Compliance
How much the lung and chest wall expand for a given change in pressure.
Definition
Compliance is the change in lung volume produced by a given change in pressure (C = ΔV/ΔP). Static compliance is measured under no-flow conditions using plateau pressure; it reflects the elastic properties of the lung and chest wall, separate from airway resistance.
Mechanism
A compliant lung expands easily for a small pressure change; a stiff lung requires a large pressure change to achieve the same volume. In ARDS, inflammation, edema, and loss of aerated lung units reduce the number of alveoli available to receive a tidal volume, so the same breath is distributed into a smaller 'baby lung' — measured compliance falls even though the aerated units themselves may be relatively normal. Compliance is calculated from tidal volume divided by driving pressure (Vt / (Pplat − PEEP)) during a no-flow condition.
Visual Physiology — Lung Compliance P-V Curve
Static pressure-volume compliance curve showing the lower and upper inflection points, safe linear compliance zone, and overdistension.
Key Visual Takeaways
- Compliance represents the volume change per unit change in distending pressure (Crs = ΔV / ΔP).
- In ARDS, loss of aerated units shifts the P-V curve downward and rightward, reducing compliance.
- Ventilating between the lower inflection point (recruitment) and upper inflection point (overdistension) minimizes lung injury.
Clinical significance
Falling compliance is one of the earliest bedside signs of worsening lung injury, often preceding a clear change in oxygenation, and a rising compliance is a useful sign of improvement or successful derecruitment avoidance.
Common ICU applications
- Trending static compliance day to day is a simple way to track ARDS severity at the bedside without recalculating full mechanics each time.
- A sudden drop in compliance should prompt a search for a new problem (pneumothorax, mainstem intubation, breath stacking) rather than assuming gradual disease progression.
- Compliance and driving pressure are directly linked — falling compliance for a fixed tidal volume shows up as rising driving pressure.
Related concepts