Alveolar surfactant aggregate conversion in ventilated normal and injured rabbits.

Alveolar surfactant can be separated into two subtypes; large aggregates and small aggregates. Large aggregates represent the surface active form of surfactant and are the metabolic precursors of small aggregates. Previous studies examined the mechanism by which large aggregates are converted into small aggregates in vitro. We used intratracheal injection of radiolabeled large aggregates in rabbits to probe the aggregate conversion in vivo. After this injection, animals were mechanically ventilated for 60 min. After the animals were killed, the lungs were lavaged, and the percentage of radiolabel present in the small aggregate fraction was determined. Our results showed that ventilation resulted in aggregate conversion and that increases in tidal volume, but not in respiratory rate, correlated with increased conversion. Aggregate conversion in rabbits with acute lung injury correlated significantly with severity of injury. We conclude that a change in surface area (i.e., respiration) is necessary for aggregate conversion in vivo and that the ventilation strategy can affect this conversion. Furthermore, increased aggregate conversion in injured lungs might contribute to increased small-to-large aggregate ratios in these lungs compared with normal lungs.