Inspiratory Capacity Calculator

Inspiratory Capacity equals Tidal Volume plus Inspiratory Reserve Volume

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What Is Inspiratory Capacity?

Inspiratory Capacity (IC) is the maximum volume of air that can be inhaled from the end-expiratory level (after a normal breath out). It combines the tidal volume (the air moved during normal breathing) with the inspiratory reserve volume (the additional air that can be drawn in with a maximal inspiratory effort). IC reflects the ability of the lungs and respiratory muscles to expand and fill.

Worked Example

A respiratory therapist measures a patient's tidal volume at 0.5 L during normal breathing and then records an inspiratory reserve volume of 2.5 L during a maximal inspiratory effort. Using the formula IC = TV + IRV, the inspiratory capacity is 0.5 + 2.5 = 3.0 L. This is consistent with a normal adult inspiratory capacity.

Normal Values

A typical IC in a healthy adult is approximately 3.0 L, composed of a tidal volume of about 0.5 L and an inspiratory reserve volume of about 2.5 L. IC decreases in hyperinflated conditions such as COPD and emphysema, where the functional residual capacity is elevated and there is less room for inspiratory expansion.

Frequently Asked Questions

Can IC be measured with spirometry?

Yes, IC can be measured directly with standard spirometry. The patient breathes normally and then performs a maximal inspiration from the end-expiratory level. The volume of that maximal inspiration is the inspiratory capacity.

Why does IC decrease in COPD?

In COPD, air trapping increases the FRC (the resting lung volume). Since total lung capacity stays roughly the same, the available space for inspiration is reduced, resulting in a smaller IC. This dynamic hyperinflation is a major contributor to exercise intolerance in COPD patients.

What is a normal inspiratory capacity?

A normal inspiratory capacity in a healthy adult is approximately 3.0 L, though it varies with age, sex, and body size. Values significantly below predicted indicate possible hyperinflation or restrictive lung disease and warrant further pulmonary function testing.

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