What does a decreased peak expiratory flow rate (PEFR) mean?

Understanding Decreased Peak Expiratory Flow Rate

A decreased peak expiratory flow rate (PEFR) primarily indicates airflow obstruction, which can occur in various respiratory conditions including asthma, COPD, upper airway obstruction, or respiratory muscle weakness. 1, 2

Clinical Significance of Decreased PEFR

Pathophysiological Meaning

• PEFR measures the maximum flow rate achieved during forceful expiration from full inspiration, expressed in liters per minute (L/min)
• Decreased PEFR reflects increased airway resistance, particularly in the large airways 1
• It's an effort-dependent parameter that depends on:
  • Lung elastic recoil
  • Respiratory muscle strength
  • Airway caliber
  • Patient effort 1, 2

Diagnostic Implications

Decreased PEFR can indicate:

1. Obstructive Lung Disease:

   • Common in asthma and COPD
   • In asthma: PEFR is classically reduced and variable 3
   • In COPD: Reduced PEFR may signal disease instability and predict worse outcomes 4

2. Central or Upper Airway Obstruction:

   • When FEV1/PEFR ratio is >8, this suggests central or upper airway obstruction 1
   • Can occur with both intrathoracic and extrathoracic lesions 1

3. Respiratory Muscle Weakness:

   • Neuromuscular disorders can reduce PEFR due to impaired expiratory muscle function 1
   • Maximum expiratory flow at large lung volumes (including PEFR) is reduced in respiratory muscle weakness 1

4. Poor Respiratory Effort:

   • As PEFR is effort-dependent, poor initial effort can affect measurements 1

Clinical Applications

Monitoring Disease Status

• Asthma Management: PEFR is valuable for monitoring asthma control and detecting early exacerbations 1, 3

  • Increased variability in daily PEFR readings suggests poor asthma control
  • Morning/evening PEFR variations >12-15% suggest unstable asthma 5

• COPD Monitoring: Greater variability in daily PEFR in severe COPD patients correlates with:

  • Fewer exacerbation-free days
  • Shorter time to first hospitalization
  • More frequent hospitalizations
  • Higher all-cause mortality 4

Distinguishing Patterns of Obstruction

• Central vs. Peripheral Obstruction:

  • Central airway obstruction may severely affect PEFR without proportionate reduction in FEV1 1
  • An increased FEV1/PEFR ratio (>8) suggests central or upper airway obstruction 1

• Fixed vs. Variable Obstruction:

  • Flow-volume loops help distinguish between fixed, variable extrathoracic, and variable intrathoracic obstructions 1
  • Maximum expiratory flow (including PEFR) is generally decreased in both intrathoracic and extrathoracic lesions 1

Clinical Pitfalls and Considerations

• Technique Dependency: PEFR measurement is highly dependent on proper technique and patient effort 6
• Reference Values Variation: PEFR varies by age, gender, height, weight, ethnicity, and altitude 7
• Complementary Testing: PEFR should be interpreted alongside other pulmonary function tests for comprehensive assessment 2
• Persistence After Remission: Even in asthma remission, mild airflow obstruction with decreased PEFR may persist 5

Conclusion

Decreased PEFR is a valuable indicator of airflow limitation that should prompt further investigation into potential causes. While particularly useful in monitoring asthma and COPD, it provides complementary information to other pulmonary function tests and should be interpreted within the clinical context of the patient's symptoms and disease history.