Inspiratory Capacity Calculator

Variables and formula

The inspiratory capacity is one of the four pulmonary capacities. It is based on the inspiratory reserve volume and on tidal volume, which are both determined during lung function tests (spirometry).

Inspiratory reserve volume represents the maximal volume of air that can be forcefully inhaled after a normal (tidal) inspiration. This is equal to approximately 3 litres.

Tidal volume is the normal amount of air circulated during normal inspiratory and expiratory processes. This is approximately 500 mL at a normal respiratory rate of 12 to 20 breaths per minute.

Inspiratory capacity formula = Inspiratory reserve volume + Tidal volume

In short: IC = IRV + TV

When determined via spirometry, normal average values for inspiratory capacity are 3.5 L for men and 2.4 L for women.

IC offers information about the volume of air that can be inhaled, during normal and forced inspiration. This is particularly useful in the diagnosis and severity evaluation of both obstructive and restrictive lung disease.

A reduction in both inspiratory and total lung capacity can be observed in obstructive respiratory diseases like COPD.

Lung capacities explained

There are four lung capacities: the IC described above, the vital capacity, the functional residual capacity and the total lung capacity.

The vital capacity is the sum of inspiratory and expiratory reserve volumes and of tidal volume and indicates the maximum amount of air possible to be expelled after a maximum inhalation.

Normal average values in adults are between 3 and 5 L and depend on patient gender, age, weight, height and even ethnicity.

Lower than normal values may indicate restrictive lung disease (e.g. pulmonary fibrosis, pneumothorax) whilst higher than normal values are characteristic for obstructive lung disease (e.g. asthma).

The functional residual capacity is based on expiratory reserve and residual volume and represents the amount of air that remains in the lungs at the end of passive expiration. An increase in FRC is consistent with emphysema.

The total lung capacity is the sum of all respiratory volumes (inspiratory reserve, tidal, expiratory reserve and residual). It is impacted by physiological changes in old age, by patient weight, by diaphragm pressure and by restrictive pulmonary diseases.

References

1. Ricard JD. Are we really reducing tidal volume--and should we? Am J Respir Crit Care Med. 2003; 167(10):1297-8.

2. Lisboa C, Leiva A, Pinochet R, Repetto P, Borzone G, Díaz O. Reference values for inspiratory capacity in healthy nonsmokers over age 50 years. Arch Bronconeumol. 2007; 43(9):485-9.

3. Zaman M, Mahmood S, Altayeh A. Low inspiratory capacity to total lung capacity ratio is a risk factor for chronic obstructive pulmonary disease exacerbation. Am J Med Sci. 2010; 339(5):411-4.