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Get a referral to a medical specialist who can conduct a helium dilution test. Consider the nitrogen washout technique. Have a body plethysmography done for best accuracy.
If your family doctor thinks you have a restrictive lung disease, they will refer you to a respiratory (lung) specialist, also known as a pulmonologist, for further testing. The pulmonologist may conduct a helium dilution test. This inert gas dilution method uses helium to directly identify your residual lung volume. To start the test, you'll exhale normally and then be connected to a closed system containing known volumes of helium and oxygen. Once connected, you breathe in the helium and the amount exhaled is measured. The difference between the two volumes of helium is a pretty accurate estimate of your residual lung volume.  Helium is an inert, colorless, odorless, tasteless gas and is non-toxic to your lungs, so there are no health concerns related to this exam. This technique may underestimate residual lung volume because it measures only the lung volume that communicates with the airways. This can be problematic for patients with severe airflow limitation. You will also need a referral to a pulmonologist to have this test done, which measures the air remaining in your conducting airways. To start the test, you'll exhale normally and then be connected to a spirometer that contains 100% oxygen. You'll then breath in deeply and exhale as strongly as you can, and the spirometer will measure the amount of exhaled nitrogen compared to the entire volume of exhaled air. The halfway point of the percentage of exhaled nitrogen allows the doctor to figure out the amount of gas you expelled, which is equal to the residual lung volume.  Recall that the air we normally breathe in is about 21% oxygen and 78% nitrogen. This test forces you to breathe 100% oxygen and then measures the amount of nitrogen exhaled, a predetermined percentage of which represents residual lung volume. Like the helium dilution technique, nitrogen washout can also underestimate residual lung volume in patients with severely restricted airflow. This very accurate method for measuring residual lung volume uses a plethysmograph, which is an enclosed instrument (a small chamber you sit in) used for recording an organ’s volume change. Once inside an airtight plethysmograph — it sort of looks like a small phone booth — you'll be asked to normally exhale, then inhale against a closed mouthpiece. As your chest wall expands, the pressure within the plethysmograph rises, which is calculated. Then you'll exhale as hard as you can out through the mouthpiece. The difference in pressures represent your residual lung volume.  Body plethysmography uses Boyle's gas law (pressure and volume of a gas have an inverse relationship when temperature is constant) to determine residual lung volume and other lung volumes. Body plethysmography is considered more accurate than gas dilution methods for calculating lung volumes, particularly if the lung is obstructed.