We went over the pulmonary function tests (PFT) the other day and since then I have found a letter that Stevo posted when I was a newbie that explains a bit more about a PFT and Spirometry test. I hope it helps some of you and it is informative for all of you.
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Pulmonary Function Tests can be confusing, especially understanding what test is measuring which lung function. I hope I can clear this up a little, so, I want to share what I have learned about these laboratory studies. There are three basic types of measurements needed to get a complete picture of lung function: volume, ventilation, and diffusion. These test results are all interpreted as a percentage of predicted results based on specific criteria.The four criteria are age, sex, race, and height.
Lung volume measurements measure the volume of air in your lungs at different
points in your breathing. A spirometer determines your lung capacity and your forced expiration both by measuring volume. These are the most frequently used to track your general condition because COPD directly effects your lung volume.
The other two types of lung function to be measured are ventilation and gas exchange. Ventilation is a dynamic test which measures resistance and airflow. Resistance must be measured in a plethysmograph which is that box with a chair in which you are enclosed. Your residual volume (the air that does not exchange
regularly) is also partially determined by resistance.
Think of ventilation as being the pressure you need to exert to move the air in your lungs. This is the test where you huff and puff quickly in and out through the tube. Although the spirometer also measures airflow, it is only in terms of volume not resistance.
Diffusion is a measurement of gas exchange which is how well you get oxygen into the blood and carbon dioxide out of it. This test is also done in the 'body box' by injecting a small amount of CO (carbon monoxide) gas into your breath and measuring what comes back out.
So, the spirometer measures volume in terms of capacity and airflow. Spirometry is usually what is done in the doctors office. The plethysmograph measures resistance and diffusion. Plethysmography is usually done in a lab at the hospital.
It is partially through these tests that your doctor can determine whether your lung disease is restrictive or obstructive.
It is also through these tests that the progress of your condition and its treatment is determined. Make your doctor explain the results of these tests to your satisfaction and ask for a copy for your own records. You pay them for that service.