Peak expiratory flow (PEF)

Expiratory peak flow (PEF) is the maximum flow generated during expiration performed with maximal force and started after a full inspiration (ref. 1). PEF is appreciably larger if the maneuver is performed without pause immediately after the inspiration than if it is performed after a pause (ref. 2). Contrary to popular belief PEF is not effort dependent: if the maneuver is performed sufficiently forcefully in most subjects PEF is determined by flow limitation in central, possibly also in more peripheral airways (ref. 3).

• Report the largest value of 3 correctly performed maneuvers, but the difference between the largest two should be less than 40 L/min; if the difference is larger then have up to 2 extra efforts performed. If even then the two largest values differ by > 40 L/min, then report the largest one with a note to the effect that reproducible measurements could not be obtained.
• Daily variability in PEF is larger in asthmatics than in healthy subjects, and is used as an index of the activity of the disease process. To that end PEF is measured daily at least at rising, at about noon, and at night before going to sleep; variability is expressed as the ratio of the difference between the highest and lowest PEF divided by the average of all measurements that day. An improved or lower level of PEF, of its variability, or both, signifies improvement or deterioration of the asthma. in non-asthmatics variability is < 20% in adults, < 31% in children (ref. 4); if monitoring of PEF is limited to subjects who are likely to have asthma this will favorably affect the proportion of false negative findings.
• Predicted values of PEF are of little use in detecting lung disease, as variability between healthy subjects is large, so that a value within the normal range certainly does not rule out the presence of airway obstruction. However, a pre-bronchodilator PEF of >70% predicted effectively rules out stages III and IV COPD of the Global Initiative for Chronic Obstructive Lung Disease (ref. 8).
• The best reference value for a subject is de best value this subject produced when in a clinically good condition.
• In severe obstructive lung disease rapid and extensive dynamic airway compression during the forced maneuver contribute appreciably to initial maximum expiratory flow from the lungs (ref. 5); the severity of airway obstruction then tends to be underestimated from PEF.
• PEF is not often used to assess bronchodilator responsiveness An increase in PEF by 60 L/min or more in adults has been regarded as a sign of significant improvement, but recent evidence suggests that PEF should not be used to that end (ref. 6). An abnormally low PEF may be due to:
  • obstructive lung disease
  • unsatisfactory cooperation
  • prior inhalation not to total lung capacity
  • restricted lung expansion (stiff chest cage, muscular or neurogenic disorder, etc.)

The original peak flow meter was not accurately calibrated, with an over-reading in the middle of the range, but errors vary over the range of the meters (ref. 7). This has bearing on measurements of within-day variation in patients with asthma, a criterion used for diagnostic purposes and in the management of asthma. Therefore, the European Commission adopted a standard (EN 13826) for these meters in 2004. As all member countries of the European must implement the standard before 2005, it is best to complete the change to the meters with the new scale (with a CE mark) in 2005, as this will lead to more consistent management of asthma and avoid confusion arising from the use of meters with the old and new scale. Also do record PEF readings with respect to the scale they were obtained with.
Predicted values for PEF have been obtained with meters with the 'old' scale. The most popular ones were those published by Nunn and Gregg, and these can be converted to the new scale.

Recommended procedures:

1. Quanjer PhH, Tammeling GJ, Cotes JE, Pedersen OF, Peslin R, Yernault JC. Lung volumes and forced ventilatory flows. Official Statement of the European Respiratory Society. Eur Respir J 1993; 6 suppl. 16: 5-40. Erratum Eur Respir J 1995; 8: 1629.
2. American Thoracic Society. Standardization of spirometry: 1994 update. Am J Respir Crit Care Med 1995; 152: 1107-1136.
3. Quanjer PH, Lebowitz MD, Gregg I, Miller MR, Pedersen OF. Peak expiratory flow. Conclusions and recommendations of a working party of the European Respiratory Society. Eur Respir J 1997; 10 suppl 24: 2S-8S.
4. Miller MR et al. Standardisation of spirometry. ATS/ERS task force: standardisation of lung function testing. Eur Respir J 2005; 26: 319-338.