Peak Flow Meter

Most people with a diagnosis of asthma have a portable peak flow meter gathering dust somewhere about the house. Many sufferers of asthma would consider that they know themselves and their condition well enough to sense when an attack is coming and use more reliever medication.  This is not always, or even often, the case.

Those with moderate to severe asthma are recommended to self-monitor at home on a weekly to daily basis by checking and recording the peak expiratory flow (PEF) using a PEF meter. PEF results should help one to adjust medication use as instructed by your doctor using an asthma action plan. PEF monitoring becomes life-saving for those who are unable to sense a worsening of their asthma.

The portable PEF meter has three main uses:

  • To regularly monitor lung function and response to treatment over the short- and long-term
  • To determine the severity of an asthma attack
  • To assess response to treatment during an attack.

So what is PEF and why should we measure it?

Peak Expiratory Flow is the highest flow rate of exhaled air from a maximal in breath.

Like most biological variables (like height, weight, blood pressure). PEF has a range of values and these are dependent on our age, gender and the actual size of our lungs (most easily indicated by our height).  Physiologists have taken measurements of lung function and compiled normal data from large groups of people with no known respiratory condition so calculation of a “predicted” PEF can be made.

Click here to calculate your Peak Flow

The PEF is not, however, as accurate a measure of lung function as the forced expiratory volume at 1 second (FEV1) and the FEV1/forced vital capacity (FVC) ratio. Lung function testing (otherwise known as spirometry) is carried out by most Asthma Societies and can be performed or arranged through the family doctor. The frequency of the testing depends on the severity of the patient’s symptoms. In mildly symptomatic, well-controlled asthmatics, the test could be completed once every 1 to 2 years. In more severe asthmatics, testing should be done more frequently to determine medication compliance and the patient’s ability to monitor and control symptoms.

Peak flow meters for individual use are available free of charge through your family doctor who obtains them through a wholesale supply order. They are funded for NZ residents through PHARMAC.  They are easy to use. However, the resulting measurements are highly dependent upon the user’s technique. It is therefore important that the family doctor periodically checks the use of the meter, and corrects any mistakes in technique.

Every person with asthma should be instructed in how to establish a baseline measure of peak flow when feeling entirely well: the “personal best” peak flow value (see instructions in box below). The personal best PEFR is then used to determine the normal PEFR range, which is between 80 and 100 percent of the personal best recording. The concept of the “personal best” PEF is a good one because it focuses on the maintenance of optimal lung function which is the aim of all asthma management.

Readings below this normal range indicate airway narrowing, a change that may occur before symptoms are noted.

Peak Flow normally changes throughout the day, normally reaching a trough in the early hours of the morning. In asthma, this undulation of the PEF is exaggerated.  Many asthma sufferers will have experienced the uncomfortable wakening in the early hours of the morning needing to reach out or rummage around in the dark for the reliever medication.  This “morning dipping” is an important sign indicating deteriorating control.

How to use your Peakflow Meter

Getting the best readings — several steps are important to make sure the peak flow meter records an accurate value:

  • The peak flow meter should read zero or its lowest reading when not in use
  • Use the peak flow meter while standing up straight
  • Take in as deep a breath as possible
  • Place the peak flow meter in the mouth, with the tongue under the mouthpiece
  • Close the lips tightly around the mouthpiece
  • Blow out as hard and fast as possible; do not throw the head forward while blowing out
  • Breathe a few normal breaths and then repeat the process two more times. Write down the highest number obtained. Do not average the numbers.

Note: The test should be repeated if the tongue partially blocks the mouthpiece or if the patient coughs or spits during the test. Most peak flow meters need to be cleaned periodically; cleaning instructions should be available when the unit is first received or purchased.

Establishing a baseline measurement — unlike a blood pressure reading or a cholesterol test, there is no PEFR that is normal for everyone. For this reason, it is important to determine what PEFR value is normal for you.

To determine your normal PEF, you should measure their PEFR when you have no asthma symptoms and have been using your asthma medications as instructed by your medical advisor and asthma nurse. Three PEF measurements should be done with the same peak flow meter two to four times daily for two to three weeks.

You should note the highest PEF measure achieved; this is the “personal best” PEFR. This number is used to determine if future PEFR readings are normal or low, and is also used to create a normal PEFR range (between 80 and 100 percent of the personal best PEFR).

Readings below the normal range are a sign of airway narrowing in the lungs. A low PEFR can occur before asthma symptoms such as wheezing or shortness of breath develop.

A personal best PEFR value should be remeasured once per year to account for growth (in children) or changes in the disease (in both children and adults). In addition, home PEFR measurements should be verified with readings taken with equipment in a healthcare provider’s office since this equipment is more sensitive. For long-term management, most clinicians will recommend PEFR testing once per day, usually in the morning.

Once you have an up-to-date knowledge of your best personal peak flow measurement, regular monitoring can indicate early changes in symptoms.

This advance warning of an attack can be very useful with modern management strategies of asthma control.  Typically, some days before an attack is apparent with the onset of cough, wheeze or shortness of breath, there is an exaggeration of the normal day-night undulation of PEF.  This is the first clue that could indicate a worsening time ahead.  Next, there is an absolute drop in PEF often occurring even though there may be no symptoms.  This “advance warning” of an attack can help prepare those with an asthma action plan to increase medications and prevent an attack.

Although not all studies looking at whether regular Peak Expiratory Flow (PEF) monitoring have shown a beneficial effect of the practice, it is generally agreed that combining subjective and objective recording of asthma control contributes to fewer days limited by asthma and fewer visits to hospital and after hours medical centres.  This short article by Dr Jim Lello (Auckland Asthma Society medical advisor, General Practice) reviews the rationale and use of PEF meters for adults with moderate to severe asthma.