Compliance (ΔV/ΔP) is the change in lung volume per unit change in pressure; it is a measure of the lung's distensibility and elastic recoil.
In a normal lung, a tidal volume of 500 mL is achieved at about 2.5 cm H2O transpulmonary pressure (static compliance about 200 mL/cm H2O).
Types and Measurement of Compliance
Two types are measured:
Static compliance: from a pressure-volume curve with airflow periodically interrupted so alveolar pressure is zero.
Dynamic compliance: recorded continuously during breathing.
Static compliance is obtained by inspiring maximally to TLC, then expiring slowly to functional residual capacity (FRC).
Lung volume (spirometer) and pleural pressure (esophageal balloon catheter) are recorded.
Airflow is periodically interrupted during this slow expiration so alveolar pressure is zero; the slope of the resulting pressure-volume curve is the compliance.
Dynamic compliance is normally lower than static compliance.
It increases during exercise, sighing and yawning, since larger tidal excursions bring more surfactant to the alveolar surface.
The inflation pressure-volume curve has four stages: