Briefly describe the factors that affect lung compliance.

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College Answer

Main points/concepts expected in answer.

  • Surfactant • increases lung compliance • decreases surface tension at alveolar air -water- interface •prevents small alveoli from collapsing •accounts for most of hysteresis in intact lung
  • Lung elastic recoil • lung compliance changes in disease states
  • Lung volume • lung compliance greatest around FRC • lung compliance reduced at low and high lung volumes • gravitational effects on regional lung compliance
  • Pulmonary blood volume • pulmonary venous congestion reduces lung compliance
  • Lung size • specific compliance = lung compliance / FRC
  • Dynamic lung compliance • influenced by airways resistance • lung compliance measured during normal breathing • less than static lung compliance • frequency dependence


This brief but comprehensive examiner comment should serve as some sort of benchmark for what the rest of them should look like. Even though the pass rate for this question was 14%, the examiners did not waste time lamenting the intellectual poverty of their trainees. They just gave you the answer they expected, and left you to lambast yourself for getting it wrong. 

There is little one can do to improve on the college answer. Using the Part One compliance section as a guide, the following tabulated alternative was produced:

Factors which Affect Respiratory Compliance
Lung complianceChest 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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