Lung compliance

Definition of compliance

Compliance

• The slope of the pressure-volume curve at a particular lung volume
  --> i.e. volume change per unit of pressure change (mL/cmH2O)
• Normal value of compliance of intact lung
  = 200mLs/cmH2O
• Lower compliance = more effort of breathing

Specific compliance

= Compliance divided by FRC (in mLs)
* Unit = /cmH2O

• Normal value = 0.05/cmH2O
• Similar values in both sexs and all ages including neonates
• Is a measurement of the intrinsic elastic property of the lung tissue

Dynamic compliance vs static compliance

Static compliance is the compliance measured when there is no gas flow into or out of the lung

Lung compliance and chest wall compliance

Compliance is made up of lung compliance and chest compliance
--> 1/Ct = 1/Cl + 1/Ccw
* i.e. Summation of elastance (=1/compliance)

Thus, 1/Ct = 1/200 + 1/200
--> Ct = 100 mLs/cmH2O

NB:

• Ct = total compliance
• Cl = lung compliance
• Ccw = chest wall compliance

 

 

Factors affecting compliance

Factors affecting compliance:

1. Lung elastic recoil
2. Lung volume
3. Disease

Elastic recoil of the lung

Due to:

• Surface tension in the alveoli
• Stretched elastic fibres in the lung parenchyma

Surface tension accounts for 70% of the elastic recoil

Lung volume

The slope of the P-V curve is not constant across different lung volumes.

At high lung volume
--> Elastic fibres already stretched
--> Greater pressure is required to inflate lung
--> Reduced compliance

At very low volumes
--> Alveoli radius reduced
--> (according to Laplace's Law), pressure required to inflate alveoli is increased
--> Reduced compliance

NB:

• At low lung volumes, surfactant concentration is higher
  --> Reduction in surface tension is greater
• But overall, not sufficient to completely offset the increased pressure
• When alveolar is enlarged, surface tension is higher because of reduced concentration of surfactant
• At FRC, compliance is the lowest
• At the base of the lung, at very low volumes, compliance is even more reduced because of positive intrapleural pressure

Other factors affecting compliance via effect on lung volume

• Posture
• Restriction of chest expansion

Other factors

Conditions that REDUCE compliance:

• Fibrosis
• Pulmonary hypertension/congestion
  --> Increases stiffness of lung
• Alveolar atelectasis
  * e.g. after prolonged period of ventilation
• Reduced surfactant (increased surface tension)
  * e.g. artificial ventilation, prematurity

Conditions that INCREASE compliance:

• Normal ageing (alteration in elastic tissue)
• Asthma
  * Reason unknown

[JW7:p97]

Special case: pulmonary emphysema

Static compliance is increased due to alteration in elastic tissues

Dynamic compliance is reduced due to increased dynamic compression of airway

Diseased lungs

Diseased lungs
--> Time constant for the alveolis are different
--> Units with higher time constants are slow to fill and empty

With higher respiratory rate, the problem worsens
--> units with high time constant hypoventilates
--> less lung units participate in volume changes
--> dynamic compliance reduced.

With collapsed alveoli
--> greatly increased surface tension
--> very high pressure is required to re-open airway/alveoli

Other notes

• Recoil pressure of the lung (transpulmonary) = alveolar pressure - intrapleural pressure
• Recoil pressure of the chest = intrapleural pressure - atmospheric pressure
• Hysteresis: property of any elastic material where the distension force is greater than the recoil force
  --> Takes more effort to stretch the material than you get back when the material recoils.

To be added later

Measurement of compliance see Nunn p54

Elastic forces - Lung vs chest wall ##20050121(10)