[box type=”download”] — Compliance as a combination of static and dynamic factors
— Appreciation of the shapes of the static and dynamic pressure-volume loops
— Static compliance as a function of age, size and sex and its reduction in fibrosis
— Dynamic compliance as incorporating an airways resistance component [/box]
Lung compliance (CL) represents the ‘stretchiness’ of the lungs.
Change in volume per unit change in distending pressure (CL = ΔV/ΔP).
The pressure distending the lungs (transmural pressure) is equal to alveolar–intrapleural pressure. Intrapleural pressure can be estimated using an oesophageal balloon. The static lung compliance is the maximum slope, generally just above functional residual capacity (FRC), and is normally ∼1.5 L/kPa, although this is dependent on age, size and sex.
Compliance is reduced by fibrosis (stiffer lungs, greater elastic recoil), and increased in emphysema where loss of structure makes them easier to stretch (reduced elastic recoil).
Dynamic compliance is measured during breathing, and therefore includes a component due to airway resistance. The dynamic pressure–volume loop is therefore much wider, reflecting the pressure required to suck in or expel air; the area of the curve is a measure of work done against airway resistance. The curve has two points where flow is zero (end of inspiration/expiration); the slope of a line between them is the dynamic compliance. Though normally similar to static compliance, this can be altered in disease.