Question 14

Define respiratory compliance (20% of marks). Describe the factors that affect it (80% 
of marks).

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College Answer

This question was generally well answered with good structure. 


It is unclear how this college comment is an improvement on the absence of any comment. And if the question was answered so well, why was the pass rate only 54%? Fortunately, in their answer Question 1(p.2) from the second paper of 2008, the examiners listed their expectations of the essential components of an answer. 


  • 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 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 connective tissue associated with age
  • Emphysema

Increased chest wall compliance

  • 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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