The effect of exogenous surfactant on alveolar interdependence

• Imaging of intact in vivo alveoli allowed to evaluate the effect of surfactant instillation.
• Surfactant instillation increased surface area of alveoli <20,000 mm2 but decreased surface area for alveoli <20,000 mm2.
• Surfactant decreased both surface tension and recoil pressure.
• Inter-alveolar mechanical equilibrium was preserved despite differences in alveolar morphology.

To investigate the nature of alveolar mechanical interdependence, we purposefully disturbed the equilibrium condition by administering exogenous surfactant in physiological non-surfactant deprived conditions.Changes in alveolar morphology induced by intra-tracheal delivery of CUROSURF were evaluated after opening a pleural window allowing in-vivo microscopic imaging of sub-pleural alveoli in 6 male anesthetized, tracheotomized and mechanically ventilated rabbits. Surfactant instillation increased the surface area of alveoli smaller than 20,000 μm2 up to ∼50% at 15 min after instillation, reflecting a lowering of surface tension due to local surfactant enrichment. Conversely, for alveoli greater than 20,000 μm2, surface area decreased by ∼5%. Opposite changes in alveolar surface are interpreted as reflecting a new inter-alveolar mechanical equilibrium modified by local surfactant distribution and by a decrease in lung distending pressure. We propose that smaller alveoli, representing the majority of alveolar population, might mostly contribute to improve the oxygenation index following surfactant replacement therapy in case of surfactant deficiency.