Forced Expiratory Volume (FEV) is a direct measurement of how much air an individual can forcibly exhale from their lungs within a specific timeframe following a maximal inhalation. This volume is a fundamental gauge of lung function and overall breathing capacity, providing insights into the mechanical properties of the airways. Healthcare professionals use FEV to assess lung health and determine how quickly air can be expelled. FEV is measured through spirometry, a standardized, non-invasive procedure routinely used in respiratory medicine.
How Forced Expiratory Volume Is Measured
The measurement of Forced Expiratory Volume is performed using a spirometer. The procedure begins with the patient inhaling as deeply as possible, filling their lungs completely with air. After this maximal inhalation, the patient must seal their lips around a mouthpiece and exhale as hard and as fast as they can.
The exhalation effort must be sustained until the lungs are completely empty, typically requiring a forceful blow lasting for at least six seconds. The spirometer records the volume and flow rate of the air being expelled. This process requires the patient’s maximal effort, and multiple attempts are often necessary to ensure the best and most reproducible results are obtained.
Defining Key Spirometry Metrics
The spirometry test generates several key numerical outputs, the most common being the Forced Expiratory Volume in 1 second (FEV1). FEV1 measures the volume of air forcefully exhaled during the first second of the maneuver. This measurement reflects the initial speed and ease with which air leaves the lungs, making it a direct indicator of potential airflow obstruction.
Another metric is the Forced Vital Capacity (FVC), which represents the total volume of air exhaled during the entire forced breath, from maximal inhalation until the lungs are empty. These two values are combined to create the FEV1/FVC ratio, a percentage calculated by dividing FEV1 by FVC. This ratio shows the proportion of the total lung capacity that can be expelled in the first second.
Interpreting FEV Results
Interpreting FEV results involves comparing the patient’s measured values to predicted values based on demographics like age, sex, and height. These predicted values establish a lower limit of normal (LLN) for each individual, which is the standard for determining abnormality. The FEV1/FVC ratio is the first value assessed, as a reduced ratio is the defining characteristic of an obstructive ventilatory pattern.
An obstructive pattern, where the FEV1/FVC ratio is below the LLN (often a fixed value of less than 70% in adults), signifies difficulty in exhaling air due to narrowing of the airways. This finding is typically seen in conditions such as asthma and Chronic Obstructive Pulmonary Disease (COPD).
In contrast, a restrictive ventilatory pattern is indicated when the FEV1/FVC ratio remains normal or high, but the Forced Vital Capacity (FVC) is significantly reduced. A low FVC with a preserved ratio suggests a reduced total lung volume, meaning they cannot fully inhale, which is characteristic of restrictive lung diseases like pulmonary fibrosis.
The severity of either an obstructive or restrictive defect is then determined by the FEV1 value itself, using a percentage of the predicted value. For example, an FEV1 of 60% to 69% of the predicted value is typically classified as a moderate severity obstruction.
Medical Applications of FEV Testing
FEV testing through spirometry is essential for the diagnosis and management of numerous respiratory diseases. It is the primary tool used to confirm airflow obstruction, such as in asthma and COPD. In asthma, the test helps demonstrate reversible obstruction, as the FEV1 often shows significant improvement after the patient inhales a bronchodilator medication.
For COPD, spirometry is used to identify persistent, non-reversible airflow limitation and to grade disease severity. The test is also routinely used to monitor the progression of chronic lung diseases and to assess treatment effectiveness. FEV testing is often included in pre-operative assessments to evaluate a patient’s risk for post-surgical pulmonary complications. It also plays a role in occupational health surveillance, particularly for workers exposed to substances that may harm lung function.