The 3-Breath Method for Measuring
RV
Refer to the figure below for illustration.
The 3-Breath method is a clever way to measure exactly how much volume is
in a person's
Residual Volume
. Remember that RV cannot be directly
measured via
spirometry
.
Here's how the 3-breath method works:
-
First, measure the subject's
Vital Capacity
using a spirometer.
-
Next, get a gas bag of volume equal to your subject's Vital Capacity.
Fill it with 100% oxygen.
-
Have your subject exhale completely, reducing his lung volume until he
reaches his Residual Volume.
-
Your subject should now inhale the entire contents of the gas bag (his
entire Vital Capacity), and then exhale entirely again. He should breathe
like this 3 full times.
-
The goal of this is to allow the bag of pure oxygen to equilibrate with
the residual lung volume filled with moist air. After 3 breaths of this
kind, the gas in the bag should have about the same mixture as the gas in
the subject's lungs.
-
Measure the concentration of Nitrogen in the gas bag. It should be about
the same as the concentration of Nitrogen in the patient's lungs.
-
Since we know the original volume of the bag, and the original
concentration of gas in the bag, and the final concentration of gas in the
bag, we can now find the original volume (the Residual Volume) of gas in
the subject's lungs.
This method directly measures RV. Note that once you find either
RV or FRC, you can calculate the other. Refer to the figure below for
illustration of this idea:
