Vital Capacity and Your Asthma Treatment

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Vital capacity (VC) is a measurement of the maximum amount of air you can fully inhale and fully exhale out of your lungs. If you have asthma, you may need to have your vital capacity measured periodically to help guide your treatment and assess the severity of your condition.

VC can be assessed with a spirometer, which is a device used for measuring a variety of pulmonary functions. Spirometry is simple and non-invasive: When you inhale and exhale into the mouthpiece of the spirometer, it measures airflow right then and there.

Signs You Need to Test Your Vital Capacity

Jessica Olah / Verywell

Purpose of Test

This test is used in asthma to assess breathing ability and lung condition. It can help to evaluate how the disease is affecting your ability to breathe as well as how well (or not) your treatment strategy is controlling your asthma.

Different pulmonary diseases affect VC. Even if you already have a diagnosis of asthma, this test can help clarify whether you also have another lung condition, such as pneumonia or pulmonary fibrosis.

You may need this test if you:

  • Are having more frequent or more severe asthma attacks
  • Experience difficulty breathing without your usual asthma symptoms and/or triggers
  • Develop new symptoms, such as coughing, chest tightness, or wheezing
  • Have a low oxygen level

Associated Tests

Your healthcare provider will likely also obtain a number of other measures as well. Some factor into the calculation of your VC, while others are considered in addition to it.

Common measures obtained with spirometry include:

  • Forced vital capacity (FVC): Your FVC is the maximum amount of air you can breathe out with maximal effort after taking a full inspiration.
  • Forced expiratory volume over one second (FEV1): FEV1 is the amount of air you can breathe out with maximal effort in one second after taking a full inspiration.
  • Tidal volume: This is a measure of the amount of air you can breathe in and out with normal inspiration and expiration.
  • Measurement is not timed

  • Involves normal breathing effort

FEV 1 and FVC
  • Measurement is timed

  • Require forceful expiration

While VC can be similar to FEV1 or FCV, it isn't always exactly the same. Your VC is likely to be slightly greater than your FEV1 because you have more time to expire when your VC is being measured than when your FEV1 is being measured. And your VC can be slightly less than your FCV due to the lack of required effort.

Before the Test

Before you have your VC measured, your healthcare provider may ask you to take your asthma medication at a certain time or to bring it with you. You might need to have your vital capacity measured before and after using an asthma inhaler to see if and how the medication changes your results.

It's a good idea to wear loose comfortable clothing that won't restrict your breathing when you have this test.

During the Test

Your diagnostic procedure will be guided by a respiratory therapist or a pulmonary technician. They will act as a coach, giving you instructions for when to inhale or exhale and how much effort you should put into each breath during your test.

Steps you can expect during this test:

  • The technician will get you seated in a chair.
  • You will be given a nose clip to prevent air from leaking out of your nose for an accurate measurement.
  • You will receive instructions about how to place your mouth on the spirometer and how to breathe into it.
  • The technician will let you know when to start and stop breathing in or out and how much effort to put into it.

Your team will have you repeat the test at least three times to ensure that your measurements are consistent and that an outlier doesn't alter the accuracy of your results. You will be given time to rest and catch your breath between each test.

If you need to take an inhaler or any other medication or treatment for part of the test, your technician will let you know when it's time to do that. You might need to wait a few minutes or longer after using the treatment before continuing with the test.

Any measurements that are repeated after medication will also be repeated at least three times to ensure accuracy and consistency.


You should feel fine after your test and you should be able to continue with your usual activities. However, if you feel any discomfort, shortness of breath, or dizziness, to tell someone from your medical team.

Interpreting Results

Your VC is the sum of your tidal volume, inspiratory reserve volume (the amount of additional air you can breathe in with maximal effort), and expiratory reserve volume (the amount of additional air you could breathe out with maximal effort).

The normal range of vital capacity for an adult is 3 to 5 liters, and your vital capacity will be compared to standardized values based on your age, gender, and height.

A person who is taller than average would be expected to have a VC on the higher side, while a person who has a higher body mass index (BMI) would be expected to have a VC on the lower side.

Lung conditions can have a major impact on your VC, but they don't all affect these test results in the same way.

Possible Diagnoses

Obstructive lung diseases may cause a slightly reduced VC. Asthma is an obstructive lung disease because the narrow airways make it difficult to get air out of the lungs; severe asthma may cause a more notable decrease in your VC. Other examples of obstructive lung diseases include chronic obstructive lung disease (COPD) and cystic fibrosis.

Additionally, if you experience a major difference (typically an improvement) in your VC shortly after taking your asthma inhaler, this could signify that you need better maintenance of your asthma. Treatment of severe asthma can improve VC.

Restrictive lung diseases make it hard for you to inhale. Conditions like pneumonia, pulmonary fibrosis, and pleural effusion (fluid in the lungs) are restrictive lung diseases. These conditions cause VC to decrease significantly.

Neuromuscular conditions such as muscular dystrophy, spinal muscular atrophy (SMA), and scoliosis, can interfere with the muscles that control your breathing. These medical illnesses may cause a decline in your VC and progress to the point that you may need treatment, such as respiratory support.


A substantially low vital capacity with asthma is an indication it's time to adjust treatment or that there needs to be further evaluation of the cause of your low VC. For example, your healthcare provider may consider treatment with a medical procedure to help reduce the effects of your asthma.

If there is a concern that you could have an infection or lung damage such as pulmonary fibrosis, you might need to have additional testing, such as a chest X-ray or a chest computerized tomography (CT) scan.

You might need to have your vital capacity measured again in a few months, especially if you have started on a new medication or received an intervention to help improve your pulmonary function.

A Word From Verywell

Maintaining the best control of your asthma involves regularly monitoring your breathing. In addition to scheduled diagnostic tests, your healthcare provider may also advise you to monitor your breathing at home using a peak flow meter. This can give you an on-the-spot measurement of your exhaled air volume so you can follow your respiratory function over time.

If your peak flow measurement changes or falls below a certain level specified by your healthcare provider, let them know. You may need to have your vital capacity and other measures of pulmonary function checked.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Langton D, Bennetts K, Noble P, Plummer V, Thien F. Bronchial thermoplasty reduces airway resistance. Respir Res. 2020;21(1):76.doi:10.1186/s12931-020-1330-5

  3. Farber HJ, Phillips WA, Kocab KL, et al. Impact of scoliosis surgery on pulmonary function in patients with muscular dystrophies and spinal muscular atrophy. Pediatr Pulmonol. 2020;55(4):1037-1042.doi:10.1002/ppul.24664

By Pat Bass, MD
Dr. Bass is a board-certified internist, pediatrician, and a Fellow of the American Academy of Pediatrics and the American College of Physicians.