How to Use Pulmonary Function Tests Effectively

This is Mayo Clinic Talks, a curated weekly podcast for physicians and healthcare providers. I'm your host, Darrell Chetka, a general internist at Mayo Clinic in Rochester, Minnesota. Pulmonary function tests are extremely useful in both diagnosing and managing patients with respiratory disease. In addition to a medical history, physical exam, and imaging studies, they give us a great deal of information of the pulmonary physiology and help us understand why patients have various respiratory symptoms. However, there are quite a number of pulmonary function tests available to us and which tests we should order can be somewhat confusing at times. Do we know when spirometry, lung volumes, or diffusion capacity will be useful for evaluating the various pulmonary conditions? Our guest for this podcast is Dr. Alexander Niven from the Division of Pulmonary and Critical Care Medicine at the Mayo Clinic and he'll provide answers to these questions and more as we discuss how to use pulmonary function tests effectively. You're listening to Mayo Clinic Talks. Alex, thank you so much for taking time to be with us today and discuss this. I think an interesting topic. Thank you, Dr. Chetka. It's truly a pleasure to be here and couldn't agree more about this being a fascinating topic. I would want to see that you say that. Well, as I mentioned, this is kind of like an extensive menu that we get. Do you want spirometry, diffusion capacity, lung volumes, do you want fries with that? It's sometimes kind of overwhelming. So I'd like to start out really basic and ask you to summarize the various components of what makeup pulmonary function tests and maybe when we should be thinking about requesting those for different conditions. Yeah, absolutely. I'd love to do that. I guess I will apologize for my field up front for the emphasis that I think we place on respiratory physiology when we give you that big long laundry list of tests to try to order in your busy primary care clinic. So let's break it down a little bit to think about the common tests that we reach for and what they're good for. By far and away, the bread and butter most common tests that I think most primary care clinicians think about when they think about pulmonary function tests is spirometry, right? A spirometry is something that has been around since the mid-1800s. That was one Hutchinson first described vinyl capacity. We're particularly proud of our former lab director here Bob Hyatt at Mayo who is largely credited as the inventor of the flow volume loop. But spirometry basically provides you with flows and volumes over time. And probably its greatest use is to give us an idea of whether or not somebody has airflow obstruction, stuff that we would expect with either chronic obstructive pulmonary disease or asthma or subsets of those conditions, including conditions like bronchiectasis. One of the other things that we can often see with spirometry is a symmetric reduction of the four-spiral capacity FEV1 or both and that can be a little bit more challenging to sort out because those findings can be associated with air trapping or true restrictive physiology due to a variety of different conditions. So when I'm faced with spirometry which is abnormal and not clearly simple obstruction, so a normal FVC with a reduced FEV1 and a decreased ratio, I will often reach for lung volumes in that setting because long volumes will tell me if true restriction is present and then can also inform things like air trapping and things along those lines. The second test that I often think about in association with spirometry is lung volumes. We most often measure either using a body plethysmograph or a body box where we close somebody into a box and measure the pressure and volume changes as they pant to estimate different lung volumes and capacities in the thorax or we can also do lung volumes using different gas dilution methods, the most common method nowadays is nitrogen washout. Generally we think about lung volumes when we're worried about physiologic restriction. So something that is making the lungs small either due to a prankimal scarring process in the lungs or inflammation in the lungs or deformity of the thoracic cage or weakness in the respiratory muscles. All of those things lung volumes are going to help us with because a low total lung capacity on lung volumes is kind of the definition of a restrictive process. One of the things that I think a lot of people have become very interested about over the course of the last decade or so is also the value that lung volumes can bring to patients with say chronic obstructive pulmonary disease who have dysnia out of proportion to their airflow obstruction on spirometry. And there we know the reductions in inspiratory capacity which is one of the measurements that we get on lung volumes is directly correlated with exertional symptoms and findings of dynamic hyperinflation during exercise. So that's another helpful thing that I walk away from with lung volumes. Now there's lots of patients that I see who have exertional dysmia and often drop their oxygen saturations when you measure their SBO2 with a pulse ox in the clinic. And whenever I'm faced with that or whenever I've got symptoms that again are out of proportion to the severity of abnormalities that I see on spirometry or lung volume testing, I'm always going to think about a diffusing capacity in that situation. So diffusing capacity is basically a test that we use to measure the gas