Dynamic compliance Cd is tidal volume divided by the difference between peak inspiratory pressure (PIP) and PEEP. Which expression correctly represents Cd?

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Dynamic compliance tells you how much tidal volume you get for each unit of pressure driving inspiration when there is flow, so it must relate the volume to the pressure change actually pushing air into the lungs. That driving pressure is the peak inspiratory pressure minus the PEEP, because PIP includes the pressure needed to overcome airway resistance plus elastic recoil, while PEEP is the baseline. Subtracting PEEP gives the net pressure that inflates the lungs during inspiration. Therefore, dynamic compliance is Vt divided by (PIP minus PEEP).

For example, with a tidal volume of 500 mL, PIP of 20 cmH2O, and PEEP of 5 cmH2O, Cd ≈ 500 / (20 − 5) = 33 mL/cmH2O.

Using plateau pressure would reflect static, not dynamic, compliance, since plateau is measured when there’s no flow. Using the sum PIP + PEEP isn’t a meaningful driving-pressure term for this purpose.