Lung compliance: Measurement

Compliance is defined as change in volume divided by change in pressure (C = dV/dP or ∆V/∆P). Normal compliance in healthy lungs is ~ 0.2-0.3L/cmH₂O. A greater compliance value means that less change in pressure is required to achieve a given change in volume. There are two types of lung compliance that are important clinically: a) static compliance and b) dynamic compliance.

1. Static Compliance: is measured when there’s no gas flow (i.e. during an inspiratory hold maneuver). Mathematically, static compliance is calculated by dividing tidal volume (VT) by the difference in plateau pressure(P_plateau) and positive end-expiratory pressure (PEEP).

C_static = VT/(P_plateau-PEEP)

See image below.

Static lung compliance is the “slope” of the graph above. Restrictive lung disease results in decreased lung compliance meaning greater pressure is required to cause incremental increases in lung volume and the “slope” of the compliance curve flattens out and shifts rightward. Conversely, in emphysema lung compliance is increased so less pressure is required to achieve an increase in volume. Static lung compliance also varies depending on which region of the lung you’re talking about as depicted above.

2. Dynamic Compliance: is measured when air is moving. It can be calculated by dividing tidal volume (VT) by [peak inspiratory pressure(PIP) minus PEEP].

C_Dynamic= VT/(P_PIP –PEEP)

Dynamic lung compliance is decreased with mucus plugging and laryngospasm but static compliance is unchanged. Both static and dynamic compliance are decreased with pneumonia, atelectasis and bronchial intubation. Static and dynamic compliance are both unchanged with pulmonary emboli.

Updated definition 2020:

Compliance is defined as the change in volume that occurs as a result of a change in pressure (C = dV/dP or C = ∆V/∆P). Compliance of a healthy adult lung is ≈ 0.2-0.3L/cmH₂O. An increase in compliance results in increased volume change per unit of pressure change. Clinically, there are two forms of compliance that are significant: a.) Static Compliance and b.) Dynamic Compliance.

Static Compliance: The compliance at a fixed volume when there is no air flow. It can be calculated as:

Cstat = VT/(Pplat – PEEP)

, where Cstat = static compliance, VT = tidal volume, Pplat = plateau pressure, PEEP = positive end expiratory pressure.

Dynamic Compliance: Is the compliance during air movement. It can be calculated as:

Cdyn = VT/(Ppip – PEEP)

, where Cdyn = dynamic compliance, VT = tidal volume, Ppip = peak inspiratory pressure, PEEP = positive end expiatory pressure.