|What is peak
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
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
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
(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
||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.
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.
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
(1) Diurnal variation less than 20%.
(2) No morning dip.
(3) Peak flow readings remain in your green zone.