27 day fetal rabbit pups were used to compare the metabolic fate of two natural surfactant preparations [term fetal rabbit surfactant (FRS) and adult rabbit surfactant (ARS)] and two commercially-available preparations[apoprotein-based Survanta® (S) and totally synthetic Exosurf® (E)] following intratracheal instillation. After anaesthetizing the doe, the pups were delivered by hysterotomy, treated with one of the surfactant preparations(FRS, ARS: 1250 μg phospholipid per pup; S, E: according to manufacturer's directions and ventilated with 100% oxygen for either 5, 60, 120 min. Treated, unventilated pups served as 0 hr controls. At the end of each observation period, pups were sacrificed, the instilled surfactant recovered by alveolar lavage and separated into the heavy aggregate, metabolically active form (H) and light-aggregate, non-surface active metabolic breakdown form (L) by high speed centrifugation (10,000 g for 30 min) and the recovery of instillate in each fraction determined.

Prior to instillation more than 90% of each surfactant preparation was in the H form. While there was little conversion to the L form (approximately 20% for all groups) following instillation and ventilation there were considerable differences in retention of the H form of each instillate. By 5 min less than 40% of the H form of S and E were recovered while virtually all of the instilled H form of ARS and FRS were recovered. By 60 min the H form of ARS decreased significantly (less than 50% recovery) but ARS recovery was nearly 90%. By 2 hr the H form of each surfactant except ARS was decreased even further and this decrease was associated with deterioration in the overall condition of the pups.

These results suggest that compared to the other surfactant preparations examined, FRS may be cleared more slowly from the preterm newborn lung. While the reason for this is not presently known, it demonstrates that a preparation with these qualities may extend the availability of the metabolically active form of surfactant in the alveoli and prolong survival of the premature newborn. (Supported by Grace Maternity Hospital Foundation.)