Pulmonary compliance

Pulmonary compliance (or lung compliance) is a measure of the lung's ability to stretch and expand (distensibility of elastic tissue). In clinical practice it is separated into two different measurements, static compliance and dynamic compliance. Static lung compliance is the change in volume for any given applied pressure. Dynamic lung compliance is the compliance of the lung at any given time during actual movement of air.

Low compliance indicates a stiff lung (one with high elastic recoil) and can be thought of as a thick balloon – this is the case often seen in fibrosis. High compliance indicates a pliable lung (one with low elastic recoil) and can be thought of as a grocery bag – this is the case often seen in emphysema. Compliance is highest at moderate lung volumes, and much lower at volumes which are very low or very high. The compliance of the lungs demonstrate lung hysteresis; that is, the compliance is different on inspiration and expiration for identical volumes.

Pulmonary compliance is calculated using the following equation, where ΔV is the change in volume, and ΔP is the change in pleural pressure:

For example, if a patient inhales 500 mL of air from a spirometer with an intrapleural pressure before inspiration of −5 cm H₂O and −10 cm H₂O at the end of inspiration. Then:

Static compliance represents pulmonary compliance during periods without gas flow, such as during an inspiratory pause. It can be calculated with the formula:

where P_plat = plateau pressure. P_plat is measured at the end of inhalation and prior to exhalation using an inspiratory hold maneuver. During this maneuver, airflow is transiently (~0.5 sec) discontinued, which eliminates the effects of airway resistance. P_plat is never bigger than PIP and is typically < 3-5 cm H₂O lower than PIP when airway resistance is not elevated. PEEP is positive end-expiratory pressure.

Dynamic compliance represents pulmonary compliance during periods of gas flow, such as during active inspiration. Dynamic compliance is always lower than or equal to static lung compliance because PIP − PEEP is always greater than P_plat − PEEP. It can be calculated using the following equation, where C_dyn = Dynamic compliance; V_T = tidal volume; PIP = Peak inspiratory pressure (the maximum pressure during inspiration); PEEP = Positive End Expiratory Pressure:

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