Elastic behavior of the lungs in healthy children determined by means of an exponential function

In order to evaluate the elastic properties of the lungs in children, a new exponential sigmoid curve fitting model was developed with the function VL = Vm + (Vm/1 + b X e-K X Pst), where VL is the lung volume, Pst the static recoil pressure, VM and Vm the upper and the lower asymptotes (limits of lung volume) and K and b are specific constants. Pressure-volume (PV) data obtained from tidal breathing cycles at different inflation and deflation levels from functional residual capacity (FRC) up to approximately 90% total lung capacity (TLC) in 16 healthy children have been evaluated by this model. K in relation to age, calculated by the least square nonlinear regression analysis decreased during childhood, whereas in b increased. It seems that K (a volume-independent index of compliance and alveolar distensibility) is influenced by the increasing number of elastic fiber bundles, changes in the surface/volume ratio and finally by changes in the composition of the surfactant. It can be seen that this model is not only restricted to higher lung volumes, the lower limb of the S-shaped curve is also represented.