Spirometry and How to Interpret It

Nov 25

3 min read

What Is Spirometry?

Spirometry is a common method used to measure how well a person’s lungs are working. It records the volume of air breathed in and out and the speed of breathing.

The results are shown on a graph called a spirometer trace, which helps scientists and healthcare professionals assess lung function and detect respiratory problems such as asthma or chronic obstructive pulmonary disease (COPD).

In Unit 5, you learn how to interpret these traces and relate them to physiological processes involved in breathing.

Understanding the Spirometer Trace

A spirometer trace usually shows volume (litres) on the y-axis and time (seconds) on the x-axis. The line moves up and down as the person breathes in and out.

The main parts of a trace are:

Term	Meaning	Description
Tidal volume (TV)	Normal amount of air inhaled and exhaled during quiet breathing	Typically around 0.5 L in adults
Inspiratory reserve volume (IRV)	Extra volume that can be breathed in after a normal inspiration	Shows lung expansion capacity
Expiratory reserve volume (ERV)	Extra air that can be breathed out after a normal expiration	Reflects how much air can be voluntarily expelled
Vital capacity (VC)	Maximum amount of air that can be exhaled after a deep inhalation	IRV + TV + ERV
Residual volume (RV)	Air left in the lungs after maximal exhalation	Prevents lung collapse
Total lung capacity (TLC)	The sum of all lung volumes	VC + RV

Interpreting Spirometer Data

By analysing a trace, you can assess:

Lung efficiency – how much air the lungs can hold.
Breathing rate – number of breaths per minute.
Depth of breathing – changes in tidal volume during exercise or rest.
Presence of disease – reduced volumes or irregular patterns may indicate respiratory problems.

For example:

A flat or reduced trace may suggest airway obstruction.
A deep and regular trace indicates normal lung function.

Example Exam Question

1. Explain what information a spirometer trace can provide about a person’s lung function.

✅ Model Answer:

A spirometer trace shows the volume of air a person breathes in and out over time. It can be used to measure tidal volume, vital capacity, and breathing rate, which indicate how efficiently the lungs are working. If volumes are lower than expected, this may show a restriction (such as lung fibrosis) or obstruction (such as asthma).Therefore, spirometry provides quantitative data about lung capacity and respiratory health.

2. A learner uses a spirometer and breathes in and out several times using the same air that is in the spirometer chamber. Explain why a carbon dioxide absorber is used

✅ Model Answer:

The spirometer is a closed system (the only gases going in an out are those being breathed in and out by the learner). Therefore any exhaled carbon dioxide stays in the spirometer. This must be removed/absorbed because carbon dioxide is toxic / acidic and if the learner breathes it back in it could cause respiratory problems.

Also, if carbon dioxide is not removed, it will add to the volume measured and therefore the learner cannot measure oxygen consumed because there will be no change of volume of air in chamber

3. In the following spirometer trace, what do the changes represented by the numbers 1-4 indicate?

✅ Model Answer:

Residual volume (RV)

Air left in the lungs after maximal exhalation

Expiratory Reserve Volume (ERV)

Extra air that can be breathed out after a normal expiration

Inspiratory Reserve Volume (IRV)

Extra volume that can be breathed in after a normal inspiration

Vital Capacity (VC)
Maximum amount of air that can be exhaled after a deep inhalation

Key Takeaways

Spirometry measures air volumes and breathing rates.
Traces show different lung volumes that reveal how well the lungs function.
Used clinically to diagnose or monitor respiratory conditions.
In Unit 5, you may be asked to interpret spirometer data, calculate lung volumes, or compare traces from healthy and unhealthy subjects.