Static compliance is measured in the absence of gas flow, and is based on plateau pressure:
Cstat = Vt/(Pplat – PEEP)
Dynamic compliance is measure in the presence of gas flow, and is based on peak pressure:
Cdyn = Vt/(Ppeak – PEEP)
Remember these three scenarios to understand how compliance curves will change:
- Pulmonary emboli do not change resistance or compliance, so both compliance curves are unchanged.
- Plugging or bronchospasm increases airway resistance (Raw) and peak pressure, so dynamic compliance curve shifts to the right and flattens. Plateau pressure and static compliance are unchanged.
- Tension pneumothorax, atelectasis, pulmonary edema, pneumonia, bronchial intubation cause reduced lung compliance which increases both peak pressure and plateau pressure, so both static and dynamic compliance fall.