4.2. Measurements
            4.2.8. Flow and volume measurement
4.2.8.2. Measurement of FRC

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

e.g.

 

Tracer gas wash-in

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

e.g.

NB:

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

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:



Table of contents  | Bibliography  | Index