Question 3

Define respiratory compliance, include its components and their normal values (25% marks). Explain the factors that affect respiratory compliance (75% marks).

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College Answer

This question covers a core principle of respiratory physiology and would be expected to have a high pass rate. Most candidates were able to provide a concise definition and distinguish between the different types of compliance. The imprecise use of terminology often created the impression of a lack of fundamental understanding of this key concept. Candidates are encouraged to be accurate and concise in their definitions. A lack of detail in describing the relevant components of compliance and the factors that influence it, immediately limited the capacity of some candidates to achieve an adequate score. Most candidates provided less than half of these factors and only provided a list rather than explaining how compliance was impacted. Marks were maximised by dividing the impacts into those that altered lung compliance versus those that impacted on chest wall compliance, and the better candidates explained how and why compliance was affected. Confusion often arose from the imprecise use of arrows with the result that candidates frequently demonstrated an incorrect fact in relation to the direction of the arrow. Candidates are reminded to take care when using abbreviations or arrows to ensure they are not relying on the examiner to interpret a cause and effect relationship.


  • 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 volume divided by change in pressure, measured during normal breathing, between points of apparent zero flow at the beginning and end of inspiration. 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:

Factors which Affect Respiratory Compliance
Lung compliance Chest wall compliance

Increased  lung compliance

  • Lung surfactant
  • Lung volume: compliance is at its highest at FRC
  • Posture (supine, upright)
  • Loss of lung conective tissue associated with age
  • Emphysema

Increased chest wall complance

  • Ehler-Dahlos syndrome and other connective tissue diseases associated with increased connective tissue elasticity
  • Rib resection
  • Cachexia
  • Flail segment rib fractures
  • Open chest (eg clamshell)

Decreased static lung compliance

  • Loss of surfactant (eg. ARDS)
  • Decreased lung elasticity
    • Pulmonary fibrosis
    • Pulmonary oedema
  • Decreased functional lung volume
    • Pneumonectomy or lobectomy
    • Pneumonia
    • Atelectasis
    • Small stature
  • Alveolar derecruitment
  • Alveolar overdistension

Decreased dynamic lung compliance

  • Increased airway resistance (eg. asthma)
  • Increased air flow (increased resp rate)

Decreased chest wall compliance

  • Structural abnormalities
    • Kyphosis / scoliosis
    • Pectus excavatum
    • Circumferential burns
    • Surgical rib fixation
  • Functional abnormalities
    • Muscle spasm, eg. seizure or tetanus
  • Extrathoracic influences on chest/diaphragmatic excursion
    • Obesity
    • Abdominal compartment syndrome
    • Prone position


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