Static compliance Cs is defined as tidal volume divided by the difference between plateau pressure and PEEP. Which expression correctly represents Cs?

• A. Cs = Vt / Plat
• B. Cs = Vt / PEEP
• C. Cs = Vt / (Plat + PEEP)
• D. Cs = Vt / (Plat - PEEP)

Answer: (D)

Static compliance measures how much volume changes for a given distending pressure when there’s no flow. The pressure that actually expands the system is the driving pressure, which is the difference between plateau pressure and PEEP measured during an inspiratory hold. So the correct expression is Cs = Vt / (Plat - PEEP).

For example, if tidal volume is 500 mL, plateau pressure is 25 cm H2O, and PEEP is 5 cm H2O, the driving pressure is 20 cm H2O, giving Cs = 500 / 20 = 25 mL/cm H2O.

Using plateau pressure alone ignores the baseline PEEP; using PEEP alone doesn’t reflect the pressure change caused by delivering the tidal volume; and using plateau pressure plus PEEP would misrepresent the actual driving pressure.