A peak flow meter is a simple, hand-held device that measures how efficiently you can move air out of your lungs. Peak flow readings can warn you of an impending asthma attack hours — or even days — before you experience any signs or symptoms. With time on your side, you can adjust your asthma medication and take other steps to help prevent an attack.

A peak flow meter allows you to objectively measure day-to-day variations in your breathing. When your asthma is under control, your airways are open and you can force more air into the peak flow meter. But when your airways are inflamed and constricted, you can't blow as hard into the meter — making your peak flow rate lower.

Your doctor may recommend using a peak flow meter at least once a day — typically before taking your asthma medication in the morning. Use the peak flow meter at the same time every day, and record your readings in an asthma diary. These readings will help you and your doctor:

• Determine the severity of your asthma
• Evaluate the effectiveness of your current treatment
• Determine when to add or stop certain medications
• Recognize an asthma attack before signs or symptoms appear
• Decide when to seek emergency care

Peak flow meters are available over-the-counter. Better yet, they're easy to use.

• Move the marker to the bottom of the numbered scale, and connect the mouthpiece to the peak flow meter.
• Stand up.
• Take a deep breath, filling your lungs completely.
• Place your lips tightly around the mouthpiece. Blow as hard and as fast as you can with a single breath.  
• Note the final position of the marker. This is your peak flow rate.
• Blow into the peak flow meter two more times. Record the highest reading of the three in your asthma diary.

For the most accurate readings, it's important to keep your peak flow meter clean. Follow the manufacturer's instructions.

Your "personal best" peak flow rate is the highest peak flow rate you can reach over a two- to three-week period when you feel good and have no asthma symptoms. This flow rate serves as a benchmark in your daily self-management plan. Because everyone's asthma is different, your personal best will be unique to you.

Your doctor will help you determine your personal best. Typically, you'll take readings twice a day for two weeks when you're not having symptoms. Ignoring any outliers, the highest consistent reading during the trial period is your personal best.

Your doctor will use your personal best peak flow rate to organize peak flow zones: green (stable), yellow (caution) and red (alert). If your peak flow readings fall too far below your personal best, you'll need to take action to prevent or minimize an asthma attack.

• Green zone. Your peak flow rates are 80 percent to 100 percent of your personal best, an indication that your asthma is under good control. You probably have no asthma signs or symptoms. Take your preventive medications as usual. If you consistently stay within the green zone, your doctor may recommend reducing your asthma medication.
• Yellow zone. Your peak flow rates are 50 percent to 80 percent of your personal best, an indication that your asthma is getting worse. You may have signs and symptoms such as coughing, wheezing or chest tightness. You may need to increase or change your asthma medications.
• Red zone. Your peak flow rates are less than 50 percent of your personal best, an indication of a medical emergency. You may have severe coughing, wheezing and shortness of breath. Stop whatever you're doing and use a bronchodilator or other medication to open your airways. Your asthma action plan will help you decide whether to call your doctor, take an oral corticosteroid or seek emergency care.

Take charge of your treatment — and your life. Identify the things that trigger your asthma symptoms or make them worse. Meet with your doctor periodically to review your asthma action plan and revise it as needed. Show your doctor how you're using your peak flow meter, just to make sure you're doing it correctly.

Your lifestyle matters, too. Eat a healthy diet. Exercise regularly, with your doctor's OK. Don't smoke. Get enough sleep. Taking good care of yourself can help you handle life with asthma.