Uncoupling the effects of pulmonary surfactant and tissue viscoelasticity using an image-based biophysical lung model.

Pulmonary biomechanical behavior is a critical determinant of the structure-function relationship in the lung under normal and pathological conditions. Pulmonary surfactant plays a key structural role in lung function and significantly contributes to the mechanical response of the lungs during respiration. Additionally, the viscoelastic behavior of the lung parenchyma contributes to the lung's mechanical function and may be altered in disease. However, the contributions of these individual mechanisms to the organ-level function of the lungs are not well understood. We developed a novel biophysical model using a compressible visco-hyperelastic formulation that incorporates pulmonary surfactant dynamics and quantifies the contributions of surfactant and parenchymal tissue viscoelasticity to lung compliance. The effect of pulmonary surfactant was modeled as a surface energy function, and the surface behavior was coupled to the bulk behavior, assuming uniform spherical alveoli. The model was used to simulate respiration and investigate the effects of altered surface tension due to surfactant dysfunction, as well as varied viscous behavior. The model captured the characteristic sigmoidal inspiratory pressure-volume relationship and hysteresis expected of lung inflation mechanics. In addition, it predicted physiologically consistent changes associated with surfactant dysfunction and alterations in tissue viscoelasticity during lung injury. We expect this work to serve as a step toward deconvoluting and predicting the respective contributions of the lung parenchyma and pulmonary surfactant to global and regional lung compliance in health and disease.