The Spirogram

The first attempt to measure lung volumes goes as far back as the second century, when Galen, the famous Greek physician had a child breathe into an animal bladder.

The spirogram defines the lung volumes and lung capacities. Static lung volumes refers to the measurement of the total lung capacity (TLC), functional residual capacity (FRC) and residual volume (RV).

You can not measure static lung volumes by spirometry.

Static lung volumes can only be measured in a lung function laboratory with specialised equipment to measure dilution of gases (helium dilution and nitrogen washout techniques), or by body plethysmography which measures the relationship between lung pressures and volumes.

Nevertheless, you should have a good understanding of the spirogram. Visualising the components of the spirogram at the back of your mind is essential when performing and interpreting spirometry. The following tutorial illustrates the normal spirogram trace and identifies the various volumes and capacities.

Definitions of lung volumes:

the normal volume of air breathed in and out during normal quiet breathing

the volume of air remaining in the lungs following a full voluntary expiration

the maximum volume of air that can be voluntarily inhaled from the top of a normal breath to the point of maximal inspiration (TLC)

the maximum volume of air that can be voluntarily exhaled from the end of a normal breath (FRC) to the point of maximal expiration (RV)

Definitions of lung capacities:

the volume of air inspired from the point of full expiration (RV) to maximal lung inflation (TLC), or from full inspiration (TLC) to complete expiration (RV). this includes the ERV, VT and IRV

the volume of air remaining at the end of a normal tidal breath. This includes the ERV and RV

The maximal volume of air within the lung at the point of maximal inspiration. It is made up of either

FRC+IC
IRV +TV+FRC
VC+RV

When performing spirometry firstly a relaxed vital capacity must be performed prior to the dynamic manoeuvres, either by performing an inspiratory vital capacity or an expiratory vital capacity. This is known as a ‘Relaxed’ Vital Capacity (RVC), or sometimes a ‘Slow’ Vital Capacity (SVC); although the latter terminology should be avoided.

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