Pulmonary Function Tests (PFTs)
Alan Baptist , MD
Pulmonary function tests (PFTs) can be of great use to a clinician. They are helpful in determining if an obstructive or restrictive disease is present. In addition, PFTs can help determine the location of a defect. However, just as neither an electrocardiogram nor chest radiograph can definitively make a diagnosis of a myocardial infarction or pneumonia without the appropriate clinical information, so too must PFTs be interpreted in their clinical setting. This site will give the reader an overview of PFTs and how they can be used clinically.
The term PFT encompasses three different measures of lung function: spirometry, lung volumes, and diffusion capacity. PFTs are effort dependent. Therefore, the lab technician who performs the PFTs plays a vital role ensuring that the patient gives a maximal effort. To assess that effort is adequate and results are reproducible, the PFT maneuver is often repeated 3 times. In addition, the lab technician will note if the effort appeared maximal. Many PFT machines will print out a plot of volume time curve, which can then be incorporated in assessing effort. This plot should go out to approximately six seconds, and should flatten, indicating maximal effort and no residual air trapping (Figure 1). However, children are rarely able to achieve six seconds due to their smaller lung volumes, and a goal of 3-4 seconds is adequate for them, depending on age.
PFTs are interpreted by comparing the patient's values to predicted values of healthy subjects with similar age, weight, and height. It is important to note that the normal predicted values which are programmed into the PFT machines are often based on Caucasian men, and therefore minorities and women may have inaccurate readings. Newer machines have attempted to correct for these errors, talk to your lab technician to determine the specifics of the machine used at your institution. Always check that the age, weight, and height of the patient have been correctly entered into the PFT machine as the predicted values are influenced by these variables.
Of the 3 PFT modalities, spirometry is the cheapest, fastest, and most readily available in outpatient clinics. It is used mainly to determine airflow. The first question that spirometry answers is if an obstructive defect is present. Obstructive defects can be seen in asthma, bronchitis, and emphysema. In order to do assess for this type of defect, the first place to look is at the flow-volume loop. If an obstructive defect is present, the flow volume loop will often have a "scooped" pattern (Figure 2). However, an early obstructive defect may not show scooping, and therefore it is critical to use the numerical values. The FEV1 (forced expiratory volume 1) is the volume of air forcefully exhaled in 1 second, whereas the FVC (forced vital capacity) is the volume of air that can be maximally forcefully exhaled - and therefore contains the FEV1 within it. If the FEV1/FVC ratio is <80%, it indicates that an obstructive defect is present. Therefore, this ratio is the first number that should be assessed when interpreting spirometry.
If the FEV1/FVC ratio is less that 80%, the next question spirometry can answer is the severity of the obstructive defect. To do this, look at the FEV1 percent predicted:
> 80% = minimal obstructive defect
65 - 80% = mild obstructive defect
50 - 65% = moderate obstructive defect
< 50% = severe obstructive defect
These numbers are approximations, and should be interpreted in the appropriate clinical setting. Finally, when assessing if an obstructive defect is present, the FEF 25-75% (the average forced expiratory flow during the mid (25 - 75%) portion of the FVC) has been found to be decreased prior to the FEV1/FVC ratio falling below 80% in small airway obstructive disease (i.e. asthma). Therefore, many clinicians will interpret a FEF 25-75% of less than 50% predicted as indication that an early obstructive disease is present, even if the FEV1/FVC is greater than 80%. Interpretation of the FEF 25-75% needs to be done cautiously, as it is the most variable of the values used.
After assessing for an obstructive defect through numerical values, it is important to look at the shape of the flow volume loop for features other than scooping. If there is flattening of the inspiratory portion, this may indicate a variable extrathoracic obstructive defect (i.e. vocal cord dyskinesia, tracheomalacia). If there is flattening of the expiratory portion, this may indicate a variable intrathoracic defect (i.e. bronchiectasis). If there is flattening of both the expiratory and inspiratory portions, this indicates a fixed defect (i.e. lung tumor, tracheal stenosis). Figure 3 shows these three different patterns.
