Peak expiratory flow (PEF) measures how
much air you or your child can breathe out using the greatest effort. It is used in
the monitoring and treatment of
asthma to determine how well your lungs are
functioning. Your peak flow drops when the tubes that carry air to the lungs
(bronchial tubes) narrow. A decrease in the peak flow can show that the
bronchial tubes have narrowed even before asthma symptoms develop. Peak
expiratory flow is measured with a
peak flow meter, an inexpensive device that can be
used at home.
Peak expiratory flow is lowest in the early morning
and highest in the afternoon. If you or your child takes the test only one time
during the day, take it first thing in the morning, before using a
bronchodilator medicine. Record the PEF in your
asthma diary(What is a PDF document?).
PEF varies throughout the day. In a person who
does not have asthma, it may vary between 10% and 15%. In a person who has
poorly controlled asthma, it may vary more than 20%.
expiratory flow rates are compared with charts that list normal values for sex,
race, and height.
A person's personal best is his or her
highest peak flow. This is a number you can use to find the correct asthma zones.
The personal best may be used in the written plan that says what to do during a
sudden increase in asthma symptoms (asthma attacks).
You determine your or your child's
personal best by taking PEF readings over 2 to 3 weeks when the asthma is under
control—when you or your child feels good and has no symptoms. During these
weeks, peak expiratory flows should be recorded at least twice daily. The
personal best is usually reached in the afternoon or evening. Personal best
measurements are never measured during an asthma attack.
personal best is the highest reading you have over this period of time.
Personal best measurement needs to be evaluated once in a while to see
whether it has changed. If it decreases, medicines may need to be changed or
increased to prevent asthma from becoming worse. If personal best measurement
improves significantly, the amount of medicine may be reduced. Children need
to have their personal best measurement checked about every 6 months to account
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