exchange in the lung and the ability for in our case we use carbon monoxide as a surrogate for oxygen diffusion across the alveolar capillary membrane. And that diffusing capacity can be reduced in a variety of different conditions, basically anything that causes ventilation or perfusion mismatch and that can be on the lung side or on the heart side in terms of the multiple different causes of pulmonary hypertension that we face. And I guess it's probably worth while mentioning that if I see pulmonary function testing that has pretty normal looking spirometry and lung volumes with a dramatically reduced diffusing capacity, then I'm going to think about pulmonary hypertension more often than not in that setting. There's a variety of other tests that we can do to answer specialized questions in a lab. Sometimes you're faced with patients where you're worried about potentially muscle weakness and the tests that we typically do use to look at respiratory muscle function in that setting is a maximum voluntary ventilation and maximal respiratory pressures. So maximum voluntary ventilation we have people breathe hard and fast for 12 to 15 seconds and then extrapolate that for a minute. The reason why we do it for 12 to 15 seconds is people would pass out if we had them do that for a minute. And then the maximum respiratory pressures we basically just have a pressure manometer and have people breathe in as hard as they can and blast out as hard as they can. Now, you can imagine those tests are very effort dependent. And so you have to look at those data with a little bit of a jaundice die, but they can be very helpful in terms of identifying folks who actually have muscle weakness as a cause of say restrictive abnormalities that you're seeing on your spirometry and your lung volumes. I think the other thing that's probably worthwhile mentioning is the variety of different exercise testing that we can do in the pulmonary function lab. The data is well established that resting measurements of lung and heart function can often underestimate the physiologic impairments that come along with exercise. And since most of our patients present to us with exercise-related symptoms of disney and things along those lines when they come with thoracic complaints, I find that six minute walk testing to objectively stratify exercise capacity and look at progress over time is very helpful for patients with say, COBD or anesthesia lung disease. And for those patients that just don't have a clear cut explanation for the cause of their exercise limitation after common testing or people who have multiple different medical conditions, let's face it, as internists, we face lots of people who present to us with multiple cardiopulmonary conditions and dysmia. Formal cardiopulmonary exercise testing in that setting can really help to determine the overall exercise capacity, what the primary limitation is, and help guide both further diagnostic and management approaches. Alright, well let's take a couple examples. I think two of the most common respiratory complaints patients bring to us as primary care providers. One is dysmia, and another is a persistent cough. So let's start with dysmia. When I see a patient with dysmia, I think of you know, could this be cardiac, a bad pump, hematologic, severe anemia or respiratory? So let's say I've ruled out the first two, and now I'm thinking respiratory, what pulmonary function tests would help us best to look at a patient complaining of dysmia? Yeah, no, fantastic question and a common one that we all face in our clinic every day. When I approach a selecting pulmonary function testing in the setting of dysmic patient, one of the things that I sort of start with is the pre-test probability of the different conditions that may be contributing to that complaint. And a lot of that can be driven by the patient that's sitting right across from me, right? So if I'm looking at somebody who is young, potentially has an atopic diathesis, but otherwise doesn't have a whole lot of cardiopulmonary conditions documented in their chart, about 40% of the time that patient's going to have asthma, another 40% of the time they're going to be deconditioned, sabbatrufact, and American society today. And about 10 to 15% of the time, they're going to have either psychogenic causes of dysmia or potentially what we use to refer to as vocal cord dysfunction. Now, I think the more popular term is inducible laryngial obstruction. And so in that sort of setting, I'm going to think about spirometry up front, because if I'm worried about asthma and I document airflow obstruction, I can give a bronchidylator and if I see a large bronchidylator response, that's going to give me the diagnosis of asthma. And if the spirometry is normal at baseline, then I'm going to think about early bronchoprovication challenge testing with something like methocholine, which is very helpful because it's very sensitive for the diagnosis of asthma. So a negative methocholine test provides a pretty high negative predictive value for asthma in most, not all, but most settings. And certainly if it's positive, you've got the diagnosis. I should mention that the American thoracic society just this year issued a new set of guidelines when it comes to the evaluation of patients with suspected or known asthma with an atopic diathesis, which actually recommends doing exhaled nitric oxide testing, at least initially, to help both confirm the diagnosis and potentially phenotype folks that have more of a TH2 phenotype, right? Because exhaled nitric oxide is something that we measure in the lab in exhaled breath. And that value is typically elevated in the setting of eosinophilic airway inflammation. So if it's elevated to begin with, it helps you to think that this is an eosinophilic airways disease asthma, and also can be helpful in terms of looking at asthma control over time after you've instituted management. So that's how I'm going to approach a young patient who comes to me with dyspia. And truthfully, if those initial tests are negative, I'll often put those folks through a cardiopulmonary exercise test early, simply because it helps me with a often difficult conversation that we have with folks in terms of deconditioning and the importance of cardiovascular conditioning as part of symptom control. And it's often helpful for me, if I'm looking for a diagnosis of induce belarigial obstruction in somebody who develops those symptoms during exercise, because at least in our lab, we can place a scope that actually directly looks at the vocal cords during exercise to make that diagnosis definitively. For an older adult who presents with dyspia, which is the red and butter, I think, of most internal medicine practices, unfortunately, the evaluation there is a little bit more complicated. We know that all comers who present with dyspia, lung problems and heart problems are going to predominate the causes of those complaints. And when I think about the lung problems that are associated, most of this falls with the common epidemiology of the lung conditions we face. So obstructive lung disease here with a mix, more of COPD, with maybe a little bit of asthma overlap in the older population, is going to be most common. There's going to be a small bit significant percentage of individuals who are going to have interstitial lung disease, who often will have other associated findings on chest radiographs or cross-sectional imaging. And then also another small population of patients, actually not so small, when you think about the overlap between pulmonary and cardiac disease, who have pulmonary hypertension as a significant contributing cause of their symptoms. I'm going to err on the side of doing what we would call a complete pulmonary function testing here. So the spirometry with a bronchidilator, if obstructed, is going to give me an idea if there's airflow obstruction that goes along with COPD. If there is a symmetric reduction in the FEC and the FEV1 with a normal ratio, ordering those lung volumes up front is going to help me to understand right away if there's a potential restrictive process due to a decreased total lung capacity. And then, because mixed causes of pulmonary hypertension are so common in folks with concominant heart and lung disease, the diffusing capacity is going to help me to understand what the physiologic impact of those heart and lung abnormalities are in terms of gas exchange. So I'll do those things up front. And then what we combine in our lab, at least, is we do a step exercise with pulse oximetry that gives a rough estimation of what the oxygen saturation is doing during exercise to hopefully help save you a few minutes in the clinic in terms of not having to walk your patient around. In patients with asthma, they can be perfectly fine one minute and have significant obstructive symptoms. And next, if you do pulmonary function tests on an asthmatic patient without symptoms are all their tests normal? They often are, but not always. I'll share an anecdote that just happened yesterday, actually, for a patient of my colleagues who was a high performing athlete who was presenting with unexplained dysmia. She had had pulmonary function testing elsewhere that was normal. He repeated the testing here, really, with plans to do a cardiopulmonary exercise test, which was the reason why I was involved. And go figure, she had significant airflow obstruction and a 26 percent response pro-sprocket dilator in terms of FUU improvement. So the reality is we've got lots of patients out there who've been living with asthma for a long period of time, who have developed a bit of a blunted sensation in terms of their dysmia symptoms, and often will underestimate the severity of airflow obstruction that they face. One of the things that we've been trying to design, and this is still a bit of a work in progress, is a bit of a result-driven protocol in the lab to shift some of the burden off the ordering provider. So if you're worried about asthma, you tell us that you're worried about asthma. We do the screening pulmonary function testing. If there's airflow obstruction, we give it a bronca dilator. There's not airflow obstruction. We do a bronchopropication challenge. I think that's an ideal situation, but I'm afraid it's still a bit of a work in progress. There's always hoops to jump through when it comes to that sort of stuff. But if we have a patient that we're concerned might have asthma, it's really wise to request a methodical in challenge, is that right? Yeah. And that's a good choice, because there's clear contraindications to the administration of methecoline, and one of those is severe airflow obstruction. So if we do baselines barometry on a patient, and there's significant obstruction there, we're not going to give that patient methecoline. We're going to give a bronchopropidylator, and then you're going to get the answer regardless. And you use a bronchopropidylator to determine the degree of reversibility of their condition? Yeah. And that's something that's probably worth digging into a little bit. And certainly when we talk about bronchopropidylator responsiveness, that is a challenging topic sort of filled with misnomers, right? When we look at a diagnosis of asthma, we look for somebody who has a compatible clinical syndrome. So an airways disease with waxing and waning, dysmia, cough, maybe nighttime symptoms or exertional symptoms, especially if those symptoms are associated with triggers like seasonal variation and things along those lines. That's what we're going to think about as an asthma syndrome. As a pulmonologist, I always like to combine that clinical diagnosis with objective testing. Number one, because it helps me to confirm the diagnosis. And number two, it helps me to classify the severity. So there, what I will think about is, again, screen sprametry, and if there's airflow obstruction, we'll look for bronchopropidylator response. Now we've traditionally been taught that a positive or reversible airflow obstruction is really the term that the American thoracic society is used is an increase of 200 cc's and 12 percent in either the FEC or the FEB1. That's been in place now for gosh, you know, a decade and a half or more. The problem with those criteria is that it didn't really factor in the differences in lung function that can exist based on gender, height, and age. And so one of the things that is a new change, that the standards committee from the American thoracic society and European respiratory society is recommending, is that we calculate bronchodilator responsiveness differently. We look at the difference in the FEB1 or the FEC pre-imposed bronchodilator. instead of dividing it by the baseline value. which is what we used to do. We divided by the predicted value for FEV-1 or FEC for that patient and then multiply by 100. It's super complicated to talk about on an audio podcast. We're actually changing our reports right now so it'll be automated in terms of how it displays. But by dividing by the predicted value rather than the absolute value, it helps us again to factor in those oh so important demographics that can really translate into important changes in terms of long function. I hope that was clear. - Sure, yeah. Now, at Mayo, we're quite spoiled in that we, when we request pulmonary function test, we get the results back not only with the volumes all the numbers but also a very nice interpretation of what happened. Is there any reason why primary care providers should really have some ability to interpret the pulmonary function test results without waiting for an official interpretation? - I'm certainly biased in that regard, but to someone who still considers himself an internist at heart in addition to a subspecialist, I always recognize the significant disadvantage to which I feel when I'm just reading somebody else's interpretation of a test rather than looking at the test myself and understanding some of the subtleties that perhaps aren't captured in those interpretations. I guess because of that, my strong biases that I do think the pulmonary function testing interpretation is kind of a core skill the primary care physicians should have. Number one, because quite honestly many primary care physicians perform spirometry in their offices. And so if you perform it, I think it's really important, number one, to understand the technical standards which first barometry have changed quite significantly over the course of the last several years. And then also be able to interpret the quality of testing in addition to looking at the numbers. Because as we know, pulmonary function testing is entirely dependent on patient efforts and without maximal acceptable and repeatable efforts. Really, the data that we get is garbage and garbage out. So if we can't look at that and say, I don't feel comfortable making a clinical decision based off these data, well, then we're potentially putting ourselves and our patients at a disservice. - Let's finish up by asking you to discuss a little bit the guidelines established for the interpretation of pulmonary function tests. And I understand there's been some changes in these guidelines recently. - It's interesting, there's been a lot of changes in the field of pulmonary function testing over the course of just the last three years that I think have been understandably overshadowed by everything that we've been dealing with with the COVID-19 pandemic. But there are a variety of new technical standards that have been published again by this ATSCRS standards committee for the performance of spirometry. And then in December of 2021 was when the new guidelines for interpretation were released by that same standards committee. And there's actually quite a few differences. I suspect that many labs are still sort of reviewing those changes and talking about how they're gonna translate those changes into their clinical practice. But these are things that if they're not already in your institution, I think we'll be coming in the not-so-distant future. Probably the first thing that's worthwhile just mentioning is the use of reference values, which has been a hot topic over the course of the last couple of years. Because so many people have recognized the challenges that we face with pulmonary function testing when it comes to finding an appropriate reference population to compare your patient's values with predicted or expected lung function for somebody of that age, height, and gender. You notice I didn't talk about race or ancestral origin as the term that we're talking about. Because