Measurement of FRC

3 techniques

1. Nitrogen wash-out
2. Tracer gas (e.g. helium) wash-in
3. Body plethysmograph

 

Nitrogen wash-out

Patient switch to breathing 100% O2

• Initial alveolar nitrogen concentration noted
• Expired air collected and quantity of total washed-out nitrogen measured

e.g.

• If initial alveolar nitrogen concentration is 80%, and total nitrogen washed out is 4L
• Initial lung volume
  = 4L / 0.8
  = 5L

 

Tracer gas wash-in

A known quantity of tracer gas introduced into lung
--> Concentration then measured
--> Lung volume can be calculated

e.g.

• 50mL of Helium introduced
  --> Final lung concentration of helium is 1%
• Lung volume
  = 50mL / 0.01
  = 5L 

NB:

• Helium is chosen because of its low solubility in blood
• Measurement needs to be made early or dissolving helium will introduce larger error

Body plethysmography

Setup

Subjects sits in an air-tight box

At end of normal expiration
--> Inspiratory effort against a closed shutter (in the mouthpiece)
--> Lung volume increase a little
--> Pressure in airway decrease and pressure in box increase

4 measurements are needed
* Pressure at the mouthpiece measured
* Pressure in the box measured
* Volume of the box measured

Principle

• Double application of Boyle's Law
• Boyle's Law:
  (Pressure x Volume) is constant given same temperature

Calculation

Pressure in the box (and airway) before inspiratory effort
= P

Pressure in the box during inspiratory effort
= P2

Volume in the box
= V

Lung volume (FRC)
= FRC

Increase in lung volume
= V'

Decrease in airway pressure during inspiratory effect
= P'

Thus,

In the box
* PV = P2 (V - V')
--> V' can be obtained

In the lung
* P x FRC = (P-P') x (FRC + V')
--> FRC can be solved and obtained

NB:

• Body plethysmography is the only method that includes gas trapped within the lung distal to closed airways
• Body plethysmography also includes gas in a pneumothorax