Pulmonary function tests (PFTs)—as the name implies— are tests designed to measure and assess lung function . PFTs were originally research tools, avail-able only in specialized academic hospital centers. They are now widely available and are frequently performed because of their usefulness in the diagnosis and treatment of asthma. Keep in mind as you read this answer that the lung function abnormalities seen in active asthma on PFT testing are, by definition, reversible. The term PFTs is used to collectively describe several different specific tests of lung function. Spirometry is the single most useful of the PFTs when it comes to asthma diagnosis and treatment.
Spirometry, in turn, includes two important subtests. The first is called the peak expiratory flow, abbreviated PEF. The second is the FEV1, the forced expiratory volume in 1 second. Measurements of PEF and FEV1 are thus parts, or subtests, of the spirometry portion of the PFTs. The availability of inexpensive, highly portable, and easy-to-use peak-flow monitors makes it possible for every person with asthma (even children) to measure his or her peak flow at home on a daily basis in order to monitor asthma activity. FEV1 measurements, on the other hand, require the use of a spirometer, which is more costly, requires special maintenance, and is not presently advised for home use.
Self-monitoring of PEF allows a person with asthma insight into his or her condition and permits an assessment of asthma control. Both PEF and FEV1 play a pivotal role in the National Asthma Education and Prevention Program’s (NAEPP’s) asthma diagnosis, classification, and treatment guidelines To perform spirometry and PEF, the patient is first asked to take a deep breath of room air. Then, that biggest single breath is forcefully and rapidly exhaled into a mouthpiece connected to the spirometer or peak flow meter. The maneuver is repeated several times during testing to ensure accurate and reproducible values. The spirometer measures the exhaled lung volume, as well as the flow of air through the mouthpiece for the time that exhalation takes place.
The spirometry measurements are recorded by the spirometer and are printed out and graphed for review and future reference. Each individual patient measurement is compared to a predicted value. The predicted values for pulmonary function tests are based on three variables: age, height, and gender. Predicted values are different for a 21-year-old, 6-foot-tall man than for a 5-foot-tall, 64-year-old woman. It follows that the PEF value (and the FEV1) that would be considered within normal limits for a short, older female with asthma would be abnormally low if obtained by a tall, adolescent male with asthma, even though they both had asthma. Because asthma is characterized as a disease of lung emptying, exhalation time is abnormally prolonged in symptomatic asthma.
Anyone with active asthma who attempts to blow out all the candles on a birthday cake with one single mighty blow of air knows about impaired lung emp-tying firsthand! Depending on the degree of asthma and other factors, such as how much airway narrowing, or bronchospasm, is present, full exhalation during spirom-etry might last as long as 14 seconds rather than the nor-mal 5 to 6 seconds. The FEV1 and PEF values reflect the efficiency and status of lung emptying, and thus provide information about how a person with asthma’s lung function is affected by his or her condition. The FEV1 measures the amount (volume) of air that is exhaled in the first second of forceful exhalation during spirometry as you breathe out as hard and as fast as you can after you have taken in a deep breath. When asthma is poorly controlled, it takes longer than predicted for the lungs to fully empty.
Since the total exhalation time is prolonged in symptomatic or inadequately controlled asthma, it follows that the amount (volume) of air exhaled during the first second of that exhalation is lower than predicted. The FEV1 decreases in symptomatic or poorly controlled asthma. With treatment, the lungs empty more efficiently, and the FEV1 value returns to a normal range. When asthma is suspected, spirometry is per-formed before and after inhalation of a short-acting bronchodilator medication to look for the normalization of the FEV1—a phenomenon called reversibility.
The most up-to-date guidelines from the third EPR (Expert Panel Report) of the National Heart, Lung, and Blood Institute define a 12% or greater increase in a person’s baseline FEV1 on spirometry after use of a bronchodila-tor as a significant response. When active or exacerbated asthma prolongs exhalation, flow of air through narrowed air passages becomes reduced. Spirometry in active asthma also reveals reduced flow rates. The peak flow is the single greatest value of flow measurement that occurs as the lungs start to empty. Peak flows reflect the flow of air through the larger, so-called conducting airways in asthma.
Peak flow generally tracks asthma activity. Monitoring peak flow at home allows for comparison of a person’s predicted PEF, with his or her actual personal best measurements obtained when the asthma is well controlled. Home-based PEF monitoring can then help identify even a mild exacerbation and guide medication adjustment up or down, depending on how the PEF value fluctuates from the personal best. Self-administered PEF measurements over time are a component of asthma action plans described in more detail. A peak-flow meter is an easy-to-use device designed to help you assess the degree of your asthma control.
Per-sons who have moderate or severe persistent asthma, persons with a history of severe exacerbations, and per-sons who have difficulty perceiving when their asthma worsens are most likely to benefit from self peak-flow monitoring. Monitoring long-term, daily peak-flow measurements detects early changes in asthma control that require an adjustment in treatment and helps gauge the responses to those treatment changes. Asthma self-monitoring should be neither a bother nor a nuisance. On the contrary, daily home peak-flow monitoring has been shown to improve asthma control, reduce exacerbations, and decrease absences from school and work.
Using a peak-flow monitor may also increase your confidence as it helps you learn how to optimize asthma control and achieve greater mastery over your asthma. Most children can accurately measure their peak flow under adult guidance starting at about 6 years of age. Peak-flow monitoring also allows for objective decisions about modifying your asthma regimen based on information contained in the written asthma action plan your physician has provided. If your physician gives you a prescription for home peak-flow monitoring, you will be asked to determine your personal best value based on measurements obtained when you feel well and are symptom-free.
An asthma action plan provides instructions on what asthma medication to take as the peak-flow value falls into one of three zones labeled green, yellow, or red. The green zone includes peak-flow measurements in the range of 80–100% of your personal best. Yellow corresponds to peak-flow measurements in the range of 60–80% of the personal best value. The red zone includes all peak flow values below 60% of your best. Peak-flow measurements in the red zone indicate that your asthma is poorly con-trolled, and that you will need to either contact your physician, proceed to the emergency room, or both.