Abstract
Compliance is defined as the change in lung volume produced by a unit change in transpulmonary pressure. Lung compliance is represented by the gradient of the pressure–volume curve.
What is lung compliance?
Compliance is defined as the change in lung volume produced by a unit change in transpulmonary pressure. Lung compliance is represented by the gradient of the pressure–volume curve:
Essentially, compliance is the property that determines the volume by which the lungs expand when pressure is applied to them: either negative pressure (as in spontaneous ventilation) or positive pressure (as in intermittent positive-pressure ventilation). For a spontaneously breathing patient:
When compliance is high, the respiratory muscles only need to generate a small transpulmonary pressure to achieve inspiration to VT. However, as less work is done in expansion, so too is less work stored as elastic potential energy. Therefore, exhalation becomes more difficult as there is less elastic recoil of the lungs.
When compliance is low, a high transpulmonary pressure is required to expand the lung to the same VT: the respiratory muscles must then work harder during inspiration and respiratory failure may ensue.
Normal tidal breathing starts from the functional residual capacity (FRC), the rest point where inward elastic recoil is equal and opposite to the force tending to spring the thoracic cage outwards. Conveniently, the lungs are at their most compliant at FRC, which means that the work of breathing at rest is minimal:
For a typical patient at FRC, compliance is 200 mL/cmH2O.
Therefore, at FRC, a VT of 500 mL is achieved with a transpulmonary pressure of just 2.5 cmH2O.
What is respiratory compliance? How does it differ from lung compliance?
Respiratory compliance refers to the compliance of the whole lung–chest unit. It is made up of two components:
Lung compliance;
Thoracic cage compliance.
Key equation: respiratory compliance
Respiratory, lung and thoracic cage compliances are mathematically related:
where RC is respiratory compliance, LC is lung compliance and TCC is thoracic cage compliance. Typical values for both lung and thoracic cage compliance are 200 mL/cmH2O. Therefore, a typical value for respiratory compliance is 100 mL/cmH2O. Thus as compliances in series add as inverses, the overall compliance is always less than the sum of its parts.
Which factors affect lung and thoracic cage compliance?
Thoracic cage compliance is affected by:
Chest wall shape, including the spine and rib cage;
Muscle tone.
Lung compliance is broadly affected by two factors:
Elastic recoil of the lung connective tissue;
Surface tension at the air–fluid interface in the alveoli.
Outline some clinical situations in which respiratory compliance is increased or decreased.
Respiratory compliance may be affected by both physiological and pathological factors.
Causes of increased respiratory compliance include:
– Emphysema, in which there is destruction of lung elastic tissue.
– Advancing age, in which there is degeneration of lung elastic tissue, similar to mild emphysema.
– Neuromuscular conditions: decreased muscular tone results in an easier expansion of the thoracic cage, as occurs in motor neurone disease.
Causes of decreased respiratory compliance include:
– Posture: the lung is generally less compliant when supine.
– Pregnancy: lung compliance is relatively unaffected, but thoracic cage compliance is reduced in late pregnancy.
– Fluid within the alveoli or lung interstitium, exemplified by pneumonia and pulmonary oedema. Owing to the effects of surface tension, fluid-filled alveoli require a much higher transmural pressure to expand than aerated alveoli do. Consequently, compliance is significantly reduced.
– Atelectasis: collapsed alveoli (i.e. alveoli with small radii) require a much higher transmural pressure than larger alveoli do to overcome surface tension forces (see Laplace’s law below).
– Pulmonary hypertension, in which the pulmonary capillaries are engorged with blood. This hinders alveolar enlargement, thus decreasing lung compliance.
– Pulmonary fibrosis: the lung interstitium becomes stiff and less easily distensible.
– Extremes of lung volume: at high lung volume, compliance is reduced because the elastic fibres are stretched to their limit. At low lung volume, lung compliance is reduced as a result of atelectasis.
– Obesity: thoracic cage compliance is reduced due to the increased weight of the chest wall opposing thoracic expansion.
– Chest wall deformity/rigidity: hinders the expansion of the thoracic cage, such as in kyphoscoliosis and ankylosing spondylitis.
How does surfactant increase lung compliance?
Surface tension is caused by forces of attraction between water molecules. These forces act to minimise the air–liquid interface by causing the water to form a spherical droplet. As the inner surface of the alveolus has a thin layer of fluid, an inward force – the surface tension – is created as the water tends to form a droplet. Opposing this inward force of surface tension is the alveolar transpulmonary pressure. As alveoli are approximately spherical, they obey Laplace’s law.
Key equation: Laplace’s law
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