Explain respiratory compliance and outline the factors that affect it.
Answers were generally well structured. Better answers described lung and chest wall compliance and the pressures which are used to calculate compliance. Better answers displayed an understanding of dynamic, static and specific compliance and provided a reasonably comprehensive list of the physiological factors affecting chest and lung compliance.
- Respiratory compliance is defined as the change in lung volume per unit change in transmural pressure gradient. It is usually about 100ml/cm H2O.
- Static compliance is defined as the change in lung volume per unit change in pressure in the absence of flow. It is composed of:
- Chest wall compliance (usually 200ml/cm H2O.
- Lung tissue compliance (also usually cm H2O.)
- Dynamic compliance is defined as the change in lung volume per unit change in pressure in the presence of flow. Its components are
- Chest wall compliance
- Lung tissue compliance
- Airway resistance (which makes it frequency-dependent)
- Specific compliance is compliance that is normalized by a lung volume, usually FRC. It is used to compare compliance between lungs of different volumes (eg. child and adult)
Factors which affect compliance can be divided into chest wall factors and lung factors:
|Lung compliance||Chest wall compliance|
Increased lung compliance
Increased chest wall complance
Decreased static lung compliance
Decreased dynamic lung compliance
Decreased chest wall compliance
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