Updated 16 September 2014

All about a peak-flow meter

A peak-flow meter enables you to measure day-to-day variations in your breathing and lung capacity.

Asthma is a condition that lends itself to self-care.

You need to discuss your asthma management in detail with your doctor and have a plan of action for altering your medication depending on how well controlled your asthma is.

To determine how well your asthma is controlled, you need to use something called a peak-flow meter. It’s not enough to work entirely on a subjective idea of how you are feeling. You can start to lose control of your asthma before noticing any changes in your breathing or exercise capacity.

Using a peak-flow meter enables you to measure day-to-day variations in your breathing. It’s a sensitive measure that allows you to respond quickly to flare-ups before they become acute attacks which may land you in hospital.

What is a peak-flow meter?
A peak-flow meter is a small, hand-held device which measures your ability to force air out of your lungs as hard as possible. This is called your peak flow and is measured as litres of air/minute.

The measure of your peak flow is a measure of your lung capacity. If your asthma is well controlled and you have no inflammation or bronchoconstriction, you can force a lot of air out of your lungs that will give you a higher reading on the peak-flow meter. As you develop inflammation in your lungs – the first sign that your control is going – you will be able to force less air out of your lungs and will get a lower reading.

Peak-flow meters are small enough to carry around with you and they are easy to use. Place your lips around the mouthpiece and blow out as hard as you possibly can for as long as you possibly can. Your peak flow is measured on a sliding scale on the side of the instrument. Your doctor will advise you on which model to buy and exactly how to use it.

What do the numbers mean?
When you first use a peak-flow meter you can determine your “personal best” score. There are also tables to show what your score should be according to your height and age. However, if you are well controlled and feeling good, then the measurement you achieve under these circumstances can be used as your personal best.

Remember that even in well-controlled asthmatics, the peak flow can vary from day to day and with the time of the day as well. It’s a good idea to always measure at the same time of day for consistency.

Interpreting the numbers is simple. Think of your peak flow as falling into three colour zones:

  • The green zone – this is when your peak flow is between 80 and 100% of your personal best. Your aim is to stay in this zone because this is where you know that your asthma is well-controlled, you can lead a normal life and your medication is working.
  • The yellow zone – this is when your peak flow is 50 to 80% of your personal best. This means “caution”. You are not as well controlled as you could be and your asthma is getting worse. You need to review your medication, reduce your exposure to something which triggers your asthma and contact your doctor to discuss a plan of action.
  • The red zone equals DANGER! Your peak flow is less than 50% of your personal best! Use your bronchodilator immediately. You may have to take oral corticosteroids and will need to visit your doctor or hospital emergency centre as soon as possible.

Discuss a plan of action for the yellow and red zones with your doctor in advance. Make a note of the actions and keep this in a safe and easily accessible place. Make sure that you always follow your “yellow” plan if you fall into this zone. Hopefully this will ensure that you never reach the red zone.

If your asthma is so bad that you can barely record a peak flow, you must seek medical help immediately.

I don’t have a peak-flow metre. How do I know which zone I am in?

The green zone
You have no or only occasional symptoms of:

  • coughing
  • wheezing
  • shortness of breath
  • chest tightness
  • interrupted sleep

The yellow zone
You may find that you are:

  • coughing
  • wheezing
  • feeling tight chested
  • using your reliever inhaler more often
  • waking at night because of your symptoms
  • taking time off work or school
  • prevented from doing some of the things you want to do by your asthma symptoms

The red zone
You may be:

  • getting increasingly breathless
  • having trouble walking or talking
  • using your reliever inhaler more frequently without effect

What are the signs of a severe attack?

  • severe breathlessness
  • too breathless to talk
  • lips may be blue
  • little or no effect from reliever inhaler
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