mechanics

Static Lung Compliance

Cst = VT ÷ (Pplat − PEEP)

Normal: 60–100 mL/cmH₂O; < 40 mL/cmH₂O indicates stiff lungs (ARDS, pulmonary fibrosis, edema)

Variables

Cst

Static compliance(mL/cmH₂O)

Normal: 60–100 mL/cmH₂O

Tidal volume delivered(mL)

Normal: 400–600 mL

Pplat

Plateau pressure (end-inspiratory pause)(cmH₂O)

Normal: < 30 cmH₂O

PEEP

Positive end-expiratory pressure (total)(cmH₂O)

Normal: 5–15 cmH₂O typically

What It Measures

Static compliance reflects the elastic recoil properties of the lung and chest wall combined. Measured during no-flow conditions (end-inspiratory pause), it reflects the true distensibility of the respiratory system without the confounding effect of airway resistance. Low static compliance means the lungs are stiff and require more pressure for the same volume.

When to Use

Measure during volume-controlled ventilation by applying a 0.5–1 second inspiratory pause to obtain plateau pressure. Used to monitor lung condition in mechanically ventilated patients, diagnose ARDS, adjust PEEP, and guide lung-protective ventilation.

Interactive Calculator

Tidal Volume (VT)(mL)

Plateau Pressure (Pplat)(cmH₂O)

PEEP(cmH₂O)

Worked Examples

Normal mechanically ventilated patient

→ 29.4 mL/cmH₂O

Normal-low range — acceptable compliance

ARDS patient

→ 23.75 mL/cmH₂O

Low compliance — consistent with ARDS; consider adjusting PEEP and TV

Healthy patient post-surgery

→ 42.3 mL/cmH₂O

Good compliance — lungs are compliant

Clinical Significance

Falling static compliance indicates worsening pulmonary disease (ARDS progression, edema, pneumonia). Rising compliance may indicate improvement or recruitment. Compliance < 30 mL/cmH₂O suggests severe restrictive physiology. Used to calculate driving pressure (Pplat − PEEP), which independently predicts ARDS mortality.

Memory Tips

💡Static = Stiff? No. High compliance = stretchy/floppy (emphysema). Low compliance = stiff (ARDS, fibrosis, edema)
💡PEEP is subtracted because it's the starting pressure — we only measure compliance during the inflation ABOVE PEEP
💡Compliance = Volume/Pressure — think of a balloon: easy to blow up = high compliance

NBRC Pearls

Plateau pressure reflects alveolar pressure — target ≤ 30 cmH₂O in ARDS
Driving pressure = Pplat − PEEP; target < 15 cmH₂O; strongest predictor of ARDS mortality
ARDS: low compliance (stiff lungs) + low P/F + bilateral infiltrates
Obesity, ascites, and chest wall edema reduce compliance WITHOUT affecting lung compliance per se

← All Formulas

What It Measures

Clinical Significance

Memory Tips

💡Static = Stiff? No. High compliance = stretchy/floppy (emphysema). Low compliance = stiff (ARDS, fibrosis, edema)

💡PEEP is subtracted because it's the starting pressure — we only measure compliance during the inflation ABOVE PEEP

💡Compliance = Volume/Pressure — think of a balloon: easy to blow up = high compliance

NBRC Pearls

Plateau pressure reflects alveolar pressure — target ≤ 30 cmH₂O in ARDS

Driving pressure = Pplat − PEEP; target < 15 cmH₂O; strongest predictor of ARDS mortality

ARDS: low compliance (stiff lungs) + low P/F + bilateral infiltrates

Obesity, ascites, and chest wall edema reduce compliance WITHOUT affecting lung compliance per se