Pulmonary compliance


 * Assistant Editor-in-Chief: Soumya Sachdeva

Overview
Compliance is the ability of the lungs to stretch in a change in volume relative to an applied change in pressure. It is an important measurement in respiratory physiology.

It is the slope of the pressure-volume curve at a particular lung volume i.e. volume change per unit of pressure change (mL/cm H2O) Normal value of compliance of intact lung = 200mLs/cmH2O . Compliance is greatest at moderate lung volumes, and much lower at volumes which are very low or very high.

Calculation
Compliance is calculated using the following equation, where ΔV is the change in volume, and ΔP is the change in pressure:

$$C = \frac{ \Delta V}{ \Delta P} $$

Factors affecting lung compliance

 * 1) Lung volume  - A person with only one lung has half the change in lung volume for a given change in pressure
 * 2) Phase of respiratory cycle - It is slightly greater when measured during deflation than when measured during inflation
 * 3) Effect of gravity - It is less at apices of the lungs as there are more distended alveoli at the apex
 * 4) Surface tension  - If surface tension is high as in deficiency of surfactant, more pressure is required to distend the lungs ,therefore,compliance is     decreased

Specific Compliance
Compliance divided byFRC (in mLs)

Unit = /cmH2O


 * 1) Normal value = 0.05/cmH2O
 * 2) Similar values in both sexs and all ages including neonates
 * 3) Is a measurement of the intrinsic elastic property of the lung tissue

Applied

 * fibrosis is associated with a decrease in pulmonary compliance.
 * emphysema/COPD are associated with an increase in pulmonary compliance.
 * Pulmonary surfactant increases compliance by decreasing the surface tension of water. The internal surface of the alveolus is covered with a thin coat of fluid. The water in this fluid has a high surface tension, and provides a force that could collapse the alveolus. The presence of surfactant in this fluid breaks up the surface tension of water, making it less likely that the alveolus can collapse inward. If the alveolus were to collapse, a great force would be required to open it, meaning that compliance would decrease drastically.