21 - 25 Physiology SBAs for the Primary FRCA
Question 21
Which one of the following is correct with regards to lung compliance?
- Compliance is the change in pressure for a given change in volume
- Compliance is the change in volume for a given change in temperature
- Compliance increases with age
- Compliance is equal for both inspiration and expiration
- Compliance is lowest at FRC
Answer
- Compliance increases with age
Compliance is the change in volume for a given change in pressure - it's a measure of stretchiness.
Compliance increases with age, production of surfactant, emphysema and acute asthma*.
Factors that decrease compliance include fibrosis, oedema, extremes of lung volumes, infection and being in a supine position
Compliance is highest at functional residual capacity.
The equation you need to know is:
1/(thoracic compliance) = 1/(lung compliance) + 1/(chest wall compliance)
Total thoracic compliance is around 100ml per cmH2O.
*This made no sense to me, because surely the lungs are stiffer in acute asthma?
Three things happen in acute asthma that affect the mechanical properties of the lung:
- The lung units become hyperexpanded, with loss of elastic recoil
- The chest wall has increased outward recoil as a result of the chest hyperexpansion
- The inspiratory muscles are working harder
The net result is that for each unit of pressure generated across the alveolar membrane, there is an increased change in volume, and therefore the compliance is higher by definition - it's just very much harder for the poor patient (or anaesthetic SHO) to generate this pressure in the first place.