Peak flow measurement animated demonstration
Apply a new disposable mouthpiece and filter if multiple patients use one device. Check that the patient has a baseline peak expiratory flow rate (PEFR). Obtain this either from their records - a personal best recorded within the last two years - or, if they have not measured their PEFR before, calculate it from a predicted PEFR chart. In the case of the latter, note the patient’s age, sex, and height.
As with any procedure, clinicians should not perform measurement of the PEFR where there is lack of consent or understanding by the patient. Although it is an entirely safe and non-invasive procedure, its accuracy is dependent on expiratory effort from the patient. Therefore, it may be unsuitable or impractical for patients who are elderly and confused or for young children.
Measurement of the PEFR is most relevant to obstructive respiratory conditions such as asthma and, to a lesser extent, chronic obstructive pulmonary disease (COPD), which intermittently or progressively inhibit the flow of air in and out of the lungs. In relation to these conditions, measurement of the PEFR may be indicated in diagnosis and management.
The British Thoracic Society/Scottish Intercollegiate Guidelines Network guideline on the management of asthma recommends using measurement of PEFR in the following situations:
When assessing the initial probability of asthma in adults, PEFR to measure symptom variability may be used to corroborate a history of recurrent episodes (attacks) of symptoms as part of a structured clinical assessment
PEFR is best used to provide an estimate of variability of airflow from multiple measurements made over at least 2 weeks; increased variability may be evident from twice daily readings
In children, however, serial measures of peak flow variability and forced expiratory volume in the first second of expiration show poor concordance with disease activity and do not reliably rule the diagnosis of asthma in or out
PEFR records from frequent readings (at least 4 per day) taken at work and away from work are the initial investigation of choice in suspected occupational asthma
Measurement of the PEFR may form part of an agreed personalised action plan for patients with asthma, although symptom-based plans are generally preferable for children
In patients presenting with an acute asthma attack, measurement of the PEFR may provide a gauge of the severity of the attack and inform management.
Spirometry is the gold standard in the diagnosis of COPD. Although research suggests a significantly reduced PEFR (less than 80% of predicted) may be sensitive in detecting COPD, the Global Initiative for Chronic Obstructive Lung Disease report on the diagnosis, management, and prevention of COPD states that: “Despite its good sensitivity, peak expiratory flow measurement alone cannot be reliably used as the only diagnostic test because of its weak specificity”. Unless the PEFR is dramatically less than predicted it should not be interpreted without recourse to formal spirometry testing.