Forced Expiratory Volume (FEV) is a measurement of respiratory mechanics that provides insight into how well a person’s lungs are functioning. This test quantifies the volume of air that can be forcibly exhaled from the lungs and the speed at which that air is expelled following a maximal inhalation. It is an objective way to evaluate the capacity and flow within the respiratory system. Analyzing FEV measurements allows medical professionals to identify patterns of impairment that suggest the presence of pulmonary disease. This non-invasive assessment helps determine the health of the airways. The data gathered helps guide treatment decisions and monitor the progression of known lung conditions.
How Forced Expiratory Volume is Measured
The data for FEV is collected using a test called spirometry. The procedure involves the patient sitting comfortably and taking the deepest breath possible to fill their lungs to total capacity. The patient then seals their lips tightly around a mouthpiece connected to the spirometer machine.
Following the deep inhalation, the patient must exhale all the air from their lungs as hard and as fast as they can. This forceful and sustained exhalation is the maneuver that generates the FEV measurements.
Achieving accurate results depends heavily on the patient’s maximal effort and proper technique. Technicians will often coach the patient to ensure the blow lasts for at least six seconds and that the exhalation is continuous without hesitation. Multiple attempts are usually required, with the highest values being selected for the final interpretation.
Understanding the Key Metrics
Spirometry yields several specific measurements, with two primary values being the foundation of FEV analysis. The first is Forced Expiratory Volume in one second (FEV1), which represents the exact volume of air forcefully exhaled during the first second of the maneuver.
The second fundamental measurement is the Forced Vital Capacity (FVC), which represents the total volume of air the person can forcefully exhale after a maximal inspiration. The FVC measures the overall lung volume that can be moved.
The most informative calculation for diagnosing respiratory issues is the FEV1/FVC ratio. This ratio is the percentage of the total exhalable air (FVC) that was expelled in the first second (FEV1). In healthy adults, this ratio typically falls between 70% and 80%.
The measured FEV and FVC values are compared to a set of predicted normal values. These predicted values are calculated using reference equations based on the individual’s age, height, gender, and sometimes ethnicity. This comparison helps determine if the patient’s lung function is lower than what is expected for a person of their physical characteristics. A value is considered lower than expected if it falls below 80% of the predicted value. This standardization allows clinicians to assess the severity of any identified lung impairment.
What Abnormal Results Indicate
Abnormal FEV results fall into two distinct patterns that indicate different mechanical problems within the lungs.
Obstructive Defect
The obstructive defect is characterized by a disproportionate reduction in the FEV1 compared to the FVC, resulting in a low FEV1/FVC ratio. This suggests difficulty in getting air out of the lungs. The airways are narrowed or blocked, which significantly slows the rate of airflow, even if the total volume of air (FVC) is normal. This narrowing is typically due to issues like inflammation, bronchospasm, or loss of elastic recoil.
Restrictive Defect
The restrictive defect is identified by a low FVC, often with a normal or high FEV1/FVC ratio. In this case, the total amount of air the lungs can hold is reduced, indicating a problem with the lungs’ ability to fully expand. The lung tissue itself may be stiff and resistant to stretching, or expansion may be limited by issues outside the lungs, such as the chest wall. Although the total capacity is diminished, the air that is inhaled is expelled quickly because there is no obstruction within the airways.
The specific relationship between the FEV1 and the FVC allows medical professionals to differentiate between these two fundamental types of ventilatory impairment. This distinction is important because the underlying causes and necessary treatments for obstructive and restrictive patterns are completely different.
Common Conditions Affecting FEV
The obstructive pattern, marked by a low FEV1/FVC ratio, is associated with conditions that narrow the airways.
Obstructive Conditions
Chronic Obstructive Pulmonary Disease (COPD) is a primary example, involving progressive and irreversible airflow limitation. COPD includes chronic bronchitis and emphysema, both of which reduce the lungs’ ability to forcefully expel air. Asthma also presents with an obstructive pattern, but the airway narrowing is often reversible. This reversibility, often seen after receiving a bronchodilator, helps distinguish asthma from COPD.
Restrictive Conditions
For the restrictive pattern, where the FVC is low but the FEV1/FVC ratio is preserved, the conditions involve reduced lung volume. Interstitial lung diseases, such as pulmonary fibrosis, cause scarring and stiffness in the lung tissue, preventing full expansion. Other restrictive causes include:
- Sarcoidosis, which leads to inflammation and scarring.
- Severe obesity.
- Spinal deformities.
While spirometry results are informative, a complete picture of a patient’s respiratory status requires integrating FEV data with their clinical history and other physical assessments.