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.
