities of phosphatidylcholine measured in alveolar washes, we found that the surfactant pool sizes as estimated by alveolar wash or by the isotope dilution technique were similar. The pool size of alveolar surfactant increased fourfold from birth to 20 min of age and doubled again by 10 h of age. The shape of the curve for the accumulation of alveolar surfactant versus time was independent of the final pool size achieved at 10 h of age. Sonicates of DPPC when mixed with either a trace amount or a treatment dose of natural surfactant were treated by the premature lung in a manner indistinguishable from the phosphatidylcholine associated with natural surfactant.