3.1.5 Spirometers

Measuring Breathing

• There are four main ways that breathing can be scientifically measured. These include:
  • Vital capacity - this is the maximum volume of air that can be breathed in or out in one breath
  • Tidal volume - this is the volume of air that is breathed in or out during normal breathing (at rest)
  • Breathing rate - this is the number of breaths taken in one minute (one breath = taking air in and breathing it back out again)
  • Oxygen uptake - this is the volume of oxygen used up by someone in a given time

Spirometers

• The breathing measurements described above can all be made using a piece of apparatus known as a spirometer
• The person (subject) being examined breathes in and out through the spirometer
• Carbon dioxide is absorbed from the exhaled air by soda lime in order to stop the concentration of carbon dioxide in the re-breathed air from getting too high, as this can cause respiratory distress
• As the subject breathes through the spirometer, a trace is drawn on a rotating drum of paper or a graph is formed digitally, which can be viewed on a computer
• From this trace, the subject's vital capacity, tidal volume and breathing rate can all be calculated
• Oxygen uptake can also be calculated using a spirometer
  • Carbon dioxide is removed from the exhaled air, meaning that the total volume of air available in the spirometer gradually decreases, as oxygen is extracted from it by the subject's breathing
  • This change in volume is used as a measure of oxygen uptake

Spirometers are used to measure different aspects of breathing. There are different types of spirometers.

Analysing Data from a Spirometer

• The results from a spirometer (either in the form of a trace drawn on graph paper or a digital graph created by a computer) can be used to calculate vital capacity, tidal volume and breathing rate. This is shown in the image below
  • A small amount of air, known as the residual volume, is always retained in the lungs

The changes in the volume of air present in the lungs are shown here. Note the residual volume, this is the volume of air left in the lungs after as much air has been breathed out as possible.

Step One: Count the number of breaths in the first 60 seconds.

One breath is shown by the trace going up and then down, so there are 12 breaths in the first 60 seconds.

Step Two: Give appropriate units.

Breathing rate should be given in breaths min⁻¹ (breaths per minute), so the breathing rate during the first minute = 12 breaths min⁻¹.

Step Three: Count the number of breaths in the second 60 seconds.

There are 14 breaths in the second 60 seconds.

Step Four: Give appropriate units.

The breathing rate during the second minute = 14 breaths min⁻¹.

Step One: For the 'at rest' phase of the trace, measure the difference between the top and bottom of the trace in terms of the volume of air in the lungs.

During rest, the tidal volume = 3 dm³ - 2.6 dm³

= 0.4 dm³

Step Two: At the peak tidal volume during exercise, measure the difference between the top and bottom of the trace in terms of the volume of air in the lungs. 

The peak tidal volume during exercise occurs right at the end of the exercise period (at around 100 seconds):

= 4.1 dm³ - 2.3 dm³

= 1.8 dm³