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Department of Mechanical Engineering and Center for Biomedical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A model is presented of
surfactant replacement therapy. An instilled bolus is pushed into the
lungs on the first inspiration, coating the airways with a layer of
surfactant and depositing some in the alveoli. Layer thickness depends
on the capillary number (µU/
, where µ, U, and
are bolus viscosity, advancing meniscus velocity, and surface
tension, respectively). Larger capillary number leads to thicker
layers, reducing alveolar delivery. Subsequently, surface tension
gradients sweep surfactant into alveoli not receiving surfactant during
the first inspiration. The effects on spreading of sorption kinetics,
bolus viscosity, initial layer thickness, initial penetration of
surfactant, gravity, and shear stress are examined. Sorption nearly
eliminates surface tension gradients in central airways but produces a
sharp transition at the leading edge of the exogenous layer. Local
thinning of the liquid layer results, trapping 95% of the surfactant
in the airways. Gravity and ventilation augment transport somewhat.
Transport to the periphery takes 4-170 s for the leading edge but
considerably longer for the bulk of the surfactant. The model
demonstrates how the various physical parameters governing surfactant
distribution might alter the response to surfactant replacement therapy.
airway liquid; modeling; respiratory distress syndrome; surface tension
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