Lung compliance, also known as pulmonary compliance, is the ability of the lungs to stretch and expand. It is defined by the relativity of the change in volume to the change in pressure. This is an important measure of respiratory function, and a consistent change in the level of compliance often indicates the presence of a disease state. Pulmonary compliance is the greatest when the volume of the lungs is at a moderate level.
In basic terms, lung compliance is the ability of the lung tissue and the alveoli, the small sacs in the lung where gas exchange takes place, to expand when inhaling. There are two types of compliance that are measured: static and dynamic. Static compliance is the measurement when the lungs are not moving. Dynamic compliance is the measurement of the lungs when in motion, at the end of inspiration and expiration.
If a person was to fill the lung with pressure and then not move it, the pressure would eventually decrease; this is the static compliance measurement. Dynamic compliance is measured by dividing the tidal volume, the average volume of air in one breath cycle, by the difference between the pressure of the lungs at full inspiration and full expiration. Static compliance is always a higher value than dynamic.
Lung compliance can indicate when there are issues with the lungs. A decreased compliance might show a condition such as fibrosis, which is a formation of excess tissue that inhibits movement. Increased compliance can indicate a state of disease where there is degeneration of tissue that causes the lungs to have to work harder to expand, such as emphysema. With emphysema, the tissue damage means that it is easier to inhale, as there is less resistance, but it is harder to exhale.
Another key component that contributes to compliance is pulmonary surfactant. A surfactant is a substance that decreases surface tension, and in the case of the lungs, there is a thin, watery liquid that coats the alveoli. This liquid has a very high surface tension and, without the surfactant, the surface tension would cause the alveoli to collapse.
There are cases of lung compliance pathology caused by problems with the surfactant. Infant respiratory distress syndrome (IRDS), a congenital defect that causes a lack of surfactant, is one example. IRDS is usually seen in premature babies born between 26 and 32 weeks of gestation and leads to pulmonary and cardiac issues, including decreased lung function. Due to the decreased surfactant, these children have difficulty breathing, but if caught in time, this condition is treatable.
