Chest wall compliance: Calculation

Chest wall compliance is the opposite of elasticity, and elasticity is the tendency of lung tissue to return to its original (or relaxed) position after an applied force has been removed. Compliance is essentially the ability of the lung tissue to “absorb” the same applied force, which generally results from a change in intrathoracic pressure.

Chest wall compliance can be calculated by this formula:

Compliance = Δ V / Δ P

Where change in volume is in liters and change in pressure is in cm H₂0.

There are two different types of compliance: static and dynamic. Static compliance is a measured during plateau (rather than peak inspiratory) pressure. PEEP should be subtracted from plateau inspiratory pressure in making this calculation. Because the pressure-volume curve (see below) is not linear during inspiration and expiration due to changes in the lung tissue, Dynamic compliance varies and is a calculated with a measurement of tidal volume at a given intrathoracic pressure during which there is airflow through the lungs at any point during inspiration or expiration.

Lungs with low compliance are stiff lungs and will require much greater pressure to reach a given volume compared to lungs that have high compliance. There are several factors that affect lung compliance including alterations in the ribs (ie. fractures), ossification of the costal cartilage, obesity, muscular or neural changes to intercostal muscles (ie. paralysis or strain/pain), position (prone/supine), structural abnormalities (ie. kyphosis or scoliosis), increased intraabdominal pressure, and age.

Normal chest wall compliance in adults is approximately 100-200ml/cm H₂O. However, children have far lower chest wall compliance at 2.5-5.0ml/cm H₂O.

Another way to determine chest wall compliance is graphically using a pressure volume curve. The slope of the line in the is equal to the lung compliance.