Compliance

Define compliance (static, dynamic and specific), its measurement, and relate this to the elastic properties of the respiratory system.

Compliance is the change in volume for a given a change in pressure
Compliance is measured in ml.cmH₂O^-1.
It occurs due to the tendency of a tissue to resume its original position after removal of an applied force
It is the inverse of elastance, which is the force at which the lung recoils for a given distension
A decreased compliance means the transpulmonary pressure must change by a greater amount for a given volume, which increases elastic work of breathing

Compliance of the Respiratory System

Compliance of the respiratory system is a function of both lung and chest wall compliance:
${1 \over C_T \ } = {1 \over C_L} + {1 \over C_W \ }$ .
The curve is not linear as compliance varies with lung volume In the normal range however, (-5 to -10cmH₂O) compliance of the lung and chest wall independently is typically stated as ~200ml.cmH₂O^-1.
- Compliance of the respiratory system as a whole is therefore ~100ml.cmH₂O^-1

Lung compliance is calculated form the alveolar-intrapleural pressure gradient
Chest-wall compliance is calculated from the intrapleural-ambient pressure gradient
Total compliance is calculated from the alveolar-ambient gradient

Measuring ambient and alveolar pressure is straightforward, as is calculating compliance of the respiratory system
- Alveolar pressure is measured by taking a plateau pressure
Separating lung and chest wall compliance requires measurement of intrapleural pressure
This is performed by measuring oesophageal pressure (using a balloon) with an open glottis, as oesophageal pressure approximates intrapleural pressure.

Measurement of compliance of each system individually determines what proportion of plateau pressure is distributed to each
- If the lung is significantly less compliant than the chest wall, a greater pressure is required to distend the lung
- Therefore, the alveolar-intrapleural gradient will be much greater than the intrapleural-ambient gradient
- This can be expressed by the equation:
  $\Delta P_{Pl.} = P_{Plat.} \times {C_L \over C_L + C_W }$

Static compliance is the compliance of the system at a given volume when there is no flow
Therefore there is no pressure component due to resistance
A static compliance curve is made by measuring the pressure across a range of lung volumes, with patient taking incremental breaths
Static compliance is a function of:
- Elastic recoil of the lung
- Surface tension of alveoli

Dynamic compliance is the compliance measured during respiration, using continuous pressure and volume measurements
Therefore, dynamic compliance includes the pressure required to generate flow by overcoming resistance forces
- This means it is also a bit of misnomer
Dynamic compliance is always less than static compliance, as there will always be a degree of airway resistance
Dynamic compliance is a function of respiratory rate
In normal lungs at normal respiratory rates it approximates static compliance.
Reduced in in lung units with unequal time constants at high respiratory rates
- Due to incomplete filling of alveoli - the portion of pressure that is used to overcome airways resistance is therefore proportionally greater

Specific compliance is the compliance per unit volume of lung, expressed as:
$C_S = {C_{Tot} \over FRC}$

In general, hysteresis refers to any process where the future state of a system is dependent on its current and previous state
Specific to the lung, it means the compliance of the lung is different in inspiration and expiration
There is hysteresis in both static and dynamic curves:
- In dynamic compliance curves:
  Airways resistance is a function of flow rate. Flow rate (therefore resistance) is maximal at the beginning of inspiration and end-expiration.
- In static compliance curves:
  There is no resistive component. Hysteresis is due to viscous resistance of surfactant and the lung.

Respiratory system compliance can be affected by changes to either lung or chest wall compliance, and can be increased or decreased.

Chest wall restriction/structural abnormalities
- Obesity
- Spastic paralysis of chest wall musculature
- Ossification of costal cartilages
- Kyphosis/scoliosis
- Scarring/constriction (e.g. circumferential burns)
Position
- Prone (60% reduced compliance)/supine
  This is due to the effect of position on lung volume.

Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
Kenny JE. Heart-Lung Interaction Lecture Series. From heart-lung.org.

Last updated 2021-08-23