Gas Volumes and Capacities

Question 1

Define closing capacity

Answer 1

Lung volume at which the small airways and alveoli in the dependent lung start to collapse
Closing capacity = residual volume + closing volume

Question 2

Describe the relationship between ageing and closing capacity

Answer 2

Increasing age = loss of elastic recoil = decreased radial traction force keeping small airways open resulting in more collapse of airways
Age 44 supine: closing capacity = FRC
Age 65 erect: closing capacity = FRC

Clinical significance
- During preoxygenation this limits denitrogenation that can occur
- You can give PEEP to keep these airways open
- increasing closing capacity with ageing is the MAIN reason for decreased paO2 with ageing