What Is Inspiratory Capacity?

Inspiratory capacity, frequently seen in literature as IC, is an important measurement of volume of air that is can be used in relation to your respiratory function or status. This is a lung volume that is captured during a pulmonary function test, which can be used to determine the mechanical function of your lungs.

Inspiratory capacity is measured as you exhale casually followed by a maximal inhalation. The normal inspiratory capacity in an adult is approximately 3 liters.

While this volume can be measure through a pulmonary function test such as spirometry, it can also be calculated. The calculation for inspiratory capacity is tidal volume (the amount of air you casually breath in) plus inspiratory reserve volume (the amount of air you forcefully breath in after a normal inhalation).

Another important and useful way to calculate the inspiratory capacity is to take the total lung capacity (TLC), which includes forceful inspiration/exhalation and any residual air volume left in the lungs, and subtract the functional residual capacity, which includes only the volume forcibly exhaled and the residual volume in the lungs after.

The average total lung capacity in an adult is approximately liters, so the average IC/TLC is around 0.5 or 50%.

How Is Inspiratory Capacity Measured?

Inspiratory capacity is measured as part of spirometry, which is a pulmonary function test. Follow your doctors instructions when preparing for this test. Common things to do before this test includes:

• No smoking for at least an hour before the test.
• Skip your breathing medications if instructed to.
• Avoid alcohol for at least four hours before the test.
• Wear loose-fitting clothing.
• Avoid large meals for at least two hours before the test.

During the test, you will breathe through a mouthpiece in different ways. Sometimes you will breathe normally in a relaxed manner, while other times you will be asked to do more forceful breathing during inhalation or exhalation.

It is important that you follow the instructions to get accurate results. If you become tired, lightheaded, or do not understand the instructions, let the person conducting the test know.

Abnormal Inspiratory Capacity Symptoms

As inspiratory capacity relates to the volume of air you can deep breath, the symptoms are closely related to how you feel when breathing. The symptoms may also not be completely related to inspiratory by itself, but in combination with another measurement from pulmonary function tests.

The most common symptoms related to inspiratory capacity includes:

• Feeling like you can't breathe deep enough
• Feeling like air is trapped (combination of other lung volumes)

Causes Related to Reduced IC

Difficulty breathing is typically related to two types of causes:

• Restrictive
• Obstructive

In restrictive airway disorders, the lungs are not able to expand sufficiently to breathe as deep. This would decrease your inspiratory capacity.

In obstructive airway disorders, you are unable to fully exhale. If you are unable to fully exhale, you will have an elevated end-expiratory lung volume. With an increased volume remaining after normal exhalation, your lungs will not be able to breathe in as deeply and have a reduction in your inspiratory capacity.

Diagnoses Related to IC

Associated conditions can be categorized as showing a reduced inspiratory capacity or an increased inspiratory capacity.

Reduction In Inspiratory Capacity

Reduced inspiratory capacities are related to several diagnoses related to the causes listed above. However inspiratory capacity is not used in the diagnosis of any breathing disorders.

Rather it is used in monitoring symptoms and can be utilized in the prognosis of some disorders such as chronic obstructive pulmonary disease (COPD) when combined with the total lung capacity ratio. Common diagnoses that decrease inspiratory capacity caused by restriction include:

• Idiopathic pulmonary fibrosis (IPF)
• Amyotrophic lateral sclerosis (ALS)

Common diagnoses that decrease inspiratory capacity caused by obstruction include:

• Chronic obstructive pulmonary disease (COPD)
• Asthma
• Cystic fibrosis (CF)

While there are certainly more diagnoses related to restrictive and obstructive lung diseases, not all have evidence of the utility of inspiratory capacity.

Increase in Inspiratory Capacity

While obesity can lead to obesity hypoventilation syndrome, obesity is typically not associated with a decrease in inspiratory capacity. It is generally believed that while obesity decreases residual air capacity in the lungs related to excess adipose tissue (fat), inspiratory capacity actually increases to help compensate for any lung restrictions.

IC in Relation to Lung Diseases

Restrictive Lung Diseases

Using inspiratory capacity has limited usefulness in the treatment of IPF. It is worth knowing that during periods of exercise there is a noted difference in inspiratory capacity to tidal volume. This suggests that difficulty breathing (particularly with exercise) is a mechanical function and not related to gas exchange of oxygen in the arteries of the lungs.

In ALS, inspiratory capacity can be used as a marker to determine declining slow-twitch muscles in the diaphragm. While forced vital capacity, which is a measure of how much air can be forced from the lungs during exhalation, is a common measure of lung function, inspiratory capacity is able to better correlated to this decrease in muscle function. It may therefore be helpful in having a pulmonologist as part of your health management team.

Obstructive Lung Disease

Inspiratory capacity is used in the management of COPD in different ways. First, the inspiratory capacity can related to the strength of your inspiratory muscles. As your muscles diminish, or atrophy, inspiratory capacity will also subsequently decrease.

Decreasing inspiratory capacity can also be used to evaluate the level of hyperinflation of your lungs. Inspiratory capacity to total volume capacity ratio is also commonly used to determine prognosis of COPD.

In both COPD and asthma, using inspiratory capacity may be useful for tracking the level of air trapping, which is when breaths stack on top of each other related to the inability to fully exhale. Air trapping is a common cause of difficulty breathing, or dyspnea, in restrictive lung diseases.

Typically air trapping is evaluated by measuring the functional residual capacity, which requires a specific test using 100% oxygen to washout nitrogen from your lungs during a pulmonary function test. Inspiratory capacity can be used as a surrogate to figure out the functional residual capacity during a pulmonary function test.

During exercise, you can anticipate that air trapping will increase in COPD and some asthmatics. After using a bronchodilator, you can expect to see the opposite, or a reduction in air trapping and a subsequent increase in inspiratory capacity.

Similar to measuring air trapping in COPD and asthma, air trapping can also be evaluated in cystic fibrosis with inspiratory capacity. In the case of cystic fibrosis, decreasing inspiratory capacity may be one of the first signs of declining lung function. Earlier detection can help escalate therapies.