Spirometers that comprise a closed space connected to a subject’s airway, record the volume changes of the lung. Their principal use is in lung function laboratories.
Spirometers with gas conditioning
Gas conditioning is mandatory if a subject breathes into a spirometer for a prolonged period of time:
- oxygen needs to be supplied
- carbon dioxide needs to be removed by chemical binding to soda lime, a process associated with the dissipation of heat
- gas circulation through the tubing is required to prevent dead space ventilation.
Spirometers with gas conditioning are mainly applied for measuring residual volume and total lung capacity by the helium dilution method, for simple spirometric tests of FEV1 and VC, but also for recording the breathing pattern, the oxygen consumption, and during exercise. The spirometer bell moves up and down in a water seal (‘water seal spirometer’). Due to constructive details they are incapable of recording very fast volume changes and are therefore unsuitable for the recording of maximum inspiratory and expiratory flow-volume curves.
The water-sealed spirometer shown schematically in the illustration has been replaced in most laboratories by spirometers in which a rolling seal replaces the water seal, and the movements of the spirometer bell are recorded electronically.
Spirometers without gas conditioning
Spirometers of this type can be designed to be suitable for recording rapid volume changes. The spirometer is usually equipped with a rolling seal rather than a water seal (‘rolling seal spirometer’). They are usually suitable for recording flow-volume curves, but in their basic form unsuitable for measurements which entail prolonged breathing into the apparatus.