What Does Expiratory Reserve Volume Measure?

Asthma Inhaler
Asthma Inhaler. Jonathan Ernst / Stringer / Getty Images

If you have breathing difficulties, your doctor may run several tests of your pulmonary function to get measurements that can help determine the severity of your condition. One of those tests is expiratory reserve volume (ERV).

If you have ever blown up a balloon, you have probably unknowingly tapped into your expiratory reserve volume. This measurement commonly used by doctors to assess people with breathing difficulties refers to the extra volume of air that can be exhaled with maximum effort beyond the level reached at the end of a normal, passive exhalation over a specific period of time (usually one second).

In other words, if you've already exhaled normally and then attempt to empty your lungs completely by intentionally pushing all the air out of them that you can, that "extra" air is your expiratory reserve volume.

Expiratory Reserve Volume and Other Lung Volumes

Your doctor won't diagnose you based only on the results of your expiratory reserve volume lung test. Your expiratory reserve volume test can, however, provide your doctor with clues about your condition. Those clues, combined with your medical history and the results of your exam, should lead your doctor to the correct diagnosis.

Expiratory reserve volume most likely will take place as part of a series of overall lung function tests, which also are likely to include multiple other measures of your lungs' volume and capacity. This series of measurements doctors use to diagnose lung diseases such as asthma, emphysema, and fibrosis.

Expiratory reserve volume is 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 the 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 and lung fibrosis might be the culprit. Or, if that ratio is very low, it could mean resistence in the lungs is resulting from asthma.

How Is Expiratory Reserve Volume Measured?

Expiratory reserve volume and other lung volume measurements are often taken using a technique called spirometry. This is one of the most commonly run tests of lung function, but there are many others that may be used to take lung volume measurements.

A spirometry test requires you to breathe into a tube attached to a machine called a spirometer. While seated and with your nostrils clipped shut, you will inhale deeply. Then, you will place your lips around the tube, or spirometer, creating a tight seal to prevent air leakage, and breathe out forcibly for several seconds. The test is often repeated three times to ensure consistent results. 

Conditions Where Expiratory Reserve Volume Is Important

There are several conditions where your expiratory reserve volume may provide clues as to the health of your lungs. These conditions include:

Your doctor may run this test as part of the process of diagnosis to determine why exactly you're having symptoms such as difficulty breathing, chronic cough, wheezing, and signs of low oxygen in your blood.

She may also run the test as part of the ongoing monitoring of your condition, to determine if you've stabilized or if your lung function has declined further.

Finally, sometimes these tests are run to screen for lung problems in smokers or in people with jobs that place them at risk for lung disease — for example, they may be exposed to toxic chemicals at work.

Sources:

Al-Ashkar F et al. Interpreting pulmonary function tests: Recognize the pattern,and the diagnosis will follow. Cleveland Clinic Journal of Medicine. October 2003. Volume 70, Number 10.

U.S. National Library of Medicine. Pulmonary Function Tests fact sheet. 

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