Pulmonary Function Tests
Pulmonary Function Tests
Richard Sheldon (Medical Advisor to the CA State Respiratory Care Board) gives expert video advice on: Why would my doctor order peak flow testing?; How is a DCLO or diffusion capacity test performed? and more...
What are pulmonary function tests?
Pulmonary function tests are the main tool that pulmonologists use to diagnose pulmonary diseases and they are easily accomplished tests. Sometimes it's done in the primary care physician's office in what we call a very simple spirometry or a blow and go, where you take a deep breath, blow into a machine, and go, but the pulmonary function studies done by a pulmonologist in a pulmonary function lab take about an hour. They require the patient to be able to sit and breathe at different rates, take deep breaths, and blow them out as fast as possible. That generates an enormous amount of information about the different compartments in the lung, we look at all that data then we can say "This patient has primarily an obstructive lung disease and this is where the obstruction occurs, small airways, large airways or he has a restrictive lung disease which would be an interstitial lung disease" and the data tells us a lot of information about severity, prognosis, and gives us an opportunity to tailor make therapy for the patient.
What is a "spirometer"?
A spirometer is a device that allows for you to take a deep breath, blow into; then it calculates, stores the data on how fast the patient was blowing, what the volumes are that they're blowing; and it allows then for this information to be fed into a computer. And the current spirometers are very accurate and very easily operated and give us a tremendous amount of information.
How is a spirometry testing performed?
A spirometry test requires a patient to arrive at the testing site, be prepared to sit for about an hour, take a deep breath and blow into the machine. The respiratory therapist that will administer the test will be very good at coaxing the patient step-by-step through the test and informing them how they want that test to be performed. The therapist will be looking for what is called a "flow volume curve" during the spirometry test. So, when the person inhales, the inspatory lube and expiatory lube, is noted against flow and volume and then the speed with which the person can exhale is also noted. For instance, in patients with asthma, do the airways collapse as they start to exhale? What happens when they inhale? We can tell whether or not the obstruction is in the upper airways or the lower airways, by the shape of the curve during the spirometry test. We can then see what happens if a person inhales a bronchial dilator. We can discover if their airway opens up and responds nicely to the application of the bronchial dilator and once again we can disvoer how that patient's lung is reacting to their particular problem with their lung.
How will spirometry feel?
Spirometry will feel painless for the most part. Some patients object to it because they don't like the period where they have to take deep breaths and blow it out and take another deep breath and blow it out. They sometimes have to huff and puff a bit, and they'll complain about that. But for the most part, patients who have had spirometry will tell you that it was really no problem, and it was just a matter of being coaxed by the respiratory therapist, and it shouldn't be uncomfortable at all.
What is a "peak flow" meter?
A peak flow meter is really a dandy little device. I pass out peak flow meters to patients constantly. It's a little plastic gadget that they can take home with them, and they open it up and unfold it and it has a little slider on it. It reminds me of going to the fair ground, where you take a hammer and you pound it down and the ball goes up has high as it can go based on how hard you hit the device at the bottom that will eject the device up the scale. Well, a peak flow meter requires you to take a deep breath and you punch it, and you want to get the peak flow, the fastest you can, to run the slider up as far as it will go. These devices are simple, they are cheap, but they will give you a enormous amount of data. This is especially the case with asthmatics, where the data from taking a peak flow meter reading will tell you that you're about to get an attack of asthma. It can be predictive of an oncoming attack, so you can increase your medications as you need to. What we try and do is get the patient to develop their personal best. That term means how far they can push that little slider up when they take a deep breath, and they exhale forcefully into it. It's not a matter of how long you inhale, its that first punch and you really get it. You want to bounce that thing as far down the scale as you possibly can. Then, you set some limits back from that, which will tell you whether you're starting to have an attack or not. It's a very, very nice, cheap inexpensive way to do home pulmonary function testing.
Why would my doctor order peak flow testing?
