Vital Capacity Calculator

Variables and formula

Vital capacity is an indicator of respiratory function that is based on three pulmonary volumes, which are measured via lung function tests (spirometry).

The three volumes are described in the table below:

The formula used to estimate VC from the above is:

Vital capacity = Inspiratory reserve volume + Tidal volume + Expiratory reserve volume

In short: VC = IRV + TV + ERV

In the light of the above, vital capacity can be defined as the maximum amount of air possible to be expelled after a maximum inhalation.

Normal values in adults are between 3 and 5 litres and depend on gender, age, weight, height and ethnicity.

Taller individuals tend to have higher vital capacities (differences of 0.8L for every 6 in of height).

Also, there is a functional difference between VC levels during 20s and 30s and those at 50s.

Lower than normal values are suggestive of a restrictive lung disease (e.g. pulmonary fibrosis, pneumothorax).

Normal values don’t necessarily guarantee the absence of respiratory condition, for example in obstructive lung diseases (e.g. asthma, emphysema or chronic obstructive pulmonary disease), the VC may be normal or just slightly decreased.

Another way to estimate VC

As part of the basic ventilator pulmonary function test, the spirometer record the lung volumes and other functional parameters such as the peak expiratory flow rate.

However, the VC can also be estimated, based on subject’s gender, age and height, via the following formulas:

Vital capacity for men (in litres) = ((27.63 –112 x Age in years) x Height in cm) / 1000

Vital capacity for women (in litres) = ((21.78 –101 x Age in years) x Height in cm) / 1000

Why is vital capacity calculated?

The VC determination contributes to the following:

■ Diagnosing certain underlying lung diseases;

■ Determination of the severity of respiratory muscle impairment in neuromuscular disease;

■ Treatment decisions in the Guillain-Barre syndrome;

■ Treatment management of myasthenic crisis.

References

1. Hutchinson J. On the capacity of the lungs, and on the respiratory functions, with a view of establishing a precise and easy method of detecting disease by the spirometer. Med Chir Trans. 1846; 29:137-252.

2. Godfrey MS, Jankowich MD. The Vital Capacity Is Vital: Epidemiology and Clinical Significance of the Restrictive Spirometry Pattern. Chest. 2016; 149(1):238-51.