Asthma and Peak Flow Monitoring


Asthma can cause narrowing of the bronchial tubes and, as a result, difficulty moving air through those tubes into and out of the lungs. How can one measure the extent to which the system of bronchial tubes has become narrowed? We routinely make such measurements when you come for an appointment at Partners Asthma Center: We ask you to blow the air from your lungs as forcefully and as rapidly as possible into a recording device, usually a spirometer. The faster the air exits your lungs, the more widely open the bronchial tubes; the slower the air exits your lungs, the more narrowed they are. This test is one very valuable measurement of how well or poorly controlled your asthma is at that moment.

The Rationale

In this pamphlet we discuss a method that enables you to measure on your own on a regular basis the extent of bronchial narrowing. Regular home peak flow measurements are simple to make, and they provide you and your doctor with an accurate record of the activity of your asthma, not just on the day of your visit to the office, but over the days and weeks preceding the visit as well. In addition, if you are having an attack of asthma, measurement of your peak flow will give you information about how severe the attack is and help you to decide what actions need to be taken to get better.

Many persons who have had asthma for a long time feel that they can judge the severity of an attack of their asthma simply by paying close attention to their symptoms of cough, wheezing, chest tightness, and shortness of breath. Although it is true that some persons become quite adept at estimating the severity of their bronchial tube narrowing, studies have shown that more than half the time persons with asthma will be mistaken in their estimates. Too often, persons with asthma will underestimate how severe their asthma attack is; they will tell themselves and others that their breathing is not all that difficult, that they will be "O.K.", even when their asthma attack is really quite severe, and potentially dangerous. Measurement of peak flow allows one to test one's guess against the reality of an actual measurement; it gives accurate information that you can then use to make good decisions about managing your asthma. It also helps you better to communicate about your asthma with your doctor; he/she will understand more clearly if you call to report that you have measured your peak flow to be 250 liters per minute, or half of your usual peak flow value, than if you simply describe "some cough and chest congestion."

Peak Flow Meters

A variety of simple devices are available that measure the highest rate at which air comes out of your lungs. They are lightweight plastic tubes, usually the size of a household flashlight or smaller, and cost approximately twenty dollars. Peak flow meters are sold at most drug stores or can be purchased by mail order (e.g., from National Allergy Supply Company, 1-800-522-1448). If undamaged, a peak flow meter should continue to function properly for at least two years without need of replacement.

Measuring Your Peak Flow

Measuring your peak flow takes less than one minute. It requires that you take a deep breath—as deep as you can—and then exhale a short forceful breath into the peak flow meter. The exhaled breath need not last more than a second or two; it is a rapid, short "puff." (Note that this is different than the complete exhalation that you may be asked to make when having your breathing measured by spirometry at the Asthma Center.) Use your chest and abdominal muscles to force the air out from your lungs as quickly as possible right from the start. You must seal your lips tightly around the mouthpiece of the peak flow meter so that no air escapes from being recorded; and you must be certain that no air is blown from your nose when making the measurement (if in doubt, simply pinch your nose with your fingers while blowing into the peak flow meter). Two pitfalls to avoid: when holding your peak flow meter, place your fingers and thumb in such a way that they do not block the movement of the plastic indicator; and do not obstruct the hole or holes where your air exits from the device.

Your peak flow meter will have a plastic indicator that marks your peak flow along a scale of numbers from zero to 600 or 800. First, set the indicator at zero before you blow into the device; next, give a rapid, forceful blow; and then, find where along the scale the indicator has been moved by your exhalation: That is the first peak flow reading. Set the indicator back to zero and repeat the process once and preferably twice more. The largest of the two or three readings that you have made is considered your peak flow.

The plastic mouthpiece on your peak flow meter is detachable and can be washed; the meter itself should simply be kept free of dirt and debris.

Recording Your Peak Flow

Most peak flow meters are sold with an accompanying diary. This little booklet has space for you to keep record of your peak flow measurements in a chart or graph; we can provide you with additional pages if needed, or you can make your own record, simply writing down the dates and the corresponding peak flow measurements on a piece of paper.

There is no precise answer to be given for the question: "When and how often should I record my peak flow?" We encourage you to do it every day at approximately the same time of day; our preference would be each morning before you take your asthma medicines. Measurements made each morning after you take your medicine would also provide an excellent record of your asthma. Additional information is gained by measuring and recording your peak flow in the morning and in the evening—to assess the variation in your breathing capacity over the course of each day—and by measurements made before and 10 minutes after your inhaled bronchodilator—to assess the extent to which your breathing improves with bronchodilator treatment. However, we recognize that for many people making more than one measurement each day may be too burdensome, and a single measurement recorded each day provides an excellent source of information for you and your doctor to review at your office visits.

In addition, it is important to make peak flow measurements at any time of day if you feel that you are having an attack of asthma. Measure your peak flow to find out how severe the attack is, and then measure it again one or more times to determine how much improvement you are getting from the asthma medicines taken to treat the attack.

Taking Action Based on Your Peak Flow Measurements

Peak flow measurements can alert you to difficulty with your breathing before you are aware or certain of the change. By making regular measurements at times when you are feeling well, you will have identified your best or "target" peak flow value. Then, if your peak flow falls significantly from this target value, you know that something should be done to bring your breathing function back toward its target value.

As a general rule, a peak flow value 80% or greater of your best peak flow value is considered within a safe range; from 50 to 80% of your best peak flow value is a moderate fall; and less than 50% of your best peak flow value is considered a severe decrease in your breathing function. Urgent action needs to be taken if you have a severe decline in your peak flow to less than half of your normal value. We encourage you to develop an Action Plan with your doctor so that you know what he/she recommends for you to do in these various circumstances. A pamphlet on Developing an Asthma "Action Plan" has been developed by the Partners Asthma Center.

To calculate a "normal" value for a healthy person of a given age and height, follow this link.


With a peak flow meter available at home, you can determine the status of your asthma by measurement, and not have to rely on an estimate or guess. Like a person with high blood pressure who can monitor his/her blood pressure at home, or a person with diabetes who can keep track of his/her blood sugar on a daily basis, so too persons with asthma can keep record of their breathing capacity with regular measurements of peak flow. This information will help your doctor assess the course of your asthma since your last visit, and it will help you to make good decisions about your asthma management at those times when you need to act on your own.