Definition of Lung Compliance
Lung compliance is the measure of the lung's ability to expand in response to pressure changes, expressed as the volume change per unit of pressure change (ml/cmH₂O), representing the elastic recoil properties of the respiratory system. 1
Basic Definition and Normal Values
- Static compliance measures lung elasticity during conditions of no airflow, such as during an inspiratory hold maneuver on mechanical ventilation, to eliminate the resistance component 1
- Normal lung compliance in healthy adults ranges from 1.2 to 2.0 ml/cmH₂O per kilogram body weight 1
- The measurement is typically performed during zero-flow conditions to accurately assess the elastic properties without interference from airway resistance 1
Lung Compliance in COPD
In COPD patients, lung compliance is typically increased (lungs become more compliant/floppy) due to emphysematous destruction of alveolar walls and loss of elastic recoil. 2
Pathophysiological Changes in COPD
- Most airflow limitation in COPD results from a combination of mechanical obstruction in small airways and loss of pulmonary elastic recoil due to emphysema 2
- The reduction of alveolar attachments around small airway walls makes these airways more likely to collapse during expiration, despite increased overall lung compliance 2
- Centriacinar emphysema (common in smokers) is associated with more small airways disease and less loss of elastic recoil for any given level of respiratory function compared to panacinar emphysema 2
Clinical Measurement Considerations in COPD
- Static and dynamic compliance measurements remain primarily research tools rather than routine clinical assessments, as they have not been shown to be of practical clinical value in COPD management 2
- While dynamic compliance may be abnormal even before detectable changes in FEV₁, neither static nor dynamic compliance measurements are easy to perform or routinely used outside research settings 2, 1
Lung Compliance in Pneumonia
In pneumonia, lung compliance is decreased (lungs become stiffer) due to alveolar consolidation, edema, and reduced aerated lung volume. 1
Mechanisms of Reduced Compliance in Pneumonia
- Alveolar edema and surfactant dysfunction reduce the volume of aeratable lung, significantly decreasing compliance 1
- Consolidation creates stiffened lung tissue that requires more pressure for a given volume change 1
- In severe cases like ARDS with pneumonia, static compliance may be reduced to less than one-fourth of normal values (approximately 20 ml/cmH₂O) 1
Clinical Implications
- Serial measurements of static compliance can track disease progression and response to treatment in pneumonia patients 1
- Decreased compliance indicates the need for higher ventilator pressures to achieve adequate tidal volumes in mechanically ventilated patients 1
Key Distinctions Summary
| Parameter | Healthy Adult | COPD | Pneumonia |
|---|---|---|---|
| Compliance | 1.2-2.0 ml/cmH₂O/kg [1] | Increased (floppy lungs) [2] | Decreased (stiff lungs) [1] |
| Primary Mechanism | Normal elastic recoil [1] | Loss of elastic recoil, emphysema [2] | Consolidation, edema [1] |
| Clinical Significance | Normal lung expansion [1] | Airway collapse on expiration [2] | Requires higher pressures [1] |
Common Pitfalls
- Do not confuse increased compliance in COPD with better lung function—the loss of elastic recoil actually contributes to airflow limitation through airway collapse 2
- Measurements should be made during zero-flow conditions; failure to do so will include resistance components and yield inaccurate compliance values 1
- In spontaneously breathing patients, esophageal pressure measurements may be needed to accurately determine transpulmonary pressure for true compliance calculation 1
- Despite clinical value, static compliance measurement is not routinely performed outside research settings due to technical challenges 1