So far, we have been systematically building up a quantitative mechanical view of the lung in terms of inverse models. Before continuing on in this upward climb toward increased model complexity, we will take a slight digression to consider in this chapter the phenomenon of expiratory flow limitation. As pointed out in Section 1.3, expiratory flow does not increase indefinitely as the expiratory muscles increase their forces of contraction. Instead, expiratory flow approaches a maximum value that can not be exceeded regardless of how much extra effort is exerted. It is convenient to think of flow limitation as yet another kind of nonlinearity that can afflict the single-compartment model, and so its discussion here makes a natural follow-on from the previous chapter. However, flow limitation has special status in terms of clinical application, so no treatise on lung mechanics would be complete without giving it due consideration. The following development is based on and.

FEV1 and FVC

It seems natural to think that the harder you try to force air out of your lungs, the faster it will come out. While this is certainly the case at low rates of flow, it is not generally true. As expiratory effort is increased, there comes a point at which further increases in effort are not rewarded with increases in flow. This maximum expiratory flow is strongly influenced by the tethering forces of the parenchyma that pull outward on the airway wall.