Pulmonary mechanics: A system identification perspective

The mechanical properties of the lungs are important determinants of its ability to function properly, and may become severely compromised in disease. Being able to assess lung mechanical function is thus crucial to the diagnosis of many lung diseases and for following the progress of therapy. Assessing lung mechanical function is essentially an exercise in system identification, whereby measurements of pressures and flows made at certain sites are linked in terms of mathematical models. Most often, the lung is assumed to function as a linear dynamic system, allowing determination of its input impedance over a range of frequencies. In mice, for example, impedance is frequently determined between 1 and 20 Hz. The interpretation of impedance in physiological terms is central to its utility as a diagnostic tool, and is best done with reference to a suitable mathematical model. Currently, the most widely used model for animal studies of lung disease consists of a single flow-resistive airway serving a uniformly ventilated alveolar region surrounded by tissue having a constant-phase impedance. This model has also been employed in human studies, and allows the mechanical properties of the lungs to be subdivided into those reflecting the conducting airway tree and those due to the behavior of the lung periphery. The parameters of this model have been shown in animals to change characteristically following interventions that mimic human pathologies such as asthma and acute lung injury. Furthermore, these changing parameter values can be linked to specific physical processes occurring within the lungs.