The effects of breathing on surfactant transport in the lung [model for surfactant replacement therapy]

We model Marangoni-driven transport of soluble surfactant in Surfactant Replacement Therapy (SRT) and as a means of liquid and surfactant clearance from healthy lungs. The effects of breathing on the transport are considered by allowing the wall supporting the thin film on which the surfactant spreads to undergo periodic stretch. The model is developed using conservation of momentum, conservation of surfactant and fluid mass, and elasticity of the wall. Area expansion due to airway branching is considered, as are axial and circumferential airway stretch. In addition to the very brief initial transient spreading phase, we consider the steady cycle-mean spreading which accounts for the majority of surfactant transport in SRT. The cycle mean flux of surfactant along the surface and in the bulk fluid are enhanced for larger strain amplitudes and slower periodic stretch. This suggests that switching to conventional ventilation (higher tidal volume, slower frequency) rather than high frequency ventilation soon after administering the surfactant dose could be beneficial, from the stand point of increased surfactant flux into the alveolar region. By reversing the surface tension gradient, we find a similar dependence on strain amplitude and breathing frequency for surfactant and liquid clearance from the normal lung