What to Know About Total Lung Capacity

Measurement and Meaning

Doctor checking lung of mature patient
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Total lung capacity, or TLC, refers to the total amount of air in the lungs after taking the deepest breath possible.

Patients with chronic obstructive pulmonary disease (COPD) are often unable to exhale fully, resulting in hyperinflation of the lungs and a greater total lung capacity.

How Is Total Lung Capacity Tested?

TLC is measured through body or lung plethysmography, one of many pulmonary function tests that help to determine how much air is present in your lungs when you take a deep breath and how much air is left in your lungs after you exhale as much as you can. Body plethysmography helps your doctor to know more about your lung disease and how to treat it.

Measured in milliliters, the maximum capacity of a healthy lung is approximately 6,000 mL. In patients with COPD, the amount of air left in the lungs during the breathing process is more than normal, a condition known as hyperinflation. 

Spirometry is the lung test usually used to diagnose COPD, but—unlike lung plethysmography—it does not on its own provide information on total lung capacity or lung residual volume (the amount of air left in the lungs after exhalation). Together, these tests can give your doctor a more complete picture of your condition. 

Lung Hyperinflation in COPD

Hyperinflation of the lungs occurs in most people with COPD. What happens is, since expiratory airflow is reduced, people begin to take their next breath before they have completely emptied their lungs of the last breath. Each time this occurs, more air becomes "trapped" in the lungs. Due to this trapped air, the lungs need to hyperinflate to take the next breaths, and since the lungs are not made to hyperinflate, these breaths require more work than would ordinarily be required with each breath.

This hyperinflation with exercise or even everyday activity, causes dyspnea, the sensation of shortness of breath. Dyspnea in people with COPD often leads to:

  • The avoidance of physical activity
  • Physical deconditioning
  • A lower quality of life
  • An elevated risk of other illnesses, such as cardiovascular disease

The type of hyperinflation discussed above in which a person begins to take their next breath before the prior breath is completely exhaled is referred to as "dynamic hyperinflation." Another type of hyperinflation referred to as "static hyperinflation" may also occur in people with severe COPD. Static hyperinflation occurs as the lungs lose their elasticity and require larger volumes of air to maintain the elastic recoil of the lungs after each breath.

Why Is Total Lung Capacity Tested?

Total lung capacity may be tested for several reasons.

When Should Lung Plethysmography Not Be Done?

You should not undergo lung plethysmography if you are mentally confused, have poor muscle control or Parkinsonism, or are on continuous oxygen support that cannot be stopped even temporarily.

How Is Body Plethysmography Performed?

If your doctor orders a lung plethysmography test to measure your total lung capacity, you can take comfort in knowing this test is relatively simple and painless.

During the test, you will sit in a clear glass booth roughly the size of a phone booth and then, wearing a nose clip, you will be instructed by a respiratory therapist to breathe through a mouthpiece and tube attached to the testing machine. Sometimes, a tracer gas such as carbon dioxide, is included in the air coming from the machine.

The test typically takes about three minutes. It measures changes in air pressure inside the booth to determine how much air you can breathe into your lungs.

To get the most accurate results, prior to the test you should not:

  • Smoke for at least one hour
  • Drink alcohol for at least four hours
  • Exercise within 30 minutes
  • Eat a large meal within two hours

Your doctor may also instruct you to not take certain medications on the day of the test. Be sure to follow your doctor's instructions precisely.

Causes of Increased Total Lung Capacity

Obstructive lung diseases are those in which air moves out of the lungs at a slower rate than normal, and includes conditions such as COPD, asthma, bronchiectasis, and cystic fibrosis. With these conditions, the total lung capacity may be increased due to hyperinflation.

Causes of Decreased Total Lung Capacity

In restrictive lung diseases, the lungs are often "restricted" from taking a deep breath and the total lung capacity is diminished. There are both extrinsic and intrinsic lung diseases depending on whether the restriction occurs outside the lungs or within the lungs themselves. Intrinsic lung diseases include those such as sarcoidosis, idiopathic pulmonary fibrosis, pneumonia, or decreased lung volume after lung surgery. Intrinsic lung diseases include obesity, scoliosis, and pleural effusions among others.

Bottom Line for Those With COPD and Increased Total Lung Capacity

If you are living with COPD, it can be helpful to understand the process discussed here which leads to hyperinflation. Shortness of breath leads to deconditioning, which worsens the factors that cause shortness of breath, and so on. It can be a vicious circle. Check out these tips for managing shortness of breath with COPD for starters. Begin exercising right away. If you're not certain where to begin, check out these best exercises for people with COPD. If you're finding it difficult to stick with an exercise program, review these five reasons why you must exercise with COPD.

Finally, if it is your loved one rather than yourself who is coping with COPD, check out these ways to help a loved one with COPD.

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