Static compliance: which statement is true?

• A. Static compliance decreases; lower values indicate stiffer lungs
• B. Static compliance increases with plateau pressures
• C. Compliance relates to airway resistance only
• D. Static compliance is unrelated to plateau pressure or PEEP

Answer: (A)

Static compliance tells us how easily the lungs and chest wall stretch when there is no air flow, measured during an inspiratory hold. It’s calculated as the tidal volume divided by the difference between plateau pressure and PEEP (Cstatic = Vt / [Pplat − PEEP]). Because this gradient represents the distending pressure needed to deliver a given volume, a higher distending pressure for the same volume means lower compliance. So when static compliance decreases, the lungs are stiffer, and that’s why lower values indicate stiffer lungs.

Plateau pressure reflects the pressure in the alveoli when flow is zero, so it mainly conveys the elastic properties of the respiratory system rather than airway resistance. If plateau pressure is high for the same tidal volume, the distending pressure is large and compliance is reduced. PEEP sets a baseline pressure, and the calculation uses the difference between plateau pressure and PEEP, tying static compliance directly to both plateau pressure and PEEP.

Thus, the statement that static compliance decreases and that lower values indicate stiffer lungs is the correct concept.