Effects on airway closure of pulmonary surfactant and changes in lung volume

Numerical simulations have been conducted to study airway closure in the small pulmonary airways due to a liquid film instability creating an obstructing meniscus, taking into account the effects of changing lung volume and the presence of pulmonary surfactant. The model consists of a tube of finite length and uniform circular cross-section, lined with a thin liquid layer. Our initial calculations simulating a linear fall in lung volume with time but in the absence of surfactant, indicated that closure would occur as soon as the conditions existed for the initiation of the fluid dynamic instability. This condition was reached at a lung volume of about 26% TLC when the liquid film thickness was 10 μm at TLC. Rates of expiration within the normal physiologic range had no significant effect on closing volume. Including an insoluble surfactant, however, changed the results significantly. Depending on the minimal surface tension reached by the liquid film in the airways during expiration, we found that airway closure could be significantly delayed. For example, with a surface tension of 20 dynes/cm at Total Lung Capacity (TLC), and falling to a minimum value of 2 dynes/cm (and using an empirically-derived relationship between surfactant concentration and surface tension for pulmonary surfactant) closure could be delayed to significantly lower lung volumes, as low as about 14% TLC with very rapid lung deflations. These results also raise the prospect of closure on inspiration, rather than expiration, due to the abrupt increase in surface tension as the film begins to expand.