What is the peak flow meter used for? PEF testing: role, process and results

A peak flow meter is a device that measures peak expiratory flow, which is a parameter of PEF testing [1]. It allows the assessment of airflow limitation due to airway narrowing, e.g. in the course of bronchial asthma or chronic obstructive pulmonary disease [1]. It is used for daily monitoring of lung function in both adults and children [2,3]. It is an essential tool for self-monitoring of asthma. It allows the detection of changes that may indicate an exacerbation of the disease. During bronchospasm (e.g. in asthma), a decrease in PEF values occurs [2]. This occurs, for example, in response to contact with allergens or as a result of infection.

The peak flow meter can be used alone and can be carried with you at all times. It is small, safe and easy to use [1]. To use it, simply blow firmly into the mouthpiece and then read the PEF result.

The peak flow meter comes in two variants – mechanical and electronic. The first one (mechanical) is cheaper (costs a few tens of zlotys), but less technologically advanced. It has an indicator that moves along the scale depending on how forcefully air is blown in – the stronger the blow, the higher the PEF value [3].

The electronic peak flow meter, generally easier to use, has a monitor instead of a scale with an indicator, which automatically displays the PEF measurement result. The role of the monitor can also be performed by a simple mobile app to be installed on a smartphone [1].

The electronic peak flow meter records the measurement results and facilitates their interpretation, which is aided by a system of qualification into one of three colour-coded categories (see below) [1]. The mobile app presents the data in graphical form, e.g. as a chart. The price of an electronic peak flow meter can range from several hundred to one thousand PLN.

Peak flow meters differ not only in their technological capabilities and price, but also in the accuracy and repeatability of their measurements [1]. Not all of them show measurement accuracy in accordance with the requirements of the International Organisation for Standardisation (ISO 23747:2015). Sometimes differences in measurements taken with different peak flow meters are over 20% [1]. And an inaccurate PEF test result can negatively affect the course and effectiveness of treatment.

PEF (peak expiratory flow) or PEFR (peak expiratory flow rate) – indicates the maximum airflow rate that can be achieved during expiration [3]. It is a parameter expressed in litres per minute and is also obtained by spirometry. However, with spirometry – in addition to measuring PEF – we also obtain a number of other indicators. The most important of these are:

VOLUMETRIC INDICES:

• FEV1 – enhanced first-second expiratory volume, i.e. the volume of air blown out of the lungs during the first second of forced expiration,
• FVC – enhanced vital capacity, i.e. the volume of air that can be blown out from the deepest inhalation to the maximum exhalation,
• FEV1/FVC – Tiffeneau test (quotient of FEV1 and FVC) – a parameter that determines whether bronchoconstriction (obstruction) or reduced lung volume is responsible for the abnormal spirometry result.

FLOW INDICES:

• MEF or FEF – parameters to assess airflow in the small bronchi,
• PEF.

PEF measurement, spirometry and the diastolic test are a set of tests to assess lung ventilation performance that can be performed by a GP in primary care [6]. With that said, it is important to remember that PEF testing is not a substitute for spirometry testing. Although it is a faster and easier procedure than spirometry, it gives a much less accurate result. Therefore, PEF testing should not serve as the only tool in the diagnosis of asthma [3]. The diagnostic value is in the series of results and not in a single PEF measurement (the idea is to observe variability) [3].

Peak expiratory flow (PEF) depends, among other things, on expiratory effort, age, sex and height of the patient [1]. In addition, it is characterised by diurnal variability. It is lowest in the morning and highest in the evening. A variability of no more than 20% is within normal limits [2,3].

Step-by-step PEF testing:

• Rest before taking the test,
• be in a standing position (!) – measurement in a lying or sitting position is the most common error affecting the PEF result; if you need to measure in a sitting position, straighten your back and neck,
• hold the device horizontally,
• if you have a mechanical peak flow meter, move the pointer to the “zero” position, and if you have an electronic peak flow meter, switch on the device and/or the mobile app,
• inhale as deeply as possible and hold your breath,
• exhale into the mouthpiece, hugging it tightly with your lips (do not block the mouthpiece with your tongue!) – do this with all your might: as quickly and as briefly as possible,
• read and record the result obtained,
• repeat the measurement three times and note the highest value, which is the target result of the test; remember that the two highest results should meet the repeatability criterion (they must not differ by more than 40 l/min) [1,2].

In order to interpret the results of PEF testing, it is necessary to start by determining the due value for the patient – based on biweekly measurements (taken twice a day – morning and evening). It is assumed that the PEF result can be considered normal if it is at least 80% of the normal value [1].

In electronic peak flow meters, the result is automatically processed and qualified into one of three categories:

• green (> 80 % of the due value)
• yellow (50-80 % of the due value),
• red (< 50 % of the due value).

With abnormal PEF test results, it is best to contact the doctor or, if adopted in the management plan, to increase the dose of the drug yourself [1,3].

Aleksandra Lipiec 

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translation: Julia Majsiak