Using A Spirometer To Measure Tidal Volume And Breathing Rate

How Does A Spirometer Work?

The general principle behind a spirometer is simple. It is effectively a tank of water with an air-filled chamber suspended in the water. It is set up so that adding air to the chamber makes the lid of the chamber rise in the water, and removing air makes it fall. Movements of the chamber are recorded using a kymograph (pen writing on a rotating drum). Tubes run from the chamber to a mouthpiece and back again. Breathing in and out through the tubes makes the lid of the chamber fall and rise. The volume of air the person inhales and exhales can be calculated from the distance the lid moves. The apparatus can be calibrated so that the movement of the lid corresponds to a given volume.

Method

1. A canister containing soda lime is inserted between the mouthpiece and the floating chamber. This absorbs the carbon dioxide that the subject exhales.
2. After calibration, the spirometer is filled with oxygen. A disinfected mouthpiece is attached to the tube, with the tap positioned so that the mouthpiece is connected to the outside air.
3. The subject to be tested puts a nose clip on, places the mouthpiece in their mouth and breathes the outside air until they are comfortable with breathing through the tube.
4. Switch on the recording apparatus and at the end of an exhaled breath turn the tap so that the mouthpiece is connected to the spirometer chamber. The trace will move down as the person breathes in.
5. After breathing normally the subject should take as deep a breath as possible and then exhale as much air as possible before returning to normal breathing.