Compliance is a measure of lung distensibility and is measured as a change in lung volume per unit change in pressure. There are two different types of lung compliance—static and dynamic. Static lung compliance is represented by the slope of the pressure-volume curve during deflation from total lung capacity. In a patient who is being mechanically ventilated, this can be measured by dividing the exhaled tidal volume by the difference between the plateau pressure (Pplat) and the positive end-expiratory pressure (PEEP) after temporary occlusion of the expiratory limb of the ventilator circuit. The peak pressure is the most accurate reflection of static compliance.

Dynamic compliance is the ratio of the change in volume to the change in pressure over a tidal breath. Insofar as there is no abnormality in the exhaled tidal volume, a decrease in dynamic lung compliance would suggest disease or narrowing of the smaller airways. The plateau pressure most accurately reflects this measurement.