Asthma is defined as a reversible obstructive defect. Therefore, a patient with an FEV1/FVC ratio < 80% can be given a bronchodilator (i.e. albuterol) and the spirometry can be repeated. If the FEV1 increases by more than 12%, it is indicative of reversible airway disease. If the FEV1 does not increase by more than 12%, it is considered nonreversible or fixed airway disease(i.e. COPD). Because asthma is a reversible obstructive defect, the spirometry may be normal at the time of evaluation. In instances where asthma is strongly considered yet the spirometry is normal, a methacholine challenge may be needed. In this test, a patient inhales one or more concentrations of methacholine, and results of spirometry before and after the inhalations are measured. The amount of methacholine needed to elicit a drop of 20% in the FEV1 (known as the PD20) is obtained. The lower the PD20, the more likely that the patient has reactive airways. The reader is directed elsewhere for a more complete review of methacholine testing.
Once the question of presence and severity of an obstructive defect are answered, the next question spirometry can be asked to answer is if a restrictive defect is present. However, a restrictive defect is defined by a decreased total total lung capacity (TLC). Spirometry cannot measure residual volume (RV) nor TLC, and therefore cannot definitively answer this question. Rather, if the forced vital capacity (FVC) is less than 80% and does not reverse with bronchodialtors, this indicates that a restrictive disease might be present, and that further testing is needed (see lung volumes below). The flow volume loop may suggest a restrictive defect by a constricted appearance (Figure 4), but as with obstructive defects may appear normal. Keep in mind that an obstructive and restrictive defect can co-exist in the same patient.
Figure 5 gives a step-by-step approach for a quick review of spirometry.
The complete interpretation of lung volumes is beyond the scope of this website. In general, lung volume measurement is usually indicated in a patient in whom a restrictive disease is suspected, whether by chest imaging or spirometry (see above). It should also be considered in a patient without reversibility of spirometry after bronchodilators. The usual method is by body plethysmography, in which a patient in a closed environment breathes in a known volume of helium. Using Boyle's law, the residual volume (RV) and TLC can be calculated. If the TLC is found to be less than 80%, a restrictive disease is present. Many diseases can cause a restrictive pattern on PFTs. These include paranchemyal defects (i.e. sacroidosis, interstitial pulmonary fibrosis, amyloidosis) or chest wall/muscle defects (i.e. muscular dystrophy, kyphoscoliosis, obesity). The diffusion capacity can be used to help sort these out (see below). For other information that lung volume/body plethysmography can provide (i.e. MIP, MEP, MVV, etc.), the reader is directed elsewhere.
The diffusion capacity (DLCO) is used to determine the gas-transfer function of the lungs - whether gas can move from the air, across the interstitium, and into the blood. It is performed by having the patient breathe in a nonreactive gas, such as carbon monoxide (CO), which binds to hemoglobin. In order to reach the hemoglogin, the CO must diffuse across the alveolar-blood barrier. If there is thickening of the aleovli or interstitium, the diffusion will be impaired, resulting in a decreased DLCO. Therefore, the DLCO is useful in a patient with a restrictive disease, in order to determine if the restriction is intrinsic or extrinsic.
Intrinsic restrictive diseases, in which the DLCO will be decreased , include:
Idiopathic pulmonary fibrosis (IPF)
Collagen vascular diseases (Wegners, Goodpastures, etc)
Extrinsic restrictive diseases, in which the DLCO will be normal , include:
Neuromuscular disease (myasthenia gravis, multiple sclerosis, etc)
The DLCO may be falsely decreased if the patient has a severe restrictive or obstructive disease as they may not be able to inspire an adequate amount of CO. Therefore, the DLCO is often adjusted by the alveolar volume (VA), and listed as the DLCO/VA. A normal DLCO/VA is considered to be > 80%. Finally, it is important to remember that other factors may affect the DLCO. For example, the DLCO may be decreased if the patient is anemic or if there is a pulmonary embolus, as both of these may decrease the ability of CO to cross the alveolar-blood barrier. Alternatively, the DLCO may be increased in conditions such as polycythemia or pulmonary hemorrhage.