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Peak Flow Measurement

A well known and often used test in general practice is measurement of Peak Expiratory Flow or peak flow. Peak flow measurement is often used for diagnosing and follow-up of astma.

A peak flow meter is a mechanical device in which the patient blows out. The force of the expiration pushes an indicator. The higher the flow is, the farther the meter is pushed.

It is important to always use the same peak flow meter to be able to compare results.

There are different types and models of peak flow meters available. The most widely used is the 'Wright peak flow meter'.

peak flow meter
a peak flow meter

 

Peak flow measurement is a simple and cheap test, but there are important limitations to measuring peak expiratory flow.

Limitation of Peak Expiratory Flow Measurement

A peak flow measurement measures per definition only one parameter. The peak flow is a parameter that can vary significantly and is very effort dependant.

Peak flow measurement historically is used for diagnosis and follow-up of asthma. For this measurement of peak flow is an acceptable - but limited - method, if different measurements at different time intervals can be done. Because of the natural varation of the peak expiratory flow, an isolated measurement has limited value.

Peak flow measurement is often used in diagnosis and follow-up of COPD as well. However, peak flow measurement is not an adequate means of diagnosis or follow-up for this pathology. Often in COPD peak flow is only affected in moderate or severe cases. The other lung function parameters can be so low at this point that the patient is experiencing respiratory symptoms. COPD is a disease that needs to be detected and treated as early as possible.

COPD
Two obstructive F/V loops are shown.
Although the black loop is showing clear signs of obstruction, PEF is normal.
The red loop is a GOLD class I COPD patient, but still shows a PEF of approx. 75% of normal value

The GOLD guidelines for COPD do not take into account peak expiratory flow for judging the severity of COPD.

The forced expiratory volume after 1 second (FEV1) and the Tiffeneau index (FEV1/VC%) are far better parameters to judge COPD severity.

Measuring Peak Expiratory Flow

During a peak flow measurement the patient needs to sit up straight or stand up.

The patient inspires as deeply as possible, takes the device in his mouth and blows out as strongly and as fast as he can. It is not necessary to blow out as long as possible, in contrast to a spirometry test. An expiration of 1 second is sufficient.

The test is repeated at least three times and the best value is withheld.

Peak flow is comared to predicted values or - even better - to the patient's best personal value.

Predicted values are dependant of sex, age, height and ethnicity.