Interpreting Spirometry for Lung Disease Diagnosis
Introduction to Spirometry Interpretation
- The European Respiratory Society recommends a step-by-step approach to interpreting spirometry charts, starting with the FEV1/FVC ratio to identify obstruction, then assessing FVC for restriction, and finally evaluating post-bronchodilator response to confirm COPD diagnosis and rule out asthma 1, 2
Key Spirometric Parameters
- The FEV1/FVC ratio is the primary parameter for identifying airflow obstruction, with a ratio < 0.7 or below the 5th percentile indicating obstruction 1, 4
- FVC is the total volume exhaled during a forced expiratory maneuver, with values below the 5th percentile or <80% predicted indicating possible restriction 2, 3
- TLC is the total volume of air in the lungs after maximal inspiration, required to confirm restrictive disease 2, 5
Step-by-Step Interpretation Algorithm
- If FEV1/FVC < 0.7 (or below 5th percentile), an obstructive pattern is present 1, 6
- If FVC is normal, lung volumes are considered normal 2
- A low FEV1/FVC ratio with normal FVC indicates pure obstruction 2, 7
- A normal FEV1/FVC ratio with low FVC indicates possible restriction, requiring TLC confirmation 5, 7
Assessing Bronchodilator Response
- A positive response to bronchodilator, defined as improvement in FEV1 or FVC ≥12% and ≥200mL in adults, suggests asthma or partially reversible COPD 1, 8
- A minimal or no response to bronchodilator is consistent with fixed airway obstruction (COPD) 1, 8
Interpreting Flow-Volume Loops
- An obstructive pattern is characterized by a concave expiratory flow curve, with normal or increased TLC 2, 7
- A restrictive pattern is characterized by a convex or normal-shaped curve with reduced volumes and low TLC 5, 7
Special Considerations
- Volume responders, with pre-BD FEV1/FVC ≥0.7 but post-BD FEV1/FVC <0.7, may indicate gas trapping 1, 8
- Flow responders, with pre-BD FEV1/FVC <0.7 but post-BD FEV1/FVC ≥0.7, require monitoring as they may develop persistent obstruction 1, 8