Inspiratory Capacity: Formula for Measuring Lung Health

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

This article will go over what inspiratory capacity is. You will learn how inspiratory capacity is measured as well as conditions that are associated with having reduced inspiratory capacity.

How Is Inspiratory Capacity Calculated?

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

While this volume can be measured through a pulmonary function test such as spirometry, it can also be calculated.

Another 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.

This equation looks like this: TLC - FRC = IC

The average total lung capacity in an adult is approximately six 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 from the test and learn about your inspiratory capacity. If you become tired, lightheaded, or do not understand the instructions, let the person conducting the test know.

Causes of Reduced Inspiratory Capacity

Reduced inspiratory capacity can happen for many reasons. Having difficulty breathing is typically related to two 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.

Conditions Related to Inspiratory Capacity

Certain health conditions are related to inspiratory capacity. These conditions are categorized as showing a reduced inspiratory capacity or an increased inspiratory capacity.

Reduced Inspiratory Capacity

Reduced inspiratory capacities are related to several diagnoses that are 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.

Summary

Inspiratory capacity (IC) measures how much air you can breathe into your lungs after you breathe out normally. Inspiratory capacity is usually measured during a pulmonary function test called spirometry.

On its own, your inspiratory capacity is not used to diagnose breathing problems but can be a useful way to monitor chronic lung diseases.