Spirometry in Pulmonary Function Tests
Objectives
- Understand FEV1, FVC, FEV1/FVC ratio, and flow volume loop.
- Use these values for diagnostic evaluations.
- Grade disease severity using FEV1.
What is Spirometry?
- Technique to measure airflow and lung volumes.
- Historical method involved breathing through a tube connected to a submerged chamber.
- Modern spirometers use digital circuits.
- Procedure:
- Inhale maximally then exhale quickly for at least 6 seconds.
- Repeat maneuver three times.
- Record highest lung volumes and air flows.
Key Measures in Spirometry
- Vital Capacity (VC): Measured during a forced exhalation as Forced Vital Capacity (FVC).
- Forced Expiratory Volume in One Second (FEV1): Volume of air exhaled in the first second.
- FEV1/FVC Ratio: Important in PFT interpretation.
- Peak Expiratory Flow Rate (PEFR): Maximum slope of the exhaled volume/time curve.
- Flow Volume Loop:
- Graph of airflow vs. volume.
- Provides insights into lung diseases, particularly airway obstruction.
Diagnosing with Spirometry
- Obstructive Lung Disease (e.g., COPD):
- FEV1 usually decreased.
- FVC normal in mild/moderate but decreases with severe obstruction.
- FEV1 decreases more than FVC; ratio decreases below 70%.
- Restrictive Lung Disease:
- FEV1 normal or decreased.
- FVC decreased; ratio normal or increased.
Diagnostic Flowchart
- Assess FEV1/FVC ratio.
- If low, assess FVC:
- Normal FVC: Obstructive lung disease.
- Low FVC: Severe obstruction or mixed.
- If normal/elevated, assess FVC:
- Low FVC: Restrictive lung disease.
- Normal FVC: Normal lung mechanics.
Additional Measurements
- FEF25-75: Average flow from 25% to 75% of FVC exhaled.
- Maximal Voluntary Ventilation (MVV): Max air inhaled/exhaled in 1 minute.
- Bronchodilator Response:
- Repeat spirometry after bronchodilator.
-
12-15% increase in FEV1 is significant.
Patterns in Flow-Volume Loop
- Mild Obstruction: Preserved FVC, late plateau in volume.
- Severe Obstruction: Reduced FVC, pronounced coving in flow-volume loop.
- Restriction: Normal airflow rate, reduced FVC.
- Upper Airway Obstruction: Variable extrathoracic, intrathoracic, and fixed obstructions affect inspiratory/expiratory flow differently.
COPD Severity Staging
- Stage 1: FEV1 ≥ 80%
- Stage 2: FEV1 50-80%
- Stage 3: FEV1 30-50%
- Stage 4: FEV1 < 30%
- Weak correlation between FEV1 and quality of life.
Conclusion
- Spirometry provides critical information for diagnosing lung diseases.
- Interpretation focuses on FEV1, FVC, and their ratio.
- Response to bronchodilators can guide treatment but not solely determine it.
Note: Next video will cover lung volume measurements.