3 techniques
Patient switch to breathing 100% O2
e.g.
A known quantity of tracer gas introduced into lung
--> Concentration then measured
--> Lung volume can be calculated
e.g.
NB:
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
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: