Pulmonary function tests and lung age: Do you know yours?
In the last post, I discussed COPD and asthma, and how to distinguish them. I introduced the topic of pulmonary function tests, or PFTs. Today, I would like to discuss PFTs in more detail. PFTs allow health care professionals to make the diagnosis of lung disease and to judge how severe it is. PFTs evaluate how well air can flow into and out of the lungs and how effective the lungs are at transferring gas from the air into the bloodstream. The ability of PFTs to diagnose lung disease or determine its severity depends a great deal on the effort of the patient who is performing the test. If the patient’s effort is poor or if the test is compromised by, say, excessive coughing, the results will be difficult to interpret, and it will be difficult or impossible to make a diagnosis or assess the severity of lung disease.
In general, PFTs can be broken up into two parts: spirometry, which measures how much air flows out of the lung, and lung volume measurements, which measure the capacity of the lung to hold air. Spirometry is performed when the patient takes a deep breath in and then blows out forcefully into a mouthpiece that measures how much air comes out. Lung volumes can be measured by two techniques. In the first, more accurate way, the patient sits in a plastic box that resembles a telephone booth, called a plethysmograph. This box seals tightly and the pressure changes that occur with breathing can be measured, and the lung volumes can be calculated. Sometimes, lung volumes are measured by having the patient breathe a gas, such as helium or nitrogen, which then washes out of the lungs over time. By measuring the time over which these gases wash out of the lungs, the lung volumes can be calculated. In addition, PFTs can examine how much lung function changes when a bronchodilator (a drug that relaxes airway muscles, usually albuterol) is given or how quickly oxygen can move from the lung into the bloodstream. PFTs can diagnose two broad categories of lung disease: obstructive and restrictive. Obstructive lung diseases, such as asthma and COPD, are characterized by a slow flow of air out of the lungs. Restrictive lung diseases usually result from inflammation or scarring of the lung tissue that leads to an inability to expand the lungs fully.
The most commonly obtained values from PFTs are the FVC (forced vital capacity) and the FEV1 (forced expiratory volume in one second). These two measurements allow a pretty good evaluation of how well the lungs are working, both how much air can flow in and out and how much the lungs can expand. These two measurements are made during spirometry, when the patient breathes forcefully through a mouthpiece. The FEV1 measures how much air can be blown out in one second, and the FVC measures how much air can be blown out in a single breath. Patients with obstructive lung diseases like asthma or COPD usually have a low FEV1 when compared to the FVC. Usually, when the results of PFTs are interpreted, the results are given as a percentage of a predicted value, based on the patient’s age, sex, height, weight, and ethnic group. Smokers with lung disease will have a low FEV1 compared to healthy non-smokers of a similar age, size, and ethnic group.
Recently, however, some health care professionals have begun to express the results of PFTs as "lung age" instead of percentages. This idea is based on the fact that all of us lose lung function as we grow older. This process begins in our late 30’s. Smokers will tend to lose lung function faster, so a 50-year-old smoker will have lower lung function than a 50-year-old non-smoker. Another way of saying this is that a 50-year-old smoker may have the lung function of a non-smoking person who is 70-years-old, or even older, due to the accelerated loss of lung function. If you had recent PFTs, you can calculate your lung age here. A recently published research article showed that smokers who were told their lung age were more likely to quit smoking than smokers who had been told their PFTs results in the standard way. In fact, those who were told their lung age were about twice as likely to quit smoking.
Unfortunately, even among the patients told their lung age, only 13.6% had quit smoking one year later. So, though knowing your lung age is a powerful tool in helping you to quit smoking, clearly, more good methods to help people quit are needed.
Are you a smoker who has performed PFTs? How did your doctor tell you about the results? Have you ever been told your lung age? Do you think knowing your lung age would help you quit smoking?
David A. Kaufman, M.D., wrote about lung disease, asthma, chronic obstructive pulmonary disease (COPD), and more for HealthCentral. He is an Assistant Professor of Medicine in the Division of Pulmonary, Critical Care and Sleep Medicine at Mount Sinai School of Medicine in New York City. He sees patients with lung diseases, including asthma, COPD, and interstitial lung diseases. He also cares for critically ill patients in the ICU at Mount Sinai.