What does an increase in Forced Expiratory Flow (FEF) in Pulmonary Function Tests (PFTs) indicate?

Interpretation of Increased FEF on Pulmonary Function Tests

An increase in forced expiratory flow (FEF) on pulmonary function tests typically indicates improvement in airway caliber and reduced airflow obstruction, most commonly seen after bronchodilator administration or with successful treatment of obstructive lung disease.

Clinical Context of FEF Changes

Post-Bronchodilator Response

• An increase in expiratory flows after bronchodilator administration reflects bronchodilation and improved airway patency 1
• Increased partial flows (flows measured during forced expiratory maneuvers initiated from volumes below TLC) often correlate better with symptom improvement than FEV1 changes alone 1
• The improvement in flow parameters within the tidal breathing range may explain decreased dyspnea even when FEV1 and FVC show minimal changes 1

Limitations of FEF Measurements

• FEF25-75% (mid-expiratory flow) is highly variable and has limited clinical utility when FEV1 and FEV1/FVC are normal 2
• FEF25-75% has greater intrasubject and intersubject variability compared to FEV1, resulting in a wider range of predicted values 2
• This parameter is highly dependent on the validity of FVC measurement and the level of expiratory effort 1

Physiological Interpretation

What Increased FEF Reflects

• Higher expiratory flows indicate reduced airway resistance and improved airway caliber 1
• In obstructive disease, increased flows after intervention suggest reversible airway narrowing rather than fixed obstruction 1
• The increase may reflect decreased bronchial smooth muscle tone, reduced airway wall edema, or decreased mucus plugging 1

Important Caveats

• Deep inhalations preceding forced expiratory maneuvers can paradoxically reduce airway caliber, especially after bronchodilator administration 1
• This volume history effect means that flows measured during tidal breathing may show greater improvement than those measured after deep inspiration 1
• FEF measurements are volume-dependent; if FVC changes, post-intervention FEF25-75% is not directly comparable to baseline values 1

Clinical Application

When to Consider FEF Changes Meaningful

• An isolated increase in FVC (>12% of control and >200 mL) not due to increased expiratory time after bronchodilator is a sign of bronchodilation 1
• Improvement in lung function parameters within the tidal breathing range, such as increased partial flows, may explain symptom improvement even without significant FEV1/FVC changes 1
• In patients with disproportionate differences between spirometric impairment and dyspnea, measurements of flow parameters may provide additional useful information 1

Practical Considerations

• FEF should not replace FEV1 and FEV1/FVC as primary measures for diagnosing or assessing airway obstruction severity 2
• The lack of acute FEV1 response does not preclude clinical benefit, as many patients show volume and flow responses without significant FEV1 improvement 3
• Responses to bronchodilator therapy are often underestimated by FEV1 and FVC compared to airway resistance or flows measured during partial expiratory maneuvers 1

Monitoring Treatment Response

• Assess clinical response using multiple parameters beyond just FEV1, including inspiratory capacity, residual volume, symptom improvement, and exercise tolerance 3
• In patients with COPD, lower FEF25-75% is associated with more severe disease manifestations including increased emphysema, functional small airways disease, and air trapping 4
• An increase in FEF parameters during treatment may indicate improvement in small airway function and reduced air trapping 4