Two-compartment modelling of respiratory system mechanics at low frequencies: gas redistribution or tissue rheology?

The mechanical properties of the respiratory system are generally inferred from measurements of pressure and flow at the airway opening. Traditionally, these measurements have been related through a single-compartment model of the respiratory system. Recently, however, there has been considerable interest in modelling low-frequency respiratory mechanics in terms of two compartments, since this gives a much improved description of experimental data. In this paper we consider two classes of two-compartment models that are compatible with pressure-flow relationships of air measured at the airway opening. One type of model accounts for regional ventilation inhomogeneity in the lung in terms of two alveolar compartments. The other type of model considers pulmonary ventilation to be homogeneous, while the tissues of the respiratory system are modelled as being viscoelastic. In normal dogs, the appropriate two-compartment model has been shown to be the viscoelastic model. In the case of abnormal physiology, however, one must invoke a model having both viscoelastic tissues and ventilation inhomogeneities. Additional experimental data are required in order to identify such a model, and to quantify these two phenomena.