Stress, Strain, and the Inflation of the Lung

The lung is an inherently mechanical organ. Central to the process of respiration are the cyclic changes in lung volume in response to changes in the transpulmonary pressure. Pressure is force per unit area and is thus a stress, while changes in lung volume and shape constitute a strain. A fully realistic description of lung mechanical behavior must contend with the fact that the relationship between stress and strain can be nonlinear and nonuniform. Moreover, depending on the timescales of interest, lung tissue exhibits viscoelastic behavior and can thus exhibit time-varying strain even when stress is constant. Fortunately, for most clinical applications, it suffices to model lung stretch during respiration as a uniform change in volume under the influence of the transpulmonary pressure, by analogy to force/length relationships in 1D models. Apart from nonlinearities, nonuniform inflation leads to important consequences in the setting of lung injury. While the viscoelastic nature of lung tissue is well established, as yet there is no convincing evidence that such effects must be routinely accounted for at the bedside.