What is the significance of NED (Normalized Expiratory Duration) 25, MEF (Maximum Expiratory Flow) 50, and MEF 75 on Pulmonary Function Tests (PFTs)?

Understanding MEF 25, MEF 50, and MEF 75 in Pulmonary Function Tests

MEF (Maximum Expiratory Flow) values at 25%, 50%, and 75% of FVC are important indicators of airway function, with MEF 75 being particularly valuable for detecting small airway obstruction.

What are MEF 25, MEF 50, and MEF 75?

MEF values represent airflow measurements at specific points during forced expiration:

• MEF 75: Flow rate when 75% of FVC remains in the lungs (equivalent to FEF 25)
• MEF 50: Flow rate when 50% of FVC remains in the lungs (equivalent to FEF 50)
• MEF 25: Flow rate when 25% of FVC remains in the lungs (equivalent to FEF 75)

Note: There are two naming conventions:

• European convention (MEF): Refers to % of FVC remaining in lungs
• US convention (FEF): Refers to % of FVC that has been exhaled

Clinical Significance

1. Small Airway Function Assessment

• MEF 75 (FEF 25): Most sensitive for detecting small airway obstruction 1
• As lung volume decreases during expiration, smaller airways are more likely to be flow-limited, making measurements at lower lung volumes (MEF 75) particularly informative about small airway function 1

2. Early Detection of Obstruction

• The earliest changes associated with small airway obstruction appear in the terminal portion of the spirogram, reflected in:
  • Concave shape on the flow-volume curve
  • Proportionally greater reduction in MEF 75 than in FEV1 1

3. Variability and Reliability

• MEF 75 and FEF 25-75 have been found to be the least variable among flow parameters 1
• However, mid-range flow measurements are not specific for small airway disease in individual patients 1

4. Response to Bronchodilators

• MEF values can show significant changes after bronchodilator administration
• In some studies, MEF parameters demonstrated greater responsiveness to bronchodilators than FEV1 1
• A significant bronchodilator response in MEF values may indicate airway hyperresponsiveness

Interpretation Guidelines

1. Normal vs. Abnormal:

   • Compare values to age, height, and gender-matched reference values
   • Values below the 5th percentile of predicted are considered abnormal 1

2. Pattern Recognition:

   • Proportionally greater reduction in MEF 75 than FEV1: Suggests small airway obstruction
   • Reduction in all MEF values: Indicates more widespread airway obstruction
   • Normal FEV1/FVC with reduced MEF values: May indicate early small airway disease

3. Disease Correlation:

   • Air trapping and bronchial wall thickening show high correlation with reduced MEF values 2
   • MEF 75 is particularly useful in detecting early obstructive changes before FEV1 is affected

Clinical Applications

• Asthma: Early detection and monitoring of airway obstruction
• COPD: Assessment of small airway involvement
• Bronchiolitis: Evaluation of small airway function
• Post-bronchodilator assessment: Evaluating reversibility of airway obstruction

Common Pitfalls to Avoid

1. Over-interpretation: MEF values have higher variability than FEV1 and FVC
2. Isolated interpretation: Always interpret MEF values in context with other PFT parameters
3. Comparison issues: If FVC changes between tests (e.g., pre/post-bronchodilator), MEF values may not be directly comparable 1
4. Technical factors: MEF values are effort-dependent and can be affected by testing technique

Summary

MEF 25, MEF 50, and MEF 75 provide valuable information about airflow at different lung volumes during forced expiration. MEF 75 (FEF 25) is particularly useful for detecting small airway obstruction and may show abnormalities before changes in FEV1 become apparent. While these measurements have higher variability than FEV1 and FVC, they add important diagnostic information when interpreted in the context of complete pulmonary function testing.