What Is Inspiratory Capacity?

Inspiratory capacity, frequently seen in literature as IC, is an important measurement of volume of air that can be used in relation to your respiratory function or status. IC 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 measured 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 breathe 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 6 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 healthcare providers' 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.

Causes Related to Reduced Inspiratory Capacity

Difficulty breathing is typically related to two types of causes:

• Restrictive airway disorders
• Obstructive airway disorders

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 tied 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.