Vital Capacity Calculator

How does the vital capacity calculator work?

This health tool determines the vital capacity based on the results from several lung function tests. This is an indicator of respiratory function, often employed in diagnosis. The three parameters used in the vital capacity calculator are:

■ Inspiratory reserve volume – is approximately 3.1 litres and represents the additional air possible to be inhaled forcefully after the normal tidal volume inspired.

■ Tidal volume – is approximately 0.5 litres and represents the amount of air which can be inhaled and exhaled during relaxed breathing. The average adult breathing rate is 12 to 20 breaths per minute.

■ Expiratory reserve volume – is approximately 1.2 litres and represents the additional air that can be extracted during forced exhalation, after the expiration of the normal tidal volume.

The formula used to extract VC from the above is:

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

In short: VC = IRV + TV + ERV

The following table introduces the average lung volumes in adult males and females:

Vital capacity is defined as the maximum amount of air possible to be expelled after a maximum inhalation. Normal values in adults depend on gender, age, weight, height and even ethnicity and range between 3 and 5 litres.

Lower than normal values may indicate restrictive lung disease (e.g. pulmonary fibrosis, pneumothorax). However, in other conditions, such as in obstructive lung disease (e.g. asthma, emphysema, COPD), the VC may be normal or slightly decreased.

VC increases during the 20s and 30s and then slowly decreases towards the 50s. Taller individuals tend to have higher vital capacities, with differences of 0.8 litres for increases of 15 cm (6 in) in height.

The spirometer, as part of the basic ventilator pulmonary function test, is used to measure the vital capacity amongst the other lung volumes and other functional parameters such as the peak expiratory flow rate.

There are also two formulas, one for females and one for males, which can be used to estimate vital capacity from the subject’s gender, age and height:

■ Vital capacity for male in litres = ((27.63 – 0.112 x Age in years) x Height in cm)/1000

■ Vital capacity for female in litres = ((21.78 – 0.101 x Age in years) x Height in cm)/1000

Along with other measurement and laboratory tests, VC contributes to:

■ Diagnosing certain underlying lung diseases;

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

■ Treatment decisions in the Guillain-Barre syndrome;

■ Treatment management of myasthenic crisis.

VC is different from residual capacity as the latter represents the volume of air present in the lungs at the end of expiration.

Example of a vital capacity calculation

Let’s take the following parameters:

■ Inspiratory reserve volume (IRV) = 2.9 L;

■ Tidal volume (TV) = 0.5 L;

■ Expiratory reserve volume (ERV) = 1.2 L.

VC = IRV + TV + ERV = 2.9 + 0.5 + 1.2 = 4.6 L

References

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

2) W. J. Dornan (1922) Transactions of the Association of American Physicians, Volume 37

3) Hutchinson J. (1846) 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; 29:137-252.