What is the normal range for static compliance?

Normal static compliance (Cstat) in a mechanically ventilated adult is approximately 60–100 mL/cmH₂O. Values below 40 mL/cmH₂O suggest significant reduction, as seen in ARDS, pulmonary edema, or fibrosis.

What is the difference between Cstat and Cdyn?

Static compliance (Cstat) is measured at no-flow conditions using plateau pressure and reflects lung and chest wall elasticity. Dynamic compliance (Cdyn) uses peak inspiratory pressure and includes the resistance component of the airways. Cdyn is always lower than Cstat in the same patient.

Why is Cdyn lower than Cstat?

PIP is always higher than Pplat because it includes the pressure needed to overcome airway resistance during active gas flow. Since Cdyn divides by a larger pressure difference, it yields a smaller compliance value. A large gap between Cstat and Cdyn points to increased airway resistance (bronchospasm, secretions, kinked tube).

How do I measure plateau pressure?

Plateau pressure is obtained by performing an inspiratory hold (pause) maneuver on the ventilator — typically 0.5–2 seconds. Flow must reach zero for an accurate reading. It should not exceed 30 cmH₂O in lung-protective strategies.

What compliance values suggest ARDS?

The Berlin ARDS definition does not include a compliance threshold, but Cstat below 40 mL/cmH₂O is common in moderate-to-severe ARDS. Very low values (< 20 mL/cmH₂O) indicate severe restriction and correlate with worse outcomes.

Respiratory System Compliance Calculator

Name: Respiratory System Compliance Calculator
Availability: InStock
Author: Nham Vu

Enter ventilator settings to calculate static compliance (Cstat) and dynamic compliance (Cdyn) for assessing lung mechanics.

Ventilator Parameters

Enter values from the ventilator display or measured during an inspiratory hold.

Tidal Volume (Vt) mL

Typical range: 300–800 mL (6 mL/kg IBW for lung protection)

Plateau Pressure (Pplat) cmH₂O

Measured during inspiratory hold; target ≤ 30 cmH₂O

Peak Inspiratory Pressure (PIP) cmH₂O

Peak pressure during active gas delivery (PIP ≥ Pplat)

PEEP cmH₂O

Positive end-expiratory pressure (0 if no PEEP)

Static Compliance (Cstat)

Cstat = Vt / (Pplat − PEEP)

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mL/cmH₂O

Dynamic Compliance (Cdyn)

Cdyn = Vt / (PIP − PEEP)

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mL/cmH₂O

Airway Resistance Gap

Cstat − Cdyn: large gap suggests increased airway resistance

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mL/cmH₂O difference

Reference Ranges

Parameter	Normal (ventilated adult)	Concern	Critical
Cstat (mL/cmH₂O)	60 – 100	40 – 59	< 40
Cdyn (mL/cmH₂O)	50 – 80	35 – 49	< 35
Resistance gap	< 10	10 – 20	> 20
Pplat target	≤ 30 cmH₂O	30 – 35 cmH₂O	> 35 cmH₂O

Reference ranges are for adults on volume-controlled ventilation. Values differ in pediatric patients and vary with body size. This tool is for educational purposes; always apply clinical judgment.

Summary