Peak flow meter revisited

Nai-ming Wong 黃乃明, Mark S H Chan 陳選豪, Albert Y F Kong 江炎輝, Yuk-tsan Wun 溫煜讚, Tai-pong Lam 林大邦

HK Pract 2009;31:30-35

Summary

Asthma is a common condition encountered in primary care. In the Guidelines for the Diagnosis and Management of Asthma, the National Heart Lung and Blood Institute of the USA recommends either peak expiratory flow (PEF) measurements or symptoms as part of the written action plan for asthma monitoring.1 The Institute also recommends long-term daily peak flow monitoring for patients with moderate to severe persistent asthma. These recommendations are based on randomized controlled trials that do not have large sample size.2-4 Other international guidelines also give similar recommendations in spite of greater emphasis on spirometry.

摘要

哮喘是基層醫療常見疾病。美國國家心肺及血液研究所的「診斷和治理指引」建議用最大呼氣流速值(PEF)或症狀作為監測哮喘病情的一部份。該研究所亦建議為中度至嚴重程度患者以每日最大呼氣流速作長期監測。這些建議都是基於實証醫學研究的結果。其他國際組織亦有類似的指引，雖然會較著重肺量的測量。

Introduction

There are recent changes in PEF measurement. Before spirometers could be widely used in the primary care setting, primary care physicians would need an update on PEF in their management of asthma.

This article discusses recent trends in assessing diabetic albuminuria, and reviews the current literature on the management of diabetic patients with albuminuria.

What is PEF?

By definition, peak expiratory flow is the maximum flow generated during expiration, when performed with maximal force after a full inspiration. To put it simply, it is an individual's maximum ability to expel air from the lungs. Its normal expected value depends on an individual's sex, age and height.

How is it measured?

It is measured by peak flow meters (PFM), which are readily available in most clinical settings. Most clinics in Hong Kong use the Mini-Wright Standard Range Peak Flow Meter manufactured by Clement Clarke International from UK. Because the measurement of PEF depends on effort and technique, patients need clear instructions, demonstrations and frequent review. The following instructions can be given to a patient on how to use a PFM:

*Do the following five steps with your peak flow meter:

1. Move the indicator to the bottom of the numbered scale.
2. Stand up.
3. Take a deep breath, filling your lungs completely.
4. Place the mouthpiece in your mouth and close your lips around it. Do not put your tongue inside the hole.
5. .Blow out as hard and fast as you can in a single blow.

*Write down the number you get.
*Repeat steps 1 through 5 two more times and take the best of the three blows.

A small study shows that PEF values do not vary significantly with the standing or sitting posture.5

The original Wright meter was developed in 1959 from airflow measurements of real patients and the Mini-Wright was designed later to provide identical readings to the original, heavy Wright meter. For many years, peak flow was measured only on the Wright.6

In USA, scientists developed another system of testing with a series of airflows of standardized waveforms generated by mechanical pumps. This is known as the American Thoracic Society (ATS) scale.7

Which PFM are you using?

The Wright scale has been previously noted to give readings that overestimate true flow by 40-80 L/min at 300 L/min and underestimate by 20-50 L/min at 720 L/min.8 From year 2004, UK and other European countries started using a new "standard" for peak expiratory flow (referred to as the new EU scale or EN13826). The PEF readings taken from a new EU scale meter are different from those taken from an old Wright-McKerrow scale or ATS scale (despite an unchanging lung function), e.g. achieving 500L/min on a new EU scale is the same as blowing 536L on an old Wright meter.

The new EU standard for PFM includes airflow waveforms additional to the ATS scale which ensure that PFM can perform with a high degree of accuracy under all circumstances. The new EN13826 Standard sets minimum acceptable levels for the overall performance of PFM, for such criteria as repeatability, accuracy, airflow resistance and reliability. In PFM with EU scale, readings change uniformly for the whole range of the meter.

PEF readings obtained on a EU scale meter should be more accurate than those from a Wright scale meter. The new standard has the advantage that readings are similar to those obtained from conventional spirometers as it is based on "absolute" airflows.

For Mini-Wright PFM, different colour codes are used on the scales (the part where PEF values are read) to show the difference:

* Standard Range (measure up to 800 L/min)

* Low Range (measure up to 400 L/min)

As the Mini-Wright design has not changed (only the scale has changed), it is possible to obtain accurate conversion between the Wright scale and EU scale using the online converter at www.peakflow.com.

Figure 1: PFM picture: first (from top), -Wright scale, low range; second, -EU scale, low range; third, -Wright scale, standard range; fourth, -EU scale, standard range

How to take care of your PFM?

