Spirometer trace interpretation

During normal breathing, only part of the air in the lungs is exchanged. Tidal volume is the amount breathed in or out in one normal breath. On this trace, it is the vertical distance from a normal expiration trough to the next inspiration peak. It is not the total amount of air in the lungs at the peak.

In the constructed resting example, the volume rises from 2.2 L to 2.7 L. The tidal volume is therefore $2.7-2.2=0.5\text{ L}$ per breath. Breathing that air out completes the same breath; it does not double the tidal volume.

You can breathe in more air after finishing a normal inhalation. This extra amount is the inspiratory reserve volume. Identify it between the normal inspiration peak and the maximum inspiration level. Here, it is $5.0-2.7=2.3\text{ L}$ at rest.

Similarly, you can force out more air after a normal exhalation. This is the expiratory reserve volume, shown between the normal expiration trough and the maximum expiration level. Here, it is $2.2-1.2=1.0\text{ L}$ at rest.

Even after breathing out as much as possible, air remains in the lungs. This is the residual volume. In the teaching diagram, it is the region from zero to the maximum expiration level: 1.2 L. The normal trough is higher than this because an ordinary breath out does not use the expiratory reserve.

What changes when exercise begins?

During exercise, working muscles require more oxygen and produce more carbon dioxide. Breathing becomes deeper and faster, helping increase the exchange of these gases.

Deeper breathing increases tidal volume. The normal inspiration peaks move towards the maximum inspiration level, while the normal expiration troughs move towards the maximum expiration level. More of the available volume is now used in each ordinary breath, leaving smaller inspiratory and expiratory reserves. Residual volume stays the same.

In the exercise section, breaths range from 1.7 L to 3.2 L. Tidal volume has increased to $3.2-1.7=1.5\text{ L}$. The inspiratory reserve has decreased to $5.0-3.2=1.8\text{ L}$, and the expiratory reserve has decreased to $1.7-1.2=0.5\text{ L}$. Lung volumes vary from person to person.

Reading and continuing a trace

The graph separates two changes that are easy to confuse. Larger vertical swings show increased tidal volume: deeper breaths. More complete cycles in the same time show increased breathing rate: faster breaths. You can determine breathing rate by counting complete breaths in a timed interval and scaling to one minute. For example, six breaths in 30 seconds gives 12 breaths per minute.

To continue a resting trace through the onset of exercise, carry the line forwards from its existing endpoint. Draw progressively larger rises and falls, with the cycles becoming closer together along the time axis. Keep the normal breaths within the maximum inspiration and expiration limits, and leave residual volume unchanged.

An explanation should connect the pattern to its meaning: the breaths become deeper and more frequent, increasing the amount of air exchanged to meet the demands of exercise.

• Tidal volume increases.
• Inspiratory and expiratory reserve volumes decrease.
• Residual volume stays the same.
• Breathing rate increases.

Interpret or continue a trace

Check the axes and direction first. Draw larger vertical swings for deeper breaths and closer cycles for faster breathing. These changes increase air exchange as working muscles require more oxygen and produce more carbon dioxide.

$\text{Breathing rate}=\dfrac{\text{number of complete breaths}}{\text{time in seconds}}\times60$