• Know how to measure RV and FRC using the helium dilution method
RV = residual volume = volume left in lungs after maximum expiration
FRC = Functional residual capacity= volume remaining in the lungs after a normal, passive exhalation.
the values stated and described above cannot be measured by using a spirometer
instead we use a different method
a subject is asked to fully exhale all the air in their lungs as far as they can go, before inhalation they are connected to a spirometer of a specified volume which contains a known concentration of inert marker gas (e.g. He) with th erest being oxygen so for example 1% concentration of He (C1) coupled with 99% oxygen for the full system 100%
the subject breathes normally at this point and the air that was inside the subjects lungs causes dilution. (remember that within a whole closed system the same concentration will be present anywhere in it) so when the equilibrium is reached the new concentration of He is measured (C2) as it will have been diluted by the air that was left within the patients lungs at the start of the procedure
RV can now be calculated
RV = (C1/C2) -1 then X this number by 5 (vol of spirometer in this example)
because 5L ^^ is the volume of the spirometer
to put values on it
say C1 = 1 & C2 = 0.78