I think over time we've recognized that expecting somebody to have different lung function based on the color of their skin is inherently problematic and overly simplified concept. There's lots of different factors that go into lung function in addition to those core things, again, age, height, and gender, nutrition, socioeconomic status, and setting not to mention the increasing complexity that we are understanding in terms of genetic determinants that drive lung function. One of the things that I think many labs are transitioning to is a set of reference equations that have been issued by the Global Lung Initiative, which is a group of investigators who've basically gone back and collected together all the data from the high-quality reference sets out there, put them together into much better reference sets than we've had before, and offered those as a standardized way of looking at lung function across the population. Now, there's some emerging data in terms of looking at what they call the, it's the GLI Global Equations, which are basically race agnostic equations, that I think is gonna generate a lot of interest in the coming years, that paper was just published within the last several months, and I think that that's gonna be really important. What's important for primary care physicians, though, is if you're seeing a patient that you've ordered pulmonary function tests on before, and you get a new test back that has different values, you know, somebody who was normal before, who now all of a sudden is just a little bit abnormal, drops below the lower limit of normal, I would really recommend stopping and questioning that before you move on to make clinical decisions, because sometimes those nasty pulmonary function lab directors like myself are changing those reference values behind the scenes and not communicating as clearly as we should. I think it just underlines the fact that mild abnormalities on pulmonary function tests are mild, and need to be interpreted within a clinical context. The second big thing that I think many people are still trying to translate into simple terms when it comes to interpretation, is we're moving away from percent predicted values, for cut points, for severity, and instead using z-scores. Now, I think for anybody who's paid attention to the guidelines and recommendations out there, there's an obvious reason why we're moving away from percent predicted values, is because all these big professional societies all recommend at different cut points, and that was just kind of a nightmare. So z-scores are really, we all know what z-scores are based on bone neural density testing and things along those lines, have been much more clearly correlated with mortality when it comes to changes in lung function, which is the reason why we're starting to use those as cut points for mild, moderate, and severe abnormalities that we see in terms of physiologic impairments. I've already talked about the changes in bronchidilator responsiveness, so I'm not going to pickle everybody's brain by trying to talk about a formula for a belief again. I think those are kind of the big things that we're seeing in terms of differences that I think would hit a primary care physician's desk and cause you to pause for a minute or two. - Okay, well, Alex, you've given us a lot of information. Can you kind of summarize our discussion on pulmonary function tests with maybe two or three key points? - Sure, I think you've illustrated with your dyspnea patients just how common it is that we face problems in primary care practice that require pulmonary function testing. And so I appreciate the opportunity to review the common types of pulmonary function testing that we do and when you might think about ordering them. And again, when I'm faced with a more complex patient, the vast majority of the time earning towards a more complete set of pulmonary function testing is going to make more sense. As opposed to a more straightforward or single problem patient, I'm going to think much more about targeted testing based on the underlying pre-test clinical suspicion for diagnosis. Being aware that there's been a lot of changes that are evolving in the world of pulmonary function testing and at least recognizing the potential clinical implications of that to your patient when it comes to changing reference values or changing interpretation standards is really important. And certainly, if you happen to do office parometry in your clinic, digging a little bit into the changes the standards committee has made with testing performance over the course of the last couple of years is going to be really important to make sure that you're number one current and number two, continuing to deliver quality results that meet standards for interpretation and then clinical decision making. - We've been discussing how to use pulmonary function test effectively with Dr. Alexander Niven from the Division of Pulmonary and Critical Care Medicine at the Mayo Clinic. Alex, thank you so much for sharing your knowledge with us today. I know I've come away with some new insights about how to order pulmonary function tests. - Well, it's an honor and privilege to be here. Thank you so much for having me. - You can now listen to over 100 different medical topics developed for primary care providers on Mayo Clinic Talks Podcasts. Find them at ce.mail.edu or your favorite podcasting app. If you've enjoyed Mayo Clinic Talks Podcasts, please follow us. Stay healthy and see you next week. (upbeat music) [BLANK_AUDIO]