Dynamic Lung Compliance

Cdyn = VT ÷ (PIP − PEEP)

Normal: 50–80 mL/cmH₂O; lower than static compliance due to airway resistance effects

Variables

Cdyn

Dynamic compliance(mL/cmH₂O)

Normal: 50–80 mL/cmH₂O

Tidal volume delivered(mL)

Normal: 400–600 mL

PIP

Peak inspiratory pressure(cmH₂O)

Normal: 15–25 cmH₂O typically

PEEP

Positive end-expiratory pressure(cmH₂O)

Normal: 5–15 cmH₂O typically

What It Measures

Dynamic compliance is measured during active gas flow and therefore reflects BOTH the elastic properties of the lung AND the resistance of the airways. It is always lower than static compliance because additional pressure is needed to overcome airway resistance during flow. The difference between static and dynamic compliance reveals the contribution of airway resistance.

When to Use

Continuous monitoring in ventilated patients during routine ventilation (no need for an inspiratory pause). Used to detect changes in airway resistance (bronchospasm, secretions, ETT kinking) or lung compliance. When Cdyn falls without a change in static compliance, suspect increased airway resistance.

Interactive Calculator

Tidal Volume (VT)(mL)

Peak Inspiratory Pressure (PIP)(cmH₂O)

PEEP(cmH₂O)

Worked Examples

Bronchospasm (resistance problem)

→ 12.5 mL/cmH₂O

Very low Cdyn with likely normal Cst — severe bronchospasm

ARDS (compliance problem)

→ 18.2 mL/cmH₂O

Low Cdyn; will also have low Cst (Pplat high) — lung stiffness is the problem

Clinical Significance

If both Cdyn and Cst fall together: compliance problem (ARDS, edema, fibrosis). If Cdyn falls but Cst is unchanged: airway resistance problem (bronchospasm, mucus plug, water in circuit, kinked ETT). This distinction is critical for targeted treatment.

Memory Tips

💡Dynamic compliance uses PIP (peak = during flow) — includes resistance PLUS elastance
💡Static compliance uses Pplat (plateau = no flow) — elastance only
💡Cdyn vs Cst: if Cdyn drops but Cst stays the same → airway problem; both drop → lung problem

NBRC Pearls

High PIP with normal Pplat = high airway resistance (bronchospasm, secretions, kinked ETT) — treat with bronchodilators or suction
High PIP with high Pplat = lung compliance problem (ARDS, pneumothorax, tension PTX) — adjust ventilator settings
Sudden rise in PIP with no change in Pplat = suspect right mainstem intubation, mucus plug, or patient biting ETT

← All Formulas

What It Measures

When to Use

NBRC Pearls

High PIP with normal Pplat = high airway resistance (bronchospasm, secretions, kinked ETT) — treat with bronchodilators or suction

High PIP with high Pplat = lung compliance problem (ARDS, pneumothorax, tension PTX) — adjust ventilator settings

Sudden rise in PIP with no change in Pplat = suspect right mainstem intubation, mucus plug, or patient biting ETT