Doctors may use a spirometer to measure lung activity to find out about people's lung capacity and how easy it is for them to breathe in and out. This is useful when checking the health of people with diseases of the lungs, such as chronic obstructive pulmonary disease.
Peak flow meters are used to detect the speed with which people can exhale the air from their lungs. This tells doctors how clear the airways are. A spirometer may be used to measure lung volumes and the rate at which oxygen is used.
At rest, air moves in and out of you lungs about 12 times per minute as your diaphragm and intercostal muscles contract and relax. Each breath refreshes some of the air in your lungs and carries away some of the CO2 generated in your body. If you exercise, or are frightened, you breathe more deeply and more quickly. This gets more oxygen-rich air into your lungs and removes more CO2-rich air out of you lungs.
Different elements of lung volume
- Tidal volume - The volume of air moved in and out of the lungs with each breath when you are at rest. Approx. 0.5dm3 & provides the body with enough safe oxygen for its resting needs while removing enough CO2 to maintain a safe level.
- Vital capacity - is the largest volume of air that can be moved into and out of the lungs in any one breath. Approx. 5dm3 but varies between men and women. Also varies with a person's size & age. Regular exercise increases vital capacity.
- Residual volume - is the volume of air that always remains in the lungs, even after the biggest possible exhalation. It is about 1.5dm3.
- Dead space - is the air in the bronchioles, bronchi and trachea. There is no gas exchange between this air and the blood.
- Inspiratory reserve volume - is how much more air can be breathed in (inspired) over and above the normal tidal volume when you take in a big breath. You call on this when exercising.
- Expiratory reserve volume - is how much more air can be breathed out (expired) over and above the amount that is breathed in a tidal volume breath.
Measuring oxygen uptake
If someone breathes in and out a spirometer for a long period of time, the level of CO2 will increase dangerously.
To avoid this, soda lime is used to absorb the CO2 that is exhaled. this means that the total volume of gas in the spirometer will go down. because the volume of CO2 breathed out is the same as the volume of O2 breathed in, as the CO2 is removed this total reduction is equal to the volume of O2 used up by the person breathing in and out. this allows us to make calculations of oxygen use under different conditions:
- Breathing rate/Breaths per minute = Number of peaks per minute.
- Mean tidal volume/dm3 = Mean of several tidal volumes from the trace.
- ventilation rate/dm3 = volume of air taken into the lungs per minute = breathing rate x mean tidal volume
- Oxygen consumption/dm3 per minute = decrease in peaks over a period of 1 minute