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 Incoming links: Static versus dynamic lung compliance

Lung compliance measures how much lung volume changes for a change in distending pressure. For a static pressure-volume relation, the local value is

C_{L} = \frac{d V}{d P _{L}}, P_{L} = P_{alveolar} - P_{pleural} .

The relevant pressure is transpulmonary pressure, not alveolar pressure or airway-opening pressure alone. The pleural pressure reference separates the lung's elastic behaviour from that of the surrounding chest wall.

OpenStax connects lung expansion to pressure changes, elastic recoil and pulmonary surfactant, which modifies surface-tension effects.^[ref] A compliant lung needs a smaller pressure increment for the same small volume increment, but that statement alone does not describe recoil, airway patency or overall respiratory function.

A finite pressure-volume ratio is a secant slope, while

d V / d P_{L}

is a local slope. In an original nonlinear example, set

x = V - V_{0}

and

P_{L} - P_{L, 0} = 5 x + 5 x^{3}

when volume is measured in litres and pressure in centimetres of water. Local compliance is

1/ (5 + 15 x^{2})

. It falls from

0.20

x = 0

0.05 L (cm H_{2} O)^{- 1}

x = 1

L. Across that entire one-litre interval, however, the secant compliance is

1/10 = 0.10

in the same units. These are constructed values, not clinical reference ranges.

Static versus dynamic lung compliance distinguishes elastic behaviour from measurements during airflow. A simplified lumped dynamic model adds an airway resistance term proportional to volume-flow rate: pressure depends on both

V

and

R \dot{V}

. A pressure ratio obtained while air is moving can therefore differ from the static elastic slope. Inflation history and hysteresis also require the direction and volume interval to be specified.

In a 1959 primary study, researchers used an intra-esophageal balloon connected to a differential pressure transducer, with a mouthpiece pressure connection, to obtain pressure differences relevant to lung mechanics.^[ref] Such a surrogate pressure measurement has its own positioning and calibration assumptions. Spirometry supplies volume information; volume measurement alone does not establish lung compliance.

Report the pressure references, volume range, flow conditions and measurement history alongside a compliance value. A single number is a property of the specified experiment and state, not an invariant description of every breath.