What is the role of peak expiratory flow (PEF) in managing asthma exacerbation?

Peak Expiratory Flow in Asthma Exacerbation Management

Peak expiratory flow (PEF) serves as the primary objective measure for classifying exacerbation severity and guiding treatment decisions in the emergency department, with percent predicted FEV1 or PEF being the primary determinant of whether an exacerbation is mild, moderate, severe, or life-threatening. 1

Role in Severity Classification

• PEF (or FEV1) percent predicted is the primary determinant for categorizing asthma exacerbations into mild, moderate, severe, or life-threatening categories, which directly determines treatment intensity and disposition decisions 1
• The National Asthma Education and Prevention Program emphasizes that objective measures of lung function are more reliable indicators of severity than symptoms alone 2
• Serial PEF measurement provides objective documentation of improvement during ED treatment 1

Clinical Application in the Emergency Department

Initial Assessment

• Measure PEF immediately upon ED arrival before initiating treatment to establish baseline severity 1
• PEF should be interpreted alongside clinical signs (respiratory rate, pulse rate, oxygen saturation, use of accessory muscles, ability to speak in sentences) 1
• Nine international guidelines recommend PEF monitoring specifically for assessing asthma exacerbations 1

Monitoring Treatment Response

• The first post-treatment PEF (after initial bronchodilator therapy) captures approximately 86% of total improvement that will occur during the ED visit 3
• Subsequent serial PEF measurements add minimal additional information, with PEF after second treatment representing only 7.5% of total improvement and third treatment only 8.6% 3
• This suggests that one post-treatment PEF measurement is sufficient for most clinical decision-making rather than multiple serial measurements 3

Important Caveats and Pitfalls

Limitations of "Personal Best" PEF

• Only 29% of inner-city ED patients can accurately report their personal best PEF 4
• Among those who report a personal best, 45% actually achieve higher measured values during follow-up, indicating their reported value was inaccurate 4
• Using inaccurate patient-reported personal best PEF could lead to inappropriate ED discharge decisions if used as the denominator for calculating percent predicted 4
• Therefore, use population-based predicted PEF values rather than patient-reported personal best in the acute ED setting 4

PEF Patterns Differ Between Exacerbations and Poor Control

• Diurnal PEF variability does not increase during exacerbations (7.7% during exacerbation vs 5.4% during stable asthma, p=0.1), unlike poorly controlled chronic asthma where variability is 21.3% 5
• During exacerbations, PEF falls and rises linearly over days rather than showing increased variability, suggesting impaired beta-agonist response 5
• This means calculation of diurnal variability may fail to detect exacerbations and should not be relied upon for this purpose 5

Target Values for Management

• Target minimum PEF of 80% predicted with daily variability ≤10% represents optimal asthma control 6
• When minimum PEF reaches 40%, daily variability increases to 46% and beta-agonist use increases to 2.6 times daily, indicating significant deterioration 6
• Daily frequency of beta-agonist inhalation correlates strongly with minimum PEF (r=0.76) and serves as a useful complementary indicator of symptom severity 6

Practical Recommendations

• Obtain baseline PEF before treatment and one measurement after initial bronchodilator therapy to assess response 1, 3
• Use percent predicted PEF based on population nomograms rather than patient-reported personal best 4
• Recognize that 95% of exacerbations occur with respiratory infections even in patients with otherwise well-controlled asthma 5
• PEF chart standardization is needed, with preference for horizontally compressed formats to facilitate pattern recognition of exacerbations and trends 7