Peak vs. plateau airway pressure

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.