(56f) Effect of Dilatational Modulus on Lung Surfactant Functionality

A definitive role of lung surfactant is to insure uniform lung inflation. The dependence of the Laplace pressure, DP = 2γ/R, on alveolar radius, R, means that interconnected bubbles or alveoli are at best metastable if γ is constant. The high pressure in the small alveoli cause them to dump their contents into the lower pressure, larger alveoli, and the small alveoli eventually deflate due to Laplace Instability. However, while not generally appreciated in the medical literature, but well known in the foam and emulsion stability community, the dynamic resistance of an interfacial film to compression can reverse the Laplace instability. The dilatational modulus, ε=A∂γ/∂A , relates the change in surface tension, γ, to the change in molecular area, A, as the interface is compressed at frequency, ϖ (ranging from 10-100 cycles/minute for normal breathing). If the dilatational modulus is large enough, the resistance to interfacial compression can overcome the Laplace pressure so that the gas pressure in the alveolus no longer increases with decreasing radius. For (2ε-γ) > 0, the Laplace pressure decreases with decreasing radius and increases with increasing radius, which reverses the Laplace instability, thereby stabilizing the alveoli against collapse. Using a newly designed capillary microtensiometer, we will show that lysolipids, a product of the inflammation induced degradation of phospholipids, can cause the dilatational modulus to decrease at low breathing frequencies, and thereby induce the Laplace instability, and perhaps acute respiratory distress syndrome. Increasing the breathing frequency can increase the dilatational modulus of lysolipids and may restore proper lung function.