When I was 14, after performing a pulmonary function test (PFT), I had a hundred questions for the respiratory therapist who performed the test. After I basically forced him to tell me, he said, “You have about a 35% lung function.”
“Cool” I said. “What does that mean?”
So, what is a PFT? Basically, it’s a test where you breath into a mouthpiece to a device called a spirometer. The spirometer measures your lung function, and helps your doctor
determine if you have asthma and how severe it is. This test is also called spirometry.
There are a lot of tests you do, yet the most important in diagnosisng asthma is performing a forced vital capacity (FVC). In this test where you take in a breath as deep as you can, and then blow out as hard as you can until you can’t exhale any more. While you’re performing the FVC you’ll note some pretty cool loops on the computer, from which the following calculations are made:
PEFR: Peak Expiratory Flow Rate. This is the maximum flow that can be generated with a forced exhalation. This is simlar to what is obtained when you blow into your peak flow meter, but more accurate and reliable.
FEV1: This is a test that measures the amount of exhaled air during the first second of the FVC. This number cannot be faked, and is the most important value for diagnosing asthma.
FEV1/FEV6: This test is used as a substitute for FEV1 in adults who have significant air trapping and who get “light headed” while trying to forcibly do spirometry.
4. FEV1/FVC: This may be a more accurate measure of severity of asthma in children as compared with FEV1.
So, what use are the above measurements? To answer this question, we must have a basic definition of what asthma is. Therefore, asthma is a reversible obstructive airway disease.
Also, before you take the test, the RT will estimate your predicted normal results for the values listed above based on your height and weight.
Airflow obstruction: If your actual FEV1 is less than 80% of your predicted FEV1, you are considered to have airflow obstruction. And your lung function will be considered to be 80%.
If you happen to be a hardluck asthmatic as I was as a kid, and your FEV1 is 35%, then you are considered to have a 35% lung function.
So that’s the first component of asthma. Now we must consider reversibility.
Reversibility: After you do your initial sequence of tests, you will take a breathing treatment with the bronchodilator of your doctor’s choice. In most cases it will be an Albuterol breathing treatment.
If your FEV1 increases by 10-15% within 15 minutes following a bronchodilator, this is indicative of airflow obstruction that is reversible.
That’s basically all you need to know about PFT results. If you have obstruction and reversibility, there’s a good likelihood you have asthma.
Note here that low VC, PEFR, FEV1/FEV6 and FEV1/FVC as compared with your predicted are also indicative of airflow obstruction. And if these improve following bronchodilator therapy you have the reversibility factor.
If your doctor suspects you might have asthma, yet your initial PFT results are normal, he may have you undergo bronchial provocation (also called a challenge test). This is where the RT will try to cause you to have an asthma attack.
There are many ways he may do this, but the most common are either having you run on a treadmill or having you inhale methacholine. If you do this and your FEV1 is now less than 80% of your predicted, and this is reversible after using a bronchodilator, you may have asthma.
Still, you must keep in mind that PFT results are not absolute. There really is no one single test that says, “you have asthma.” Although most asthma experts believe PFT testing, particularly the FEV1 results, aren the best indicator that you probably have asthma.
Thus, If your doctor suspects you have asthma, you should have a PFT done to confirm his suspicion.
The only exception here would be little children who are less likely to cooperate with the test, and the results may not be accurate. In many cases, a PFT test cannot be completed until a child is at least five years old.
In this case, a doctor will have to use other means to diagnose asthma, such as patient and family history.
Another neat thing your physician can do with PFTs is determine if you have exercise induced asthma, and what medicine works best as a pre-treatment to prevent EIA. Likewise, follow-up PFTs can help your doctor monitor the course of your asthma over time, and how well you are responding to your current medicines.
As I noted earlier, when you’re doing the PFT you’ll see some cool loops on the computer screen. The one created as you do your FVC is called a flow volume loop. If you have airflow obstruction and reversibility, the loops will show this. By analyzing the loops and calculations together, your physician can obtain a good idea of what’s going on in your lungs.
If you want to see some loops, I provide a brief loops 101 course using some PFT results from my medical records, and one I did recently on myself. You can check this out by clicking here.
Personally I think PFTs are hard to do. When I’m done I’m exhausted. I don’t know if this is because I have asthma or if it’s just always exhausting. Yet it’s still an interesting series of test, and knowing how to interpret the results is definitely good asthma wisdom to have.
A Registered Respiratory Therapist and asthmatic