What Is an Expiratory Reserve Volume (ERV) Test?

An expiratory reserve volume (ERV) is the amount of air that can be breathed out after a normal exhalation. In other words, if you were breathing in and out normally, but then forcefully pushed out as much additional air as possible after an exhale, the extra air would represent your expiratory reserve volume. This is measurement is obtained during lung volume testing through spirometry, a type of pulmonary function test, or PFT.

ERV can be useful in a variety of ways, from diagnosing lung disease to evaluating a person's lungs prior to surgery.

Purpose of Test

Expiratory reserve volume is an important measurement of lung function testing that, when combined with results from other PFTs, is primarily used to diagnose and distinguish between lung diseases such as chronic obstructive pulmonary disease (COPD), asthma, and pulmonary fibrosis.

Because imaging of the lungs, such as X-rays or computerized tomography (CT) scans, often cannot make these distinctions, PFTs are highly useful.

The results of pulmonary function testing, including ERV measurements, can also contribute information used to:

• Determine if existing lung disease is improving or getting worse
• Check to see how well treatment for lung disease is working
• Evaluate a patient before surgery, such as a lung transplant
• Monitor for toxic side effects of certain medications
• Screen for lung problems in smokers or people whose jobs put them at risk for lung disease (through exposure to toxic chemicals, for example)

Risks and Contraindications

Pulmonary function tests such as spirometry are very safe procedures, but some people do experience shortness of breath, lightheadedness, or coughing. There's a very small risk of an asthma attack for those who have asthma.

There also are a handful of people for whom PFTs are contraindicated, including those who've recently had a heart attack or stroke or abdominal, chest, or eye surgery; people who have a collapsed lung (pneumothorax) or an aneurysm in any part of the body; and anyone who has tuberculosis or a respiratory infection.

Women who are pregnant and people experiencing stomach bloating, extreme fatigue, or muscle weakness may not get accurate results from PFTs.

Before the Test

Lung volume testing is done to determine expiratory residual volume can be done in the office of a pulmonologist (lung doctor) or at a hospital as an outpatient procedure.

It is a noninvasive technique and preparation is minimal. Because it requires taking a series of very deep breaths, it's advisable to eat lightly beforehand, avoid alcohol and smoking, and dress in non-restrictive clothing.

If you use a short-acting inhaler, it may be necessary to cease using it for six to eight hours prior to undergoing pulmonary function testing.

During the Test

For the actual test, you will be seated in a chair. So that all of your breathing takes place through your mouth, a technician will put a clip on your nose to keep it gently closed.

You'll be given a small hand-held device with a sterile mouthpiece that connects to a spirometer, a machine that's about the size and shape of a printer that will both register the force of your breath and print out the results.

Once you're situated, the healthcare provider will give you specific instructions about how to hold the mouthpiece and breathe into it. Once you've mastered the technique, you'll be asked to breathe in as deeply and fully as possible, and then to exhale as hard and fast as you can in order to completely empty your lungs.

You'll likely be asked to do this at least three times.

Interpreting Results

Again, ERV is not a separate test but rather one of several measurements of lung function obtained from spirometry. It is determined after a key measurement of lung volume—functional reserve capacity (FRC)—has been measured. FRC refers to the volume of air left in the lungs after a normal, passive exhalation and used to evaluate the elasticity of the lungs and chest wall.

ERV is added to another measurement, residual volume (RV), which refers to the volume of air left in the airways after maximal exhalation, to determine FRC.

Expiratory reserve volume is also often measured along with vital capacity (the total amount of air that can be exhaled, including the ERV) and inspiratory reserve volume, which—as you might imagine—measures the amount of extra air you can intentionally draw into your lungs after you've breathed in normally.

Often, various ratios are calculated using these measurements. For example, if the ERV to vital capacity ratio is high, it suggests that the lungs are stiff and unable to expand and contract properly; lung fibrosis might be the culprit. Or, if that ratio is very low, it could mean resistance in the lungs is resulting from asthma.

The average ERV volume is about 1200 mL in men and 700 mL in women.

ERV is generally reduced with obesity, abdominal swelling (ascites), or after upper abdominal surgery. You may also have decreased ERV if you are shorter or live in a location with a lower altitude.

A Word From Verywell

ERV testing is very safe, noninvasive, and can provide information about your breathing status and lung function in a way that X-rays and CT scans can't. Remember that ERV is just one measurement performed in a pulmonary function test that can help your healthcare provider narrow down a diagnosis of whether your condition is obstructive or restrictive in nature. This will, in turn, help inform an effective treatment plan.