Dynamic behavior of lung surfactant

We have previously developed an adsorption-limited model to describe the ex change of lung surfactant and its fractions to and from an nir-liquid inter face in oscillatory surfactometers. Here we extend this model to allow for diffusion in the liquid phase. Use of the model in conjunction with experim ental data in the literature shows that diffusion-limited transport is impo rtant for characterizing the transient period from the start of oscillation s to the achievement of steady-state conditions. Matching previous data sho ws that upon high levels of film compression, large changes occur in adsorp tion rate, desorption rate, and diffusion constant, consistent with what on e might expect if the subsurface region was greatly enriched in DPPC. Colla pse of the surfactant film that occurs during compression leads to a signif icant elevation of surfactant concentration immediately beneath the interfa ce, consistent with the subsurface depot of surfactant that has been postul ated by other investigators. Modeling studies also uncovered a phenomenon o f surfactant behavior in which the interfacial tension remains constat at i ts minimum equilibrium value while the firn is compressed but without colla pse of the film. The phenomenon was due to desorption of surfactant from th e interface and termed "pseudofilm collapse. " The new model also gave impr oved agreement with steady-state oscillatory cycling in a pulsating bubble surfactometer.