Pulmonary - Critical Care Associates
of East Texas

Jeffrey M. Shea, M.D., F.C.C.P.
                              Venkatesh Donty, M.D.

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Peak Flow Meter

What Is a Peak Flow Meter?

A peak flow meter is a small, easy-to-use instrument that enables you to measure lung function at home, at work " wherever you go. The peak flow meter measures how fast a person can blow out air after a maximum inhalation. This is called the peak expiratory flow rate, or PEFR.

Why Use a Peak Flow Meter?

People with asthma cannot always feel the early changes taking place in their airways because these changes often occur gradually. By the time symptoms of asthma develop, a person can be experiencing a 25 percent or greater decrease in lung function.

The peak flow meter can serve as an early warning sign and in some cases may show a decrease in lung function one to three days before other respiratory symptoms become evident. This is important because once you know your lung function is declining, you can take steps to prevent an episode. The peak flow numbers, along with early warning signs, can be used to make decisions about asthma treatment. Most asthma experts agree that people with asthma need an objective means of assessing the severity of their condition. Pulmonary function studies are costly and impractical for home use; the peak flow meter provides a good alternative. It is an inexpensive, practical way to measure lung function.

The peak flow measures how fast you can blow air out after taking a deep breath. This measurement, which is read as a number, may reflect the amount of obstruction in the airways. Monitoring the peak flow numbers can help you and your clinician assess your lung function and the state of your asthma. It is a valuable number to use in making decisions about the following:

1.Effectiveness of asthma medications. 

2.Adding or stopping medication(s). 

3.When to seek emergency care. 

4.Environmental control measures. 

5.Personal assessments.

Using Peak Flow Numbers to Make Treatment Decisions

Height, sex and age determine predicted peak flow values. Manufacturers usually supply graphs with standard values. You can use these as initial guides until you and your clinician define your "personal best." Determine your "personal best" by recording the values for two weeks when your asthma is under good control. Use the highest number you can regularly blow (after a bronchodilator treatment if prescribed.) Asthma is controlled when you do not experience asthma symptoms (including nighttime symptoms) and you maintain a normal level of activity. Peak flow values typically are highest in the midday or early evening, so use these readings to determine your personal best.

Once you've determined your personal best, it may be helpful for you and your clinician to establish zones. Zones will cue you about how well you are breathing. The zone system can be compared to the colors of a traffic light:

Green Zone (80%-100% of personal best) signals ALL CLEAR.

  • This indicates good lung function. Follow the routine treatment plan for maintaining asthma control.

Yellow Zone (50%-80% of personal best) signals CAUTION.

  • You may need more aggressive medical management for asthma. This may include a temporary increase in bronchodilator and inhaled steroid medicines, an oral steroid burst or other medicines as prescribed by your physician.

Red Zone (50% or less of personal best) signals a MEDICAL ALERT!

  • You need immediate treatment with an inhaled bronchodilator. Notify your physician or seek emergency care if peak flow numbers do not immediately return and stay in the yellow or green zones.

You can mark your peak flow meter with colored tape, dots or lines to indicate the green, yellow and red zones. Remember that the peak flow percentages we suggest are guidelines only. Establish your peak flow zones with your clinician.

It can be helpful to record peak flow values on a graph or in a diary. The peak flow information should supplement record keeping of your asthma symptoms, use of inhaled medications, activity level and nighttime awakenings due to asthma. This allows you and your clinician to monitor trends that may indicate changes in lung function. Persons whose numbers frequently fluctuate or slowly drop may need a change in their daily medications.

When to Use a Peak Flow Meter

The frequency that you record peak flow numbers depends upon the severity of your asthma, the season, your pattern of symptoms and other factors specific to you. Persons with moderate, severe or unstable asthma may need to record peak flow values before and after bronchodilator treatment twice a day (morning and evening). Individuals with mild or stable asthma may only need to use their peak flow meters two to three times a week. However, peak flow readings should be taken daily when you:

Are exposed to asthma triggers (such as allergens, smoke and other environmental irritants), or have a respiratory infection, or have changes in medical therapy.

How to Use the Peak Flow Meter

Slide the marker down as far as it will go. This sets the meter at zero.





Stand up.





Take a deep breath with your mouth open.





Place the meter in your mouth and close your lips around it with your tongue away from the hole. Keep your fingers away from the markings. Blow out once as hard and fast as you can.




Don't touch the marker then write down the number you get.





  1. Repeat twice. First reset the marker to zero each time. Write down the number each time. Your peak flow is the highest of these three numbers.





It is important to know that peak flow numbers are effort dependent. This means you must put forth a good effort to have reliable, consistent results.

To clean your peak flow meter, follow the manufacturer's directions.


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