Interpreting Spirometer Traces

A spirometer is a device used to measure the volume of air entering and leaving the lungs. Understanding spirometer traces (graphs) is a key skill for AQA GCSE PE (3.1.1.2), as the exam frequently presents traces and asks students to identify lung volumes, interpret changes, and explain how exercise affects breathing patterns. This lesson covers the key lung volumes you need to know, how to read a spirometer trace, and how the trace changes during exercise.

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What Is a Spirometer?

A spirometer is a medical instrument that measures the volume of air inhaled and exhaled by the lungs. The person breathes into the spirometer through a mouthpiece, and the device records the volume of air moved in each breath. The results are displayed as a graph called a spirometer trace (or spirogram).

Spirometry is used in:

• Sport — to assess an athlete's lung function and respiratory fitness
• Medicine — to diagnose and monitor respiratory conditions such as asthma and chronic obstructive pulmonary disease (COPD)
• PE lessons and exams — as a practical demonstration of lung volumes

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Key Lung Volumes

You must know the following lung volumes for AQA GCSE PE:

Lung Volume	Definition	Typical Resting Value
Tidal volume (TV)	The volume of air breathed in or out in one normal, relaxed breath	~500 ml (0.5 litres)
Inspiratory reserve volume (IRV)	The extra volume of air that can be forcefully inhaled after a normal inhalation	~3,100 ml (3.1 litres)
Expiratory reserve volume (ERV)	The extra volume of air that can be forcefully exhaled after a normal exhalation	~1,200 ml (1.2 litres)
Residual volume (RV)	The volume of air that remains in the lungs after the most forceful exhalation — it can never be breathed out	~1,200 ml (1.2 litres)
Vital capacity (VC)	The maximum volume of air that can be exhaled after a maximum inhalation (TV + IRV + ERV)	~4,800 ml (4.8 litres)
Total lung capacity (TLC)	The total volume of air the lungs can hold (VC + RV)	~6,000 ml (6.0 litres)

Exam Tip: You do not need to memorise exact numerical values for each lung volume, but you should know approximate values and, most importantly, be able to identify each volume on a spirometer trace. AQA commonly asks you to label a trace or calculate a volume from a graph.

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Understanding the Spirometer Trace

A spirometer trace is a graph with:

• x-axis: Time (usually in seconds or minutes)
• y-axis: Volume of air in the lungs (usually in litres or millilitres)

The trace shows a wavy line that rises during inhalation and falls during exhalation. Here is how to read the key features:

Tidal Volume (TV)

Tidal volume is shown as the height of the normal, regular breathing waves. It is measured from the lowest point (end of normal exhalation) to the highest point (end of normal inhalation) of the regular pattern.

Inspiratory Reserve Volume (IRV)

IRV is the additional volume above the peak of a normal tidal breath up to the maximum inhalation point. On the trace, it is the space from the top of a normal breath up to the very top of the deepest possible inhalation.

Expiratory Reserve Volume (ERV)

ERV is the additional volume below the trough of a normal tidal breath down to the maximum exhalation point. On the trace, it is the space from the bottom of a normal breath down to the lowest point that can be reached by forceful exhalation.

Residual Volume (RV)

Residual volume cannot be measured directly by a spirometer because this air never leaves the lungs. On a spirometer trace, there is always space below the lowest point of exhalation — this represents the residual volume. It must be estimated using other techniques (such as gas dilution).

The residual volume exists because the lungs never fully collapse — even after the most forceful exhalation, some air remains to keep the alveoli open and prevent them from sticking together.

Vital Capacity (VC)

Vital capacity is the total range from maximum exhalation to maximum inhalation. On the trace, it is measured from the very lowest point (after maximum forced exhalation) to the very highest point (after maximum forced inhalation).

VC = TV + IRV + ERV

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Reading a Spirometer Trace — Key Skills

When examining a spirometer trace, you should be able to:

1. Identify tidal volume — Look at the regular wave pattern and measure the height of one wave (peak to trough).
2. Identify IRV — Look for a maximal inhalation above the normal peak; the distance from the normal peak to the maximum peak is the IRV.
3. Identify ERV — Look for a maximal exhalation below the normal trough; the distance from the normal trough to the maximum trough is the ERV.
4. Calculate vital capacity — Add TV + IRV + ERV, or measure from the maximum exhalation to the maximum inhalation.
5. Identify residual volume — Note the volume below the maximum exhalation line (cannot be directly measured by the spirometer).
6. Count breathing rate — Count the number of complete breath cycles (one full wave = one breath) in a given time period.
7. Calculate minute ventilation — Multiply tidal volume by breathing rate.

Worked Example

A spirometer trace shows the following at rest:

• The peak of normal breathing reaches 3.0 litres on the y-axis
• The trough of normal breathing reaches 2.5 litres on the y-axis
• A maximum inhalation reaches 5.5 litres
• A maximum exhalation reaches 1.5 litres
• There are 15 complete breaths in one minute

Tidal volume = 3.0 − 2.5 = 0.5 litres

IRV = 5.5 − 3.0 = 2.5 litres

ERV = 2.5 − 1.5 = 1.0 litres