What Is Forced Vital Capacity (FVC)?

spirometer, a device used to measure forced vital capacity or FVC
Istockphoto.com/Stock Photo©dina2001

Forced vital capacity, or FVC, is defined as the amount of air that can be forcibly exhaled from the lungs after taking the deepest breath possible. This number, which averages around three liters (but depends on height, weight, and other factors), may be decreased either temporarily or permanently due to a number of different lung issues. Measurements may be used to help diagnose lung disease, determine its severity and progression, assess one's ability to tolerate surgery, and more.

One of the important uses of FVC is to help distinguish obstructive lung diseases, such as asthma and COPD, from restrictive lung diseases, such as pulmonary fibrosis and sarcoidosis.

You may also hear about a test called vital capacity (VC). Forced vital capacity and vital capacity record similar information, but FVC refers to the amount of air you can exhale forcefully, while VC records the maximum amount of air that can be exhaled when breathing normally.

Purpose and Indications

Forced vital capacity (FVC) is a very important pulmonary function measurement that is used to evaluate a wide range of lung diseases. By determining the amount of air that can be forcibly exhaled, doctors can gain valuable information that gives clues about how the lungs are functioning, both initially, and how they may change over time. 

The FVC and similar pulmonary function tests are used to establish the status of your lung function by comparing your measurements to what would be expected of other adults of your age, gender, race, height, and weight.

There are many reasons why your doctor may choose to measure your FVC, including:

  • To evaluate symptoms that could indicate lung diseases, such as shortness of breath, a persistent cough, or wheezing
  • To monitor lung diseases already diagnosed to see if they are getting worse, or see if they are improving with treatment
  • To evaluate your ability to tolerate surgery: It's important to ensure that your breathing is stable prior to a procedure, especially if you will be having lung surgery.
  • To evaluate your ability to tolerate an exercise program
  • To assess heart disease risk: In recent years, it's been found that the lower your FVC, the greater your risk of heart disease—even if your FVC measurement is within normal limits.
  • As an endpoint in clinical trials: Clinical trials looking to assess how well a medication is working may measure FVC in large numbers of people to compare a drug with conventional treatments.
  • To estimate prognosis: FVC is people with chronic lung diseases can be used to help predict survival.

The Test

Spirometry is the pulmonary function test used to determine forced vital capacity. There is no special preparation needed, but you should wear comfortable, loose-fitting clothing the day of your test, as you will be asked to take as deep a breath as possible.

Before the test, your doctor will want to review your history, findings on a physical exam, and any imaging tests you have had. The test is non-invasive and only takes a few minutes. Other pulmonary function tests will be likely be performed.

How FVC Spirometry Is Performed

  1. You are seated in a chair and asked to breathe comfortably.
  2. A clip is placed over your nose.
  3. You are given a tube to breathe into.
  4. Sealing your lips tightly over the tube, you are asked to inhale as deeply as possible and exhale as forcefully as you can.
  5. The procedure is repeated at least three times.

The accuracy of an FVC test is limited by a person's ability to exhale forcefully. If this is not possible, the results will be far more difficult to interpret.

Interpreting the Results

The FVC result can be looked at in two different ways. The total volume can be used as a comparison with what your FVC should be in relation to your age, sex, height, etc. Your FVC can also be compared with previous measurements to determine whether a pulmonary condition is progressing or if your lung function improving under treatment.

Forced vital capacity will be reported as both a number in liters for an absolute value and on a linear graph to chart the dynamics of your exhalation.

The normal FVC range for an adult is between 3.0 liters and 5.0 liters.

Factors can influence the FVC including your body mass index (BMI), physical condition, and smoking status. Lung capacity tends to decrease with age beginning in your mid-30s.

For children, the expected FVC can be predicted using reference tables that incorporate the child's height, body weight, and other factors. For example, the average FVC for an "average-size" preschool boy is 1.16 liters and 1.04 liters for an "average-size" preschool girl.

Conditions Diagnosed

Forced vital capacity can be decreased in a number of different conditions and may be temporary or permanent. Some of these include:

Obstructive vs. Restrictive Lung Diseases

If your FVC is decreased, the next step is to check the FEV1/FVC ratio to determine whether the condition is obstructive (characterized by the obstruction of airways) or restrictive (caused by a narrowing of the air passages).

FEV1 is the amount of air that can be forcefully expelled from the lungs in one second. Therefore, the ratio of FEV1 to FVC compares the amount of air that can be forcefully expelled in one second to the amount that can be expelled in total.

A normal FEV1/FVC ratio is 70 percent to 80 percent or higher in adults, and 85 percent or higher in children. While restrictive diseases may limit air intake, people can generally exhale with the same force. By contrast, obstructive diseases make it difficult to exhale, causing shortness of breath and rapid breathing.

With restrictive diseases, such as pulmonary fibrosis, the FEV1 and FVC will be decreased proportionally, so that the ratio of FEV1/FVC is equivalent. With obstructive diseases, such as COPD, the FEV1/FVC ratio will be less than 70 percent.

Follow-Up

If FVC (and the FEV1 ratio) are done to help diagnose a lung disease, further testing will depend on the ratio. If the FEV1/FVC ratio suggests a restrictive pattern, full pulmonary function tests will likely be needed.

If the ratio suggests obstructive disease, the next step may be repeating the test after using a drug called a bronchodilator to determine if the obstruction is reversible. (Conditions such as asthma are reversible, whereas conditions such as COPD are largely irreversible).

Sometimes with a low FVC, the FEV1/FVC ratio is unable to distinguish different lung diseases, and tests such as lung diffusion tests (DLCO) and body plethysmography may be needed.

If FVC is being measured to monitor a known disease, follow-up will depend on the particular disease and its severity.

A Word From Verywell

While forced vital capacity is an important measurement in evaluating lung disease, it is important to remember that this is just a number. Your doctor will want to look at your medical condition, your general health, and other findings to understand what your FVC measurement means with regard to both your treatment options and prognosis.

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