Many common diseases of the lung involve alterations in lung mechanics. Being able to characterize these alterations is thus of great importance. Most research involving the model-based estimation of lung mechanics involves nothing more than what has just been described in the previous chapter. That is, the lung is assumed to behave like a single linear compartment characterized by only two parameters, resistance (RL) and elastance (EL). These two parameters are typically evaluated first under baseline or control conditions and then following whatever intervention is being investigated. This raises the question of how to interpret R and E, and their changes, in physiological terms. In this chapter we consider what these two parameters can tell us about events happening within the lungs.

We saw in the previous chapter (Fig. 3.4) that RL is made up of two distinct components – one from the airways (Raw) and one from the lung tissues (Rt). This raises questions about how these two components arise. How, for example, could the airway tree, with all its structural complexity, give rise to a particular value of Raw? Why is it that when energy is imparted to the tissues during lung inflation, some portion of this energy is dissipated to give rise to Rt while the remainder is stored to be reflected in the value of EL? Answering these questions is clearly important in being able to assign Raw, Rt, and EL their appropriate physiological interpretations.