What is respiratory 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.
What are the different types of respiratory compliance?
  • Static compliance
  • Dynamic compliance
  • Specific compliance 
What is the difference between static and dynamic compliance?
  • 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)
How does compliance change with lung volume?

Static lung compliance diagram from Rahn et al, 1946

  • Compliance is poor at low and high volumes, but optimal just above the FRC, i.e. in the range of the normal tidal volume
What are the factors which influence compliance?
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 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
How does lung surfactant influence lung compliance?
  • Lung surfactant increases lung compliancecompliance of normal and lavaged lung (Lachmann et al, 1980)
What is hysteresis?
  • "The energy applied to the lung in inspiration is not recovered in expiration. The property of dissipating energy receives the name of hysteresis."
What are the mechanisms which cause hysteresis?
  • Recruitment and derecruitment: Collapsed alveoli have walls which are stuck together and which require added mechanical energy to open. In contrast, well-inflated alveoli are relatively elastic and require relatively little energy to inflate further. Because of this, the pressure-volume relationship of alveoli changes after they have been fully inflated. 
  • The effect of alveolar surface tension: surface tension in a deflated lung is lower than in a fully inflated lung because the molecules of alveolar surfactant are packed closer together, increasing their concentration at the gas-liquid intereface and thereby decreasing surface tension. These phospholipid molecules on the surface of well-stretched alveoli are further apart, which increases the surface tension and makes the lung less compliant.  Thus, after fully inflating the lung, the deflation curve has a lower compliance, i.e. there is little change in volume over a substantial change in pressure
  • Stress relaxation refers to the loss of energy in the lung parenchyma which occurs with stretch. This resembles the classical definition of hysteresis, as the quantity of unrecovered energy which results from something being imperfectly elastic. The imperfect lung stretches, consumes energy, and then wastes it on changing the shape of its collagen and elastin fibres instead of storing it for later release. 
  • Gas absorption during measurement is not really a property of the lung parenchyma itself but rather an artifact of measurement. As mentioned above, measurement of static lung compliance has a certain built-in pause in every step, which allows some of the gas to become absorbed in living systems, leading to an apparent change in volume and pressure.


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