3. Physiology
        3.13. Respiratory
            3.13.2. Ventilation and perfusion
           Mechanics of breathing Lung compliance

Lung compliance

Definition of compliance


Specific compliance

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

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




Factors affecting compliance

Factors affecting compliance:

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

Elastic recoil of the lung

Due to:

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


Other factors affecting compliance via effect on lung volume

Other factors

Conditions that REDUCE compliance:

Conditions that INCREASE compliance:


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

To be added later

Measurement of compliance see Nunn p54

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



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