What is PEF (Peak Expiratory Flow) in the context of asthma management?

What is PEF (Peak Expiratory Flow)?

PEF is the highest flow achieved from a maximum forced expiratory maneuver started without hesitation from a position of maximal lung inflation, expressed in L/s (or L/min with portable devices), and represents a simple measure of airway obstruction. 1

Technical Definition and Measurement

• PEF measures the maximum speed at which air can be forcefully exhaled following full inspiration, with the defining characteristics being the rise time (RT) - time for flow to rise from 10% to 90% of PEF - and dwell time (DT) - duration that flow remains >90% of PEF 1

• The measurement requires maximal lung inflation to maximize elastic recoil pressure and minimize upstream frictional pressure loss, with maximum expiratory effort causing wave speed limitation at higher lung volumes 2

• When obtained from flow-volume curve data, PEF is expressed at BTPS (body temperature, pressure, saturated) in L/s, while portable monitoring instruments express it in L/min 1

Clinical Role in Asthma Management

Limitations as a Clinic Measurement

• PEF is inferior to FEV1 as a clinic-measured parameter of airways obstruction because it confers no advantage in reproducibility, lacks accurate reference values for many populations, and may underestimate airway obstruction in individuals with airway remodeling 1

• The European Respiratory Society recommends that PEF should not be relied upon as a substitute for FEV1 in assessing the degree of airway obstruction, especially in severe emphysema 3

Optimal Use: Home Monitoring

• PEF measurement is most suitable for ambulatory monitoring for within-patient comparisons over time, providing objective day-to-day assessment of airway obstruction 1, 4

• Home lung function measurement helps detect deterioration and guide treatment changes, particularly in patients with refractory asthma who often have reduced perception of their airflow limitation 4

Diagnostic Utility

• A 20% or greater variability in amplitude % best (highest - lowest/highest × 100) with a minimum change of at least 60 L/min over time is highly suggestive of asthma 1

• However, many patients with asthma demonstrate variability below 20%, making this a reasonably specific but insensitive diagnostic test - marked variability confirms asthma, but smaller changes do not exclude it 1

• Sequential measures of PEF variability correlate with increased airway responsiveness, and diurnal variability >13% is an important diagnostic characteristic of asthma 4

Practical Implementation

Measurement Technique

• The subject must exhale completely to residual volume (RV), then inhale maximally to total lung capacity (TLC), and blow as vigorously as possible with the neck in neutral position (not flexed or extended) 1

• PEF is dependent on effort and lung volume, with patient cooperation being essential - the subject must be encouraged to blow as rapidly and vigorously as possible 1

• A nose clip is not necessary, and the patient must not cough during the maneuver 1

Home Monitoring Protocol

• The same instrument should be used on each occasion if possible to ensure consistency 1, 4

• Patients should record PEF before taking routine medication and preferably before reliever medication, with most studies using morning recordings 4

• Adequate training is mandatory, with technique checked at every clinic visit to ensure good within-session reproducibility 4

• Patients should complete symptom diaries before measuring PEF to avoid bias, and the highest value from three maneuvers is usually analyzed 4

Important Caveats

• In children aged 4-8 years, approximately 50% are unable to perform PEF maneuvers adequately, representing a significant limitation in this population 4

• The frequently cited cut-point for diurnal variability of >20% for diagnosing asthma is not applicable to twice-daily monitoring - this criterion originated from four-times-daily PEF data 1

• Compact electronic devices now make daily home monitoring of FEV1 a realistic and successful alternative to PEF monitoring 1, 4

• When serial office PEFs are recorded, the same instrument should be used on each occasion as PEF is sensitive to meter resistance (e.g., resistance of 0.25 kPa·L⁻¹·s⁻¹ decreases PEF by approximately 8%) 1