PFM does not require calibration for three years following its first use.9 At the end of three years, it should be replaced. When not in use, it should be stored in a clean, dry area away from hazardous substances. It should be cleaned at least once every six months when in use. Fungal growths may occur if the instrument has not been cleaned for some time.

Cleaning can be done by immersing the PFM in warm, mild detergent solution for 2-3 minutes. It should be agitated to ensure thorough cleaning, then rinsed in clean warm water followed by shaking gently to remove excess water and allowed to dry completely before using again.

What is the current recommended use?

Although PFM are simple to use, there are wide variations even in the published predicted PEF reference values, which is specific to each brand of PFM. Suchbrand-specific values are currently not available for most brands.

We summarize the recommendations from five international guidelines in Tables 1 and 2 . Some recommendations are not explicitly supported by evidences and these guidelines have minor variations in grading the levels of evidence. We re-arrange the evidences provided by them into a simpler grading scale as:

Level A: Randomized controlled trials (RCTs) of rich body of data Evidence is from endpoints of well designed RCTs that provide a consistent pattern of finding in the population for which the recommendation is made. Level A requires substantial numbers of studies involving substantial numbers of participants.

Level B: Randomized controlled trials (RCTs) of limited body of data Evidence is from endpoints of intervention studies that include only a limited number of patients, posthoc or subgroup analysis of RCTs, or meta-analysis of RCT. In general, Level B pertains when few randomized trials exist, they are small in size, they are taken in a population that differs from the target population of the recommendation, or the results are somewhat inconsistent.

Level C: Non-randomized trials or observational studies Evidence is from outcomes of uncontrolled or non-randomized trials or from observational studies.

Level D: Panel consensus judgment This level is used only in cases where the provision of some guidance was deemed valuable but the clinical literature addressing the subject was insufficient to justify placement in one of the other categories. The Panel Consensus is based on clinical experience or knowledge that does not meet the above-listed criteria.

Discussion

We have outlined briefly the introduction of the new EU scale since 2004 in European countries and the steps in the use and care of the PFM. We have also briefly mentioned the current recommendations of the various authorities in the use of the PFM for the diagnosis and monitoring of patients with obstructive airway diseases. For a further in-depth discussion on this topic, please refer to the article by Timothy R Myers in the June 08 issue of Respiratory Care.14

PEF is a simple tool with its limitations for usage. It is effort dependent and insensitive to the obstruction in small and peripheral airways. It finds its usefulness in serial repetition and self use by asthmatic patients at the workplace and home, using results plotted against time. The pattern of the graph may identify aspects of the patient's disease. Isolated falls may locate the trigger event and specific allergen exposure. A downward trend over a few days may indeed indicate a worsening of asthma and hence can act as a warning sign for early medical attention.15

With regard to current recommendations by the authorities, there are several points worth noting:-

Diagnosis

All guidelines except the one from NHLB (USA) still regard PFM as an aid to the diagnosis of asthma. Patient Monitoring

Limitation

1. 1. PEF is effort dependent and insensitive to obstruction of the small and peripheral airways.
2. Many of the recommendations are based on opinion from panel of experts and score low in their level of evidence.
3. Spirometry is the lung function test of choice for the diagnosis and monitoring control of obstructive airway diseases in adults. PFM cannot replace the role of spirometers.

In practice, when recording PEF, the scale used should be specified and it is preferable for a family physician and his/her patient to use the same scale. PEF is particularly helpful in those patients who have poor perception or understanding of their symptoms and in those who have suspected occupational causes. It should be noted that isolated single PEF measurements are not adequate for use in routine asthma management and careful instruction is required to reliably measure PEF. Interestingly, despite theeasy availability of PFM, PEF in the management of asthma patients is reported to be underused.16 Finally, patients?previous best measurements are preferred for use in monitoring. Predicted PEF values, due to its wide variability in different localites and the PFM used, will only give us a rough guide in the absence of known previous best. A large scale local study on the PEFR predicted value is underway and this data will be available soon.

Key messages

1. All guidelines find long-term PEF useful in the monitoring of asthma control in adult patients and it is equally effective as symptom-based monitoring.
2. PFM finds its use with serial repetition and self use by asthmatic patients at their workplace and home using results plotted against time.
3. The scale used should be specified and it is preferable for a doctor and his patient to use the same scale. PEF is particularly helpful in those patients who have a poor perception or confusion of symptoms and in those who have suspected occupational causes.
4. Isolated fall may locate the trigger event and specific allergen exposure. A downward trend over a few days may indicate a worsening of asthma and hence acts as a warning sign for early medical attention.