Which expression is not part of the dynamic compliance formula?

Which expression is not part of the dynamic compliance formula?

• A. Plateau pressure - PEEP
• B. PIP - PEEP
• C. Vt
• D. PIP

Answer: (A)

Dynamic compliance describes how much volume changes for a given driving pressure during actual air flow. It is defined as Vt divided by the driving pressure, where driving pressure is the difference between peak inspiratory pressure and PEEP (PIP − PEEP). This reflects the combined effect of lung and chest wall compliance in the presence of airway resistance. Plateau pressure minus PEEP is used to calculate static compliance, not dynamic. Plateau pressure is measured during no-flow conditions and isolates the elastic properties of the lung and chest wall, removing airway resistance from the equation: static compliance = Vt / (Plateau − PEEP). Therefore, that expression does not describe dynamic compliance. Vt is the tidal volume, the numerator in the dynamic formula, so it is part of the expression. PIP − PEEP is the driving pressure in the dynamic formula, so that component is also part of dynamic compliance (though PIP alone is not the expression).

Dynamic compliance describes how much volume changes for a given driving pressure during actual air flow. It is defined as Vt divided by the driving pressure, where driving pressure is the difference between peak inspiratory pressure and PEEP (PIP − PEEP). This reflects the combined effect of lung and chest wall compliance in the presence of airway resistance.

Plateau pressure minus PEEP is used to calculate static compliance, not dynamic. Plateau pressure is measured during no-flow conditions and isolates the elastic properties of the lung and chest wall, removing airway resistance from the equation: static compliance = Vt / (Plateau − PEEP). Therefore, that expression does not describe dynamic compliance.

Vt is the tidal volume, the numerator in the dynamic formula, so it is part of the expression. PIP − PEEP is the driving pressure in the dynamic formula, so that component is also part of dynamic compliance (though PIP alone is not the expression).