lungcompliance

Lung compliance, or respiratory compliance, is a measure of the lung’s distensibility. It is defined as the change in lung volume produced per unit change in transpulmonary pressure. In adults, the respiratory system’s static compliance is commonly about 0.2 liters per centimeter of water (approximately 200 mL per cm H2O). Compliance is the inverse of elastance, and it reflects how readily the lungs and chest wall expand during inspiration.

Compliance arises from two main components: the elastic properties of the lung tissue and the chest wall. The total compliance of the respiratory system is influenced by both and is mathematically the result of their interaction: 1/Ctotal = 1/Lung compliance + 1/Chest wall compliance. Surfactant within the alveoli increases compliance by reducing surface tension, whereas surface tension tends to decrease it. Conditions that stiffen the lung parenchyma, such as fibrosis or edema, reduce compliance, while conditions that destroy elastic recoil, such as emphysema, can increase it. Age, body position, and pleural pressures can also affect overall compliance.

Measurement in clinical settings often differentiates static and dynamic compliance. Static compliance is calculated from the change in volume divided by the plateau pressure minus positive end-expiratory pressure (PEEP): Cstatic = ΔV/(plateau − PEEP). Dynamic compliance uses peak inspiratory pressure: Cdyn = ΔV/(PIP − PEEP). In mechanically ventilated patients, low compliance implies the need for higher pressures to deliver the same tidal volume and can increase the risk of barotrauma; high compliance can accompany hyperinflation risks in diseases like emphysema.

See also: respiratory mechanics, ventilation strategies, alveolar surface tension.