From the Guidelines
Lung compliance is lowest at extreme lung volumes due to the mechanical properties of the respiratory system, which are significantly influenced by the changes in pleural pressure and transpulmonary pressure, as described in the study by experts in intensive care medicine 1. The mechanical properties of the respiratory system at extreme lung volumes are key to understanding why compliance is lowest at these points. At very low lung volumes, near residual volume, the elastic recoil of the chest wall and the increased surface tension forces due to alveolar collapse make it difficult to expand the lungs, thus requiring more pressure for a given volume change. On the other hand, at very high lung volumes, near total lung capacity, the lung tissue and chest wall are stretched to their limits, with collagen fibers becoming taut and resisting further expansion.
Key Factors Influencing Compliance
- The pressure-volume curve of the respiratory system, which forms an S-shape, indicates that the steepest portion (highest compliance) occurs at normal tidal breathing volumes, while the flatter portions at both extremes represent areas of low compliance where more pressure is needed to achieve volume changes.
- Changes in pleural pressure (Ppl) and transpulmonary pressure (TP) play a crucial role in determining lung volume and compliance, as noted in the study 1.
- The effects of mechanical ventilation on hemodynamics, including the impact of positive end-expiratory pressure (PEEP) on venous return and cardiac output, further highlight the complexities of lung compliance at extreme volumes 1.
Clinical Implications
- Understanding the factors that influence lung compliance at extreme volumes is essential for managing patients with respiratory conditions, such as acute respiratory distress syndrome (ARDS), where lung compliance is significantly reduced 1.
- The use of PEEP and other ventilatory strategies must be carefully considered to optimize lung recruitment while minimizing the risk of overdistension and decreased compliance 1.
- The interdependence of the right and left ventricles, as well as the impact of increased pulmonary vascular resistance on right ventricular afterload, must also be taken into account when managing patients with compromised lung compliance 1.
From the Research
Lung Compliance at Extreme Volumes
Lung compliance is the ability of the lungs to expand and fill with air in response to pressure changes. It is an important factor in respiratory health, and its measurement can provide valuable insights into lung function.
- Compliance is typically measured as the change in lung volume per unit change in pressure.
- At extreme lung volumes, compliance is often lower due to the physical properties of the lung tissue and the airways [ 2, 3 ].
Factors Affecting Compliance
Several factors can affect lung compliance, including:
- The elasticity of the lung tissue and airways [ 4 ]
- The surface tension of the surfactant lining the alveoli [ 5 ]
- The presence of any lung disease or injury, such as acute respiratory distress syndrome (ARDS) [ 6 ]
Compliance at Low Volumes
At low lung volumes, the lungs are more prone to collapse, and the airways are narrower, which can reduce compliance [ 3 ].
- This can lead to increased resistance to airflow and reduced gas exchange.
- In patients with lung disease, such as ARDS, low lung volumes can exacerbate these problems [ 6 ].
Compliance at High Volumes
At high lung volumes, the lungs are more fully expanded, and the airways are wider, which can increase compliance [ 2 ].