This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.
Compliance and driving pressure
In a patient on a ventilator, the pressures the machine displays tell you how stiff the lungs and chest wall are. Compliance is the change in volume for each unit of pressure: high compliance means the lungs inflate easily, low compliance means they are stiff, as in ARDS, pulmonary edema or fibrosis.
The formulas
- Static compliance = tidal volume ÷ (plateau pressure − PEEP)
- Dynamic compliance = tidal volume ÷ (peak pressure − PEEP)
- Driving pressure = plateau pressure − PEEP
- Airway resistance = (peak pressure − plateau pressure) ÷ inspiratory flow (L/s)
The plateau pressure is read during an inspiratory hold of 0.5 to 1 second, when no air is moving: it reflects the elastic properties of the respiratory system alone. The peak pressure also includes the pressure needed to push air through the tube and airways, so the gap between peak and plateau reflects resistance. Formulas for resistance assume constant (square-wave) flow in volume-control mode.
Example
A tidal volume of 450 mL with a peak of 25, a plateau of 20 and a PEEP of 8 cmH2O at 60 L/min gives:
- static compliance 450 ÷ 12 = 37.5 mL/cmH2O
- driving pressure 12 cmH2O
- dynamic compliance 26.5 mL/cmH2O
- resistance (25 − 20) ÷ 1 L/s = 5 cmH2O/L/s
Interpreting the values
| Measure | Usual or target value |
|---|---|
| Static compliance, healthy lungs on a ventilator | 50 to 100 mL/cmH2O |
| Static compliance in ARDS | often 20 to 40 |
| Plateau pressure | 30 cmH2O or less |
| Driving pressure | 15 cmH2O or less |
| Resistance in an intubated adult | about 5 to 15 cmH2O/L/s |
A large analysis of ARDS trials (Amato, 2015) found that driving pressure was the ventilation variable most strongly associated with survival.
Pitfalls
- Use the total PEEP: measure auto-PEEP with an expiratory hold in patients with obstruction.
- Active breathing efforts during the hold make the plateau unreliable.
- A falling dynamic compliance with a stable static compliance points to the airways (secretions, bronchospasm, a kinked tube), not the lungs.