How is static respiratory compliance calculated?

• A. VT / (Pplat - PEEP)
• B. (Pplat - PEEP) / VT
• C. Pplat / VT
• D. VT / (PEEP - Pplat)

Answer: (A)

Static compliance reflects how much volume the respiratory system can move for a given elastic pressure when airway flow is zero. That means we measure with an inspiratory hold so the pressure reflects elastic recoil only, giving plateau pressure. The pressure that actually opposes inflation is the difference between plateau pressure and PEEP. Tidal volume divided by that pressure difference yields static compliance: VT / (Pplat - PEEP). In other words, how many milliliters per centimeter of water the system expands for each unit of elastic pressure. If you use the peak pressure (dynamic scenario) or omit subtracting PEEP, you’re mixing in airway resistance or baseline pressure, which isn’t static compliance.