Most of us pulmonologists will order peak flow testing because we want the asthmatic patients, primarily, to keep a day by day, if not twice a day, log of what their treatment is doing to their airflows and how they're responding to their environment with their asthma. So, I'll order a peak flow test not as a single test, but I'll ask them to do it on a daily basis to help them understand how well their medications are doing and number two, whether or not they're developing an asthma attack and they need to be positioned in such a way that they get better control, maybe in escalation of their medicines, somewhat, so they'll have better control over their airways. If you're an asthmatic and using peak flow properly, you'll know that your peak flow personal best may be 400 liters per minute flow. If you suddenly see that you, for the next 12 hours, drop your flow rates down to, let's say 300 or 250, you pretty well can bet that you're on your way to having a major attack. Therefore, you should increase your medications, or let's say that it would go to 200, which is fifty percent of your personal best at 400. With 200, you should pick up your phone and say "Can I have an appointment tomorrow morning with my pulmonologist?", or you might decide it's time for you to go to the emergency room. Because, certainly if your personal best is 400 and you're down to 200 you will be very short of breath and you will be very uncomfortable.
What is a "diffusion capacity lung" test or "DCLO" test?
A diffusion capacity lung test is a specialized test that helps us understand the health of the alveolar-capillary membrane. Now remember that the alveolus is a thin membrane that allows for oxygen and CO2 to move across this membrane. Well this membrane is tested best by the diffusion capacity, and there are several ways of doing diffusion capacity but they all require that you inhale a minute amount of carbon monoxide. Carbon monoxide moves across the alveolar-capillary membrane in such a way that it is not blocked at all by, it's not blocked at all by its diffusion ability. It is rapidly absorbed across the membrane into the red blood cells, but if that membrane becomes diseased it will in some way then limit the ability of the carbon monoxide to move, to move across. So that the only thing that is a variable there is the state of health of the alveolar-capillary membrane. Now, if in, like, for instance, emphysema, emphysema is a disease where these alveoli fuse together. Where you used to have one aveloli you now have--you used to have two, you now have one. The surface area of two alveoli is greater than the surface area of one when you combine those two together, and so you'll start to lose diffusion capacity because the surface area is being lost, and that's what emphysema does--one of the many things it does--to damage your lung. So diffusion capacities will help you not only help you establish the health of the alveolar-capillary membrane using the inhalation of carbon dioxide--not enough to poison you of course, but just minute amounts--and it'll also tell you about the size of your alveolar-capillary membrane as an aggregate whole. Are you losing surface area? We lose surface area as we age naturally, however, we'll not lose it as fast as if you are a significant smoker who then develops emphysema and then start to lose surface area, and that'll be picked up by diffusion capacity.
Why do I need a DCLO or diffusion capacity lung test?
The reason for taking a diffusion capacity test would be to help you establish the health of the alveolar-capillary membrane. The standard pulmonary function test in almost all pulmonary function labs that a pulmonologist will run, or order tests to be performed at, includes diffusion capacity in the test. If you are getting your test done simply in a primary care physician's or a general practitioner's office, he won't have the machinery available to him and the testing devices available to him to do a diffusion capacity. But if a full pulmonary function test is done, diffusion capacity is part of that test, always.
How is a DCLO or diffusion capacity test performed?
The diffusion capacity test is performed in several ways, depending on how you're set up to do it. There's really three different ways of doing it. There's the single breath method where you take a deep breath and hold it for ten seconds. There's also a rebreathing technique, where you rebreathe the carbon monoxide. There's a steady state test, where you keep breathing it until you reach what's called "steady state." All of which are quite painless, quite easily performed, and it will depend a lot on the lab and the type of patient you have whether you do a steady state, single breath, or a rebreathing form of the diffusion capacity test.
How will a DCLO test feel?
When people take a diffusion capacity test as part of their pulmonary fuction test, they really don't feel anything. They don't feel the gas, they don't taste the gas, it doesn't make them dizzy, it doesn't do anything that would be noxious. If they have trouble holding their breath for ten seconds they may find that to be uncomfortable, but by and large there's nothing to feel.
What do abnormal DCLO results mean?
An abnormal result on a diffusion capacity could mean several things. It could mean that you've lost surface area, such as would be encountered in patients with emphysema. It could also mean that the health of the alveolar capillary membrane has been diminished and so you have a damaged alveolar capillary membrane. It may be the very first thing that you would pick up in interstitial lung diseases. Where every thing else is normal, one of the earliest findings in some of the interstitial lung diseases such as, for instance, sarcoi, would be an abnormal diffusion capacity.