One of my personal pet peeves is when someone is told they have a certain percentage of “lung function” left. I know it is done with a good intention, trying to take a complicated test and make the result simple and easy to understand. But I believe it actually goes a bit too far and over-simplifies things, which can just make things confusing again.
To my mind, your lungs and respiratory system have three main functions. They move air in and out, they hold air for a certain amount of time, and they allow oxygen and carbon dioxide to travel in and out of your blood. In order to have a clear picture of your lung health, we need to look at all three. Fortunately, we have PFTs that can do exactly that.
The most simple and common PFT is called spirometry. You are hopefully very familiar with this one. It is the one where you blow through a tube while a respiratory therapist yells at you to keep blowing and blowing as hard as you can. This test looks at how well the air moves in and out of your lungs. We look at how much air comes out in the first second of blowing (called the forced expiratory volume in 1 second, or FEV1) and how much air comes out during your entire breath (called the forced vital capacity, or FVC). We then compare those numbers both to each other and to average values from people of the same age, sex, and height. If your FEV1 is less than 70% of your FVC, you are said to have an obstruction in your airflow. If this does not go away with a bronchodilator like albuterol, you are said to have a chronic obstruction. We compare the FEV1 and FVC to the average values so that we can look apples-to-apples at what we would expect your lungs to be doing. Looking at what your lungs are actually doing compared to those predicted values gives us an idea of how bad the obstruction is. This is where a lot of the confusion comes in. If you have, for example, an FEV1 that is only 50% of the predicted number, that does NOT mean you only have 50% of your lung function left. As someone with COPD once told me, “it’s not like a gas tank. It doesn’t go all the way down to zero.” It gives us an idea of what therapy we might want to use, and then we can use later measurements to see if that therapy is helping.
Another kind of PFT is lung volume measurement. This test uses either a phone-booth like box or special gas mixtures to see how much air your lungs can hold. These tests are considered part of a “full PFT” exam, but they tend to be more helpful in the treatment of restrictive lung problems like pulmonary fibrosis.
The third kind of PFT is called diffusion capacity of the lung for carbon monoxide, or DLCO. DLCO measurements tell us how well oxygen can get across from the air sacs in your lungs (called alveoli) into your blood. You take one deep breath of another gas mixture containing a very small amount of carbon monoxide, hold that breath for 10 seconds, then exhale. The PFT machine measures how much of the carbon monoxide is left in your breath and calculates what crossed over into your blood. We use carbon monoxide because it attaches to the same part of the blood oxygen does (the hemoglobin in your red blood cells) but does so much faster than oxygen. That way the test can be done quickly and easily. Your values are again compared to predicted ones, which can then give us clues about the actual structure of your lungs.
I hope this clears up a bit about how lung function tests work without poor analogies! When you had a PFT done, was it explained to you? How did your clinician talk to you about it? Do you have any other questions? Ask away in the comments below!