What Spirometry Measures

Spirometry is a simple, painless lung function test that measures how much air you can breathe in and out, and how quickly you can exhale. It is the most important diagnostic tool for Chronic Obstructive Pulmonary Disease (COPD) and is also used to monitor disease progression and response to treatment. This article explains what spirometry measures, how the test works, and what the results mean.

What Is Spirometry?

Spirometry is a type of pulmonary function test that assesses the function of your lungs. During the test, you breathe into a device called a spirometer, which records the volume and speed of your breath. The test is non-invasive and typically takes about 15 to 30 minutes to complete.

Spirometry measures two key values that are essential for diagnosing COPD: Forced Vital Capacity (FVC) and Forced Expiratory Volume in one second (FEV1). These measurements help determine if there is airflow obstruction, which is the hallmark of COPD.

How Spirometry Works

During a spirometry test, you will be asked to take a deep breath in and then exhale as forcefully and completely as possible into the spirometer. The test is usually repeated at least three times to ensure accurate results. You will be asked to wear a nose clip to prevent air from escaping through your nose, and you will be seated in a comfortable position.

Your healthcare provider will guide you through the process and may ask you to perform the test before and after using a bronchodilator medication. This helps determine if your airway obstruction is reversible, which is more typical of asthma than COPD.

Key Measurements

Forced Vital Capacity (FVC)

FVC is the total amount of air you can exhale forcefully after taking the deepest breath possible. It is measured in liters. A reduced FVC can indicate restrictive lung disease or poor effort during the test. In COPD, FVC is often reduced because air becomes trapped in the lungs, making it difficult to exhale completely.

Forced Expiratory Volume in One Second (FEV1)

FEV1 is the volume of air you can exhale in the first second of a forced exhalation. It is the most important measurement in COPD diagnosis. A reduced FEV1 indicates airflow obstruction, which is characteristic of COPD.

FEV1/FVC Ratio

This ratio is calculated by dividing FEV1 by FVC. In healthy individuals, the FEV1/FVC ratio is typically above 0.70. In COPD, the ratio is less than 0.70, confirming the presence of airflow obstruction. The lower the ratio, the more severe the obstruction.

Understanding Your Results

Your spirometry results will be compared to predicted values based on your age, height, sex, and ethnicity. The results are often expressed as a percentage of the predicted value:

• Normal: FEV1 and FVC are greater than 80% of predicted values
• Mild COPD (GOLD Stage 1): FEV1 is 80% or more of predicted, with FEV1/FVC less than 0.70
• Moderate COPD (GOLD Stage 2): FEV1 is 50-79% of predicted
• Severe COPD (GOLD Stage 3): FEV1 is 30-49% of predicted
• Very Severe COPD (GOLD Stage 4): FEV1 is less than 30% of predicted

It is important to remember that spirometry is just one piece of the puzzle. Your healthcare provider will consider your spirometry results along with your symptoms, medical history, and other tests to make a comprehensive assessment.

Other Measurements

In addition to the core measurements, spirometry can also provide other useful information:

• Forced Expiratory Flow (FEF): Measures the flow rate at different points during exhalation
• Peak Expiratory Flow (PEF): The maximum flow rate achieved during exhalation
• FEV6: The volume of air exhaled in six seconds, sometimes used as an alternative to FVC
• FEV1/FEV6 ratio: Can be used when FVC is difficult to obtain

What a Spirometry Test Feels Like

Many people find spirometry to be straightforward and well-tolerated. You may feel slightly lightheaded or dizzy after the forced exhalations, but this is usually temporary. Your healthcare provider will allow you to rest between attempts and will stop the test if you become uncomfortable.

It is important to follow your healthcare provider's instructions before the test. You may be asked to avoid using your bronchodilator medication for several hours before the test, to avoid heavy meals, and to avoid smoking or alcohol before the test.

When to Have Spirometry

Spirometry is recommended for anyone who has symptoms of COPD, such as chronic cough, sputum production, or shortness of breath, especially if they have a history of smoking or exposure to lung irritants. It may also be used to monitor the progression of COPD and assess the effectiveness of treatment.

Regular spirometry testing can help you and your healthcare provider make informed decisions about your care. It can also be helpful in identifying when your condition is worsening or when treatment adjustments may be needed.

Limitations and Considerations

While spirometry is a valuable tool, it does have limitations:

• Effort-dependent: The test requires full effort, and results can be affected by fatigue or lack of cooperation
• Not a standalone diagnosis: Spirometry results should be interpreted in the context of your overall health and symptoms
• Variability: Results can vary between visits, so trends over time are often more informative than individual measurements