Closing capacity
Closing capacity
= the lung volume at which the small airways in (usually the dependent part of) the lung first start to close
--> Impairs gas exchange and increase venous admixture
--> Decrease PaO2
NB:
- Closing capacity
= RV + closing volume
Variation of closing capacity
Closing capacity increases with age
FRC does not change with age
- Young subject, closing capacity = 10% of VC
- In erect position, closing capacity = FRC = 40% of VC at 66 y.o.
- In supine position, closing capacity = FRC at 44 y.o.
- In neonates, lung elastic recoil is reduced
--> More airway closure
--> Closing capacity > FRC
--> Reduced PaO2
Measurement of closing volume
Closing volume is measured with single breath nitrogen test
- A single breath of 100% O2 from residual volume
- Slow expiration with a rapid nitrogen analyzer
- Late in expiration when airway closure starts to occur, expired [N2] starts to rise about the plateau.
Closing volume = the volume expired, from the start of this rise to the end of maximal expiration
Rationale
When inspiring from residual volume
Airway is closed in the base
- Because initial part of the inspired gas (which is dead space gas, with high [N2]) goes preferentially to apex,
AND,
- Because apical alveoli were more expanded at the start and expands less during inspiration, [N2] is less diluted
--> Thus apical [N2] is higher
When basal airway starts to close at closing capacity
--> The portion of gas that is coming from apical alveoli increases
--> Expired [N2] starts to rise