Chronic Obstructive Pulmonary Disease (COPD) is a progressive lung disease that makes breathing difficult. The diagnostic process relies on measurements of lung function obtained through a test called spirometry. These tests provide objective data that allows doctors to confirm a diagnosis and determine an appropriate course of management. This guide explains one of the key measurements, how it is obtained, and how it is used to evaluate COPD.
What is Forced Vital Capacity?
Forced Vital Capacity (FVC) is a measurement of lung function. It represents the total amount of air a person can forcefully and completely exhale after taking the deepest possible breath. The goal of the FVC maneuver is to measure how much air your lungs can hold in total.
This measurement provides a clear indication of a person’s lung size or volume. A low FVC value can suggest a restrictive lung disease, where the lungs are unable to fully expand.
While FVC measures the total volume, it does not measure the speed at which that air can be expelled. This distinction is important for obstructive lung diseases like COPD, where the issue is difficulty exhaling quickly due to narrowed airways.
The Spirometry Test Experience
FVC is measured using a non-invasive procedure called spirometry. Before the test, patients are advised to avoid things that could affect lung function, such as smoking, using certain inhaled breathing medications, or consuming caffeine. Following these instructions helps ensure the test results are as accurate as possible.
During the test, a technician will ask the patient to sit upright and will place soft clips on their nose. They will be given a mouthpiece connected to the spirometer, and the nose clips ensure all exhaled air is measured.
The core of the test involves a breathing maneuver that is repeated several times to ensure consistency. The technician will coach the patient to take the deepest breath possible, seal their lips around the mouthpiece, and then blast the air out as hard and fast as they can until their lungs feel empty.
Diagnosing COPD with the FEV1/FVC Ratio
While FVC measures total lung capacity, another value measured during the same spirometry maneuver is the Forced Expiratory Volume in 1 second (FEV1). FEV1 is the volume of air that can be forcefully exhaled in the first second of the FVC effort, and it is a measure of how quickly the lungs can be emptied. For a COPD diagnosis, the relationship between them, expressed as the FEV1/FVC ratio, is what matters.
In healthy lungs, a person can exhale a large portion of their total lung capacity in the first second. In COPD, the airways are narrowed, creating an obstruction that slows the flow of air, significantly reducing the FEV1.
To confirm a diagnosis, the test is performed again after the patient inhales a bronchodilator medication to open the airways. A post-bronchodilator FEV1/FVC ratio of less than 0.70 is the standard criterion used to confirm the diagnosis of COPD, as it indicates a fixed airflow limitation.
Determining COPD Severity
After the FEV1/FVC ratio confirms a COPD diagnosis, doctors use the FEV1 measurement by itself to classify the severity of the airflow obstruction. This staging helps in creating an appropriate management plan. The most widely used framework for this is the Global Initiative for Chronic Obstructive Lung Disease (GOLD) system.
The GOLD classification uses the post-bronchodilator FEV1 value and compares it to the predicted FEV1 for a healthy person of the same age, height, gender, and ethnicity. The result is expressed as a percentage of the predicted value. The GOLD system has four stages of severity based on this FEV1 percentage:
- GOLD 1 (Mild): FEV1 is 80% or more of the predicted value.
- GOLD 2 (Moderate): FEV1 is between 50% and 79% of predicted.
- GOLD 3 (Severe): FEV1 is between 30% and 49% of predicted.
- GOLD 4 (Very Severe): FEV1 is less than 30% of the predicted value.