TAKE CONTROL - Q&A to Understanding Peak Flow
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What is peak flow?
Peak flow is a measure of how hard and fast you can blow the air out of your lungs. If you are well and your airways are open you will be able to blow out fast and you will achieve a high peak flow reading. If your asthma is not so good your airways will be narrowed; you will not be able to blow out so fast and you peak flow will fall. Peak flow then, works in a similar way to a thermometer when you have a fever. The higher the fever the higher the thermometer reading. In this case, the worse the asthma, the lower the peak flow reading.

How is it measured?
Peak flow is measured using a hand held instrument called a peak flow meter. When you blow down the meter it measures the speed of the air as it passes through. The meters are hollow with a scale down the side in units of litres per minute (abbreviated to l/min). Peak flow meters are now on prescription in the UK. In addition, many pharmacies also sell them. If you buy your own meter, it is very important that you are shown how to use it by your doctor or asthma nurse.

How should a meter be used?
It is essential that you are taught how to use a peak flow meter correctly. Accurate readings depend on accurate technique. Even when you have been taught to use the meter, your technique should be reviewed when you visit the surgery. It is very easy to fall into bad habits! This is why it is so important to have your technique checked. The steps to using a meter are:

(1) Move the marker to the zero point.
(2) Stand up and hold the meter horizontally in front of you. Make sure your fingers do not touch the scale or markers.
(3) Breathe in as fully as possible.
(4) Place your lips firmly around the mouthpiece.
(5) Blow out as hard and fast as possible.
(6) Take a note of the reading and move the marker back to the zero point.
(7) Repeat this sequence twice more.
(8) The highest of the three readings is your peak flow at that time.


Who can use a peak flow meter?
Generally, children of five years and under are too young to use a peak flow meter. Although the older ones of this age group may produce one or two 'real' readings, more often than not, they cannot put the same amount of effort in every time; accurate peak flow readings depend on effort. In addition, many of the inhaler devices containing medications for asthma require you to breathe in. This technique is quite enough for most young children. Getting them to do the opposite by blowing out down a peak flow meter can complicates things! It is obviously more important they get the medication than measure peak flow if there has to be a choice. Some adults, particularly elderly people, are also unable to master the technique.

What is a normal reading?
Peak flow values depend on a number of factors including your age, sex, height and ethnic origin. Your peak flow increases until you are about 40 years old then decreases after that time. There are two kinds of 'normal' reading you might come across; these are outlined below.

  Predicted Peak Flow
Peak flow readings have been taken on people who do not have asthma, of different age, sex and height and the same ethnic origin. The readings obtained are commonly used as a reference point and are called predicted peak flows. Your predicted peak flow is based on the peak flow of someone who is the same age, height, sex and ethnic origin, but who does not have asthma. The problem with comparing readings to these values, is that you can fall quite a long way above or below your predicted peak flow (which is an average) and still be normal! This is why it is much better to compare your readings to your personal best peak flow.
  Personal Best Peak Flow
This is your highest peak flow when you are well and your asthma is under control. It is usually measured in the middle of the day after using your immediate reliever medication. Your doctor or nurse can help you establish this value which can be done over two or three weeks when using home monitoring. You should compare all your peak flow readings to your best, as this shows how well you are at any one time compared to how well you can be.

When should peak flow be measured?
Peak flow can be measured when you visit your doctor or nurse to see how well you are at the time of the visit. Peak flow monitoring at home on a daily basis gives a wealth of information if it is carried out first thing in the morning before taking any asthma medications, and in the evening, at about the same time each day. The individual readings and the difference between the morning and evening readings are both indicators of how well you are. Monitoring peak flow at home in this way can help build up a picture of your asthma on a day to day basis.

How is peak flow expressed?
There are a number of ways in which peak flow readings are expressed; the main ones are summarised below.

  Peak Flow Reading
This is the peak flow value you achieve as read off the scale on your meter. The units are in litres per minute, abbreviated to l/min.
  Diurnal Variation
This is the difference between the width of the airways in the morning and evening on one day measured about 12 hours apart. The airways are narrower at different times over 24 hours even in people who do not have asthma. Research has shown them to be widest at 4 p.m. and narrowest at 4 am. In people with asthma, this difference is exaggerated. Instead of varying by about 10%, the airway width can vary by 20% or more. This is a sign that asthma is present and unstable: that is, the airways are twitchy and irritable. Diurnal variation of the airways can therefore be measured by taking morning and evening peak flow readings approximately 12 hours apart. It follows, then, that the bigger the difference between the morning and evening peak flow readings, the greater the diurnal variation, the more unstable the asthma.
  Morning Dip
It is not uncommon for morning peak flow to be much lower than evening peak flow in people with asthma. This is called the morning dip. The exact reason why this happens is unknown. There are many ideas as to why it may occur, including acid leaking from the gullet at night, posture during sleep, low levels of body steroids at night and other theories; It still, however, remains a mystery! A pronounced morning dip indicates poor or worsening asthma control. If you show signs of a morning dip but your evening peak flow remains high, then it follows that your diurnal variation will be high (see above).
  Peak flow as a Percentage of Personal Best Peak Flow
As a general rule, the further away a reading is from your personal best value, the worse your asthma at that time. To make peak flow easier to understand, readings are often expressed as a percentage of personal best peak flow. For example, if your personal best peak flow is 400 l/min and you blow a peak flow of 300 l/min, this can be calculated as a percentage of your best value as follows:

300/400 x 100=75%

This means that you peak flow reading is 75% of your best value.

This is a useful way of expressing peak flow readings. Peak flow readings expressed in this way are then often divided into zones. These zones are widely used in asthma management as guidelines as to how well you are. An examples of the use of such zones is shown below:


(a) Green Zone - Under Control
If your peak flow remains in your green zone you are well. The range used here is usually between 100% and 80% of your personal best peak flow.
(b) Yellow Zone - Caution
If your peak flow falls into your yellow zone it may mean your asthma is getting worse. The range used here is usually 80% to 50% of your personal best peak flow.
(c) Red Zone - Severe
If your peak flow falls into your red zone it usually means your asthma is very severe and you need to get help. You should not spend any of your time in your Red Zone!

The number of zones and the zone ranges used may vary from person to person depending on their individual asthma. These zones are often used in plans when people are managing their asthma at home; see the page Having a Self-Management Plan for more information.

Why is peak flow so useful?
Peak flow is a very useful indicator as to how well you are. It is the only true objective measure of your asthma which can be done at home. It gives a true reflection of how wide or narrow your airways are, rather than just relying on how you feel. Studies have shown that many people cannot really assess how well they are. Some seem to have no apparent symptoms, say they feel fine, yet can have a peak flow as low as 50% of their best value! This is dangerously low and shows their asthma is severe and they are at risk of a life-threatening attack. Peak flow monitoring at home can thus provide much more information than symptoms alone. Many people now have their own meters for this very reason. If you are well, your peak flow should be stable, this means:

(1) Diurnal variation less than 20%.
(2) No morning dip.
(3) Peak flow readings remain in your green zone.
Topic "Take Control" Updated 14th August 